La Metallurgia Italiana - Numero 5, maggio 2020

La

Metallurgia Italiana

International Journal of the Italian Association for Metallurgy

n. 5 maggio 2020 Organo ufficiale dell’Associazione Italiana di Metallurgia.

Rivista fondata nel 1909

La Metallurgia Italiana International Journal of the Italian Association for Metallurgy Organo ufficiale dell’Associazione Italiana di Metallurgia.

House organ of AIM Italian Association for Metallurgy.

Rivista fondata nel 1909

Direttore responsabile/Chief editor: Mario Cusolito Direttore vicario/Deputy director: Gianangelo Camona Comitato scientifico/Editorial panel: Livio Battezzati, Christian Bernhard, Massimiliano Bestetti, Wolfgang Bleck, Franco Bonollo, Bruno Buchmayr, Enrique Mariano Castrodeza, Emanuela Cerri, Lorella Ceschini, Mario Conserva, Vladislav Deev, Augusto Di Gianfrancesco, Bernd Kleimt, Carlo Mapelli, Jean Denis Mithieux, Marco Ormellese, Massimo Pellizzari, Giorgio Poli, Pedro Dolabella Portella, Barbara Previtali, Evgeny S. Prusov, Emilio Ramous, Roberto Roberti, Dieter Senk, Du Sichen, Karl-Hermann Tacke, Stefano Trasatti Segreteria di redazione/Editorial secretary: Valeria Scarano Comitato di redazione/Editorial committee: Federica Bassani, Gianangelo Camona, Mario Cusolito, Carlo Mapelli, Federico Mazzolari, Valeria Scarano Direzione e redazione/Editorial and executive office: AIM - Via F. Turati 8 - 20121 Milano tel. 02 76 02 11 32 - fax 02 76 02 05 51 met@aimnet.it - www.aimnet.it

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Editoriale / Editorial

AIM reagisce al lock-down

Ing. Mario Cusolito ............................................................................................................................ pag.04

Memorie scientifiche / Scientific papers

Metallurgia Additiva / Additive Metallurgy Microstructure based strengthening model of a biocompatible WE54 alloy reinforced by SiC

M. Cabibbo, F. F. Průša......................................................................................................... pag.08

Knock (and pre-ignition) damage on engine components: case studies

E. Balducci, Parisi, F. Boccia, M. Barichello, L. Ceschini.............................................................. pag.20

n. 5 maggio 2020

Anno 112 - ISSN 0026-0843

Detection of Non-metallic Inclusions in 12Mn Steel Continuous Casting Round Billets

X. Wang, J. Wei, S. Qiu......................................................................................................... pag.27

Study on Heat Transfer Characteristics during Solidification of 18-ton Steel Ingot with Large Ratio of Height to Diameter

Z. Yu, H. Zhang, X. Wang, X. Wu ........................................................................................... pag.37

Attualità industriale / Industry news

Il nuovo acciaio da utensile da lavorazione a caldo THERMODUR E40K SUPERCLEAN

indice

a cura di: E. Prati, A. Magistrelli ............................................................................................. pag.48

Scenari / Experts' Corner

LA METALLURGIA NEI GRANDI POEMI DELL’ANTICHITÁ

a cura di: Giuseppe G. Casarini .............................................................................................. pag.58

Atti e notizie / AIM news

Eventi AIM / AIM events ........................................................................................ pag.66 Comitati tecnici / Study groups........................................................................... pag.67

editoriale - editorial

AIM reagisce al lock-down

La situazione sanitaria dovuta alla pandemia ha colpito con gravità inaspettata tutto il mondo, compreso quello scientifico e produttivo che ruota intorno allo studio e alla fabbricazione dei metalli.

L’Associazione Italiana di Metallurgia ha dovuto reagire e adat-

Mario Cusolito Direttore de La Metallurgia Italiana

tarsi alle mutevoli realtà che di giorno in giorno si delineano.

Il lock-down imposto in Italia e in molti altri paesi ha segnato l’inizio di un periodo di grande incertezza: non sappiamo quando potremo ricominciare a frequentare luoghi affollati come i congressi e le riunioni, non sappiamo quando potremo ricominciare a viaggiare senza restrizioni. Tutto ciò ci impedisce, per ora, un normale rapporto con le persone che incontriamo e

“

che vorremmo incontrare.

Una delle attività principali di AIM è l’organizzazione e la gestione di iniziative che hanno lo scopo di diffondere la conoscenza metallurgica. L’Associazione ha dovuto quindi modificare i pro-

Facciamo e faremo il possibile per tenere alto il livello scientifico degli articoli e l’interesse dei lettori.

pri programmi differendo gli eventi previsti a nuove date nelle

Mario Cusolito

– European Conference on Continuous Casting” – si terrà nella

quali, presumibilmente, si starà tornando verso la normalità.

Il maggiore evento internazionale del prossimo futuro, l’”ECCC

medesima location di Bari dal 21 al 23 ottobre, mentre il “38° Convegno Nazionale AIM” resta, per il momento, confermato dal 16 al 18 settembre a Napoli. Il 27° Convegno Nazionale Trattamenti Termici” è stato differito al 18 e 19 novembre, mantenendo la prestigiosa location al Porto Antico di Genova.

Le numerose Giornate di Studio previste in questo periodo vengono rimandate alla prima data utile, tenendo conto anche della disponibilità delle eventuali aziende ospitanti, per salvaguardare uno dei principali valori aggiunti che è quello della visita ai

La Metallurgia Italiana - maggio 2020

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editoriale - editorial

reparti produttivi e ai laboratori.

Anche l’offerta formativa didattica dei Corsi organizzati da AIM ha subito importanti modifiche: in questo caso però AIM sta seguendo nuove strade, da affiancare a quelle tradizionali. E così, il “Corso itinerante sulla macchina fusoria” sarà necessariamente ripianificato appena possibile, perché la visita alle acciaierie è parte integrante del programma, mentre il corso “Metallurgia di base propedeutico ai trattamenti termici” sarà ripensato per una fruizione online delle lezioni.

Altra importante innovazione è la nuova proposta di formazione ed aggiornamento a distanza, denominata FaReTra – Fair Remote Training, che va arricchendosi costantemente di contenuti grazie all’impegno dei diversi comitati tecnici. A fare da apripista è stato il Comitato Tecnico Acciaieria con il “Corso Acciai ad Alto Carbonio”, che ha che ha ottenuto un notevole successo di pubblico in streaming. Ulteriori proposte all’interno del contenitore FaReTra sono i corsi “in pillole”, lezioni online di diversa durata per una formazione di base sui vari argomenti in attesa della didattica frontale: sarà affrontato in questo modo il corso di “Failure Analysis in pillole”, così come è già in fase di svolgimento “Siderurgia in pillole” e come presto saranno disponibili “Tecnologie Additive in pillole” e “Tecnologie Press & Sinter non tradizionali in pillole”.

L’obiettivo principale del progetto FaReTra è mantenere il contatto con il proprio pubblico e non fare mancare sul mercato l’offerta in cui AIM è specializzata, oltre a poter affinare uno strumento che completi, anche ad emergenza conclusa, la formazione in presenza.

La Rivista dell’Associazione Italiana di Metallurgia è fedele testimone delle attività dell’Associazione: da diversi anni ogni singolo numero della Rivista, oltre a aggiornare sulle attività di AIM, tende ad avere un filo conduttore preciso, praticamente monografico, con articoli basati su eventi quali convegni, conferenze e seminari. Lo spostamento di questi eventi ci obbliga ad una nuova pianificazione che non sempre ci permetterà numeri monografici: nonostante questo facciamo e faremo il possibile per tenere alto il livello scientifico degli articoli e l’interesse dei lettori.

Ci auguriamo quindi di avervi con noi, lettori sempre più attenti, e vi auguriamo di superare al più presto questa emergenza per tornare ad una vita lavorativa e affettiva priva di tutti gli attuali condizionamenti.

The health situation due to the pandemic has hit the whole world with unexpected severity, including the scientific and productive situation that revolves around the study and manufacture

La Metallurgia Italiana - May 2020

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editoriale - editorial

of metals.

The Italian Metallurgy Association had to react and adapt to the changing realities that are emerging day by day. The lock-down imposed in Italy and in many other countries marked the beginning of a period of great uncertainty: we don't know when we will be able to start again to attend crowded places like congresses and meetings, we don't know when we will be able to start travelling without restrictions. All this prevents us, at the moment, from having a normal relationship with the people we meet and whom we would like to meet.

One of the main activities of AIM is the organization and management of initiatives aimed at spreading metallurgical knowledge. The Association has therefore had to change its programs by postponing the planned events to new dates when, presumably, the situation will be back to normal.

The biggest international event of the near future, the "ECCC - European Conference on Continuous Casting" - will be held in the same location in Bari from October 21st to 23rd, while the "38th AIM National Conference” remains, for the moment, confirmed from September 16th to 18th in Naples. The “27th National Conference on Heat Treatment" has been postponed to November 18th and 19th, maintaining the prestigious location at the Porto Antico of Genoa.

The numerous Study Days planned in this period are postponed to the first useful date, also taking into account the availability of possible host companies, with the aim of safeguarding one of the main added value that is the visit to the production departments and laboratories.

The didactic training offer of the Courses organized by AIM has undergone important changes too: in this case, however, AIM is following new paths, to be added to the traditional ones. And so, the "Travelling course on the melting machine" will necessarily be re-planned as soon as possible, because the visit to the steelworks is an integral part of the program, while the course "Basic Metallurgy preparatory to heat treatments" will be rethought for an online use of the lessons.

Another important innovation is the new proposal for distance training and updating, called FaReTra - Fair Remote Training, which is being constantly enriched with content thanks to the commitment of the various technical committees. The Steelmaking Technical Committee has been the forerunner with the “High Carbon Steels Course”, which has achieved considerable success with the streaming audience. Further proposals within the FaReTra container are the "in pills" courses, online lessons of different duration for a basic training on the various topics

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editoriale - editorial

waiting for frontal teaching: the "Failure Analysis in Pills" course will be dealt with in this way, just as "Steelmaking in Pills" is already in progress and "Additive Technologies in Pills" and "Non-traditional Press & Sinter Technologies in Pills" will soon be available.

The main objective of the FaReTra project is to maintain contact with AIM audience and not to miss on the market the offer in which AIM is specialized, in addition to being able to refine a tool that will complete training in presence, even after the emergency finishes.

The “International Journal of the Italian Association for Metallurgy� is a faithful witness of the Association's activities: for several years now, every single issue of the magazine, besides updating on AIM's activities, tends to have a precise, practically monographic thread, with articles based on events such as conventions, conferences and seminars. The displacement of these events obliges us to a new planning that will not always allow us monographic issues: despite this, we do and will do our best to keep the scientific level of the articles and the interest of the readers high.

We therefore hope to have you with us, increasingly attentive readers, and we wish you to overcome this emergency as soon as possible in order to return to a working and emotional life devoid of all current conditioning.

La Metallurgia Italiana - May 2020

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Memorie scientifiche - Miscellanea

Microstructure based strengthening model of a biocompatible WE54 alloy reinforced by SiC M. Cabibbo, F. Průša A large number of magnesium alloys and magnesium-based composites are nowadays used as biocompatible light

metallic materials. Example of their applications include bone-tissue screws, cardiac valves, orthodontic screws and

components. In this sense, the biocompatibility, durability, and corrosion resistance and blood compatibility are key factors for the full availability of magnesium based alloys in the bioengineering field. On the other hand, minimal necessary mechanical properties necessary for their potential application in such a filed were investigated in the last three

decades. With this respect, not only magnesium based alloys, but also magnesium composite alloys were tested for their biocompatibility. Oxides such TiO 2, MgO, ZnO, ZrO 2, TiB 2, Al 2O 3, and also SiC showed sufficient biocompatibility

and in addition, composite magnesium alloys added with such oxides or SiC are known to possess higher mechanical

properties compared to their magnesium alloy counterparts. Among the different available metallurgical technologies to produce magnesium alloys, the powder metallurgy (PM) is surely one of the most promising one. With this regard,

squeeze casting is one of the most reliable and cost-effective PM technique of production of magnesium based alloys and composites.

In the present work, the microstructure and mechanical properties of WE54+15vol.%SiC under various compression temperature conditions, up to 300°C, were investigated by transmission electron microscopy (TEM). Microstructure inspections revealed the formation of stable cuboid secondary phase particles, and lamellae and irregular-shaped in-

termetallic phases. A microstructure-based strengthening model was proposed and compared to the experimentally obtained compression stress carried out at temperatures ranging 50-to-300°C. The most effective strengthening term

was found to be the one coming from the refined grain structure. A further important strengthening contribution was constituted by the secondary phase particle precipitation within the Mg-matrix.

KEYWORDS: BIOCOMPATIBLE MG-BASED ALLOYS; COMPRESSION; STRENGTHENING MODEL; TEM. Introduction Mg-RE (WE) alloys have driven much interest as biomedical applications due to their outstanding high strength at both room temperature and high temperatures, excellent precipitation hardening and good biocompatibility [1-3]. With this respect, commercial alloys such the WE43 and WE54 have been widely used in various structural and biomedical applications due to their good balance on performance and cost. This interest favored a number of investigations

Marcello Cabibbo

DIISM / Università Politecnica delle Marche, Ancona, Italy

Filip Průša

University of Chemistry and Technology, Prague, Department of

Metals and Corrosion Engineering, Technická 5, 166 28 Prague 6, Czech Republic

on the mechanical properties [4-6], deformation behavior [7,8], precipitation sequence [9,10] of the WE series alloys.

Mg-Gd-based alloys with some Y or Nd addition exhibited

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Scientific papers - Miscellaneous remarkable aging-hardening response and good ductility

ly reduced as compared to unreinforced alloys. Bio-com-

even after T6 treatment [11]. With this respect, Wang et al.

posites have been developed and used in tissue enginee-

[4] reported that over 2 wt% Gd additions significantly im-

ring, drug delivery, dentistry and bone implants because of

proved the ductility of the as-cast Mg-5Y-3Nd-0.6Zr alloy.

their high performance. However, their low stiffness, poor

Many researchers reported formation of particle, namely

mechanical properties, and inflammation issues during the

cuboid-shaped phases Mg24RE5 and Mg5RE ([12] to cite but

implantation period have limited their use in the biomedi-

one).

cal field [13].

The biodegradable metallic materials can be divided into

The composite magnesium alloys usually contain at least

three categories, such as pure metals, alloys, and composi-

two components, which are known as matrix and reinfor-

tes. Commercially available AZ91D Mg alloy is considered

cement. All the components of the composite must be

the most suitable bio-engineering magnesium alloy, and it

biocompatible and nontoxic in any physiological environ-

has been so far deeply investigated in vivo and vitro envi-

ment. The composite material allows the combination of

ronments. On the other hand, composite materials must

matrix and reinforcement properties such as sufficiently

possess the highest level of mechanical strength, corro-

high mechanical properties (tensile strength, elastic mo-

sion resistance and biocompatibility to be qualified as a

dulus, yield strength, compression strength), corrosion re-

biomaterial implant.

sistance and biocompatibility. Thence, selection of matrix

Ceramic-based implants were also developed due to their

component and reinforcement component are very critical

better mechanical performance, lack of an inflammation

to obtain the desired properties [23,24].

reaction, and optimum degradation rate. However, bio-ab-

In the present work a WE54 alloy added with 15%vol.SiC

sorbable ceramics also have fast degradation rates com-

was tested by compression at high temperatures. Micro-

pared to non-absorbable ceramics. Mg-based bio-alloys

structure evolution was inspected by electron microscopy

are considered to absorb within the human body at an ap-

(TEM). The work presents a microstructure based stren-

propriate resorption rate, but they also have a problem of

gthening model that was compared to the experimental

a fast degradation rate during tissue remodelling, which li-

compression tests carried out at temperatures ranging

mits their applications in clinical fields [13]. For this reason,

50-to-300째C.

Mg-based alloys are still considered to be a good choice in the fields of tissue engineering, orthopaedics and cardio-

Experimental procedure

vascular stents because of their suitable mechanical pro-

The material used in this study was a WE54 magnesium ma-

perties, reasonable biodegradation and lower toxicity [13].

trix composite. Commercial WE54 (Mg-5wt.%Y-4wt.%RE,

Researchers have developed Mg-based bio micro-com-

mainly Nd) alloy was reinforced by 15vol.% silicon carbide

posites and bio nano-composites to achieve the desired

particles. Composite was prepared by squeeze casting te-

mechanical properties, optimized corrosion resistance,

chnique. In particular, a two-step squeeze casting process

minimum cytotoxicity, and high biocompatibility [14-21].

was followed to fabricate the WE54-SiC composite. This

Numerous opportunities and challenges still exist in deve-

was constituted by a first stage of very low pressure for in-

loping Mg-based biodegradable composites and alloys for

filtration applied to the matrix melt, followed by melting

biomedical applications. In this sense, Mg-RE alloys (such

and solidification at quite high squeezing pressure. The SiC

as the WE series) provide better mechanical properties and

particles were embedded in the alloy through preheating

corrosion resistance at both room and high temperatures

in a furnace at 400-450째C, before further maintenance at

[22].

900째C for 2 h. During squeeze casting, the applied pressu-

Considerable improvement of the mechanical properties

re and time for infiltration were 0.4-to-0.5 MPa and 15-25

can also be achieved by reinforcement with ceramic par-

s, respectively. Then, the WE54-SiC composite alloy was

ticles or fibres. Metal matrix composites (MMCs) provide

melted in an electric furnace with a graphite crucible using

a substantial increase in strength and stiffness as well as

a specific flux of inert gas protection (namely argon). The

creep resistance. The ductility of composites is significant-

casting temperature was fixed at 750째C. For all experimen-

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Memorie scientifiche - Miscellanea tal runs, the squeezing pressure after infiltration and the

lected-area diffraction pattern (SAEDP).

dwelling of the squeezing pressure were 100 MPa and 90

All the here reported mean values and statistical evalua-

s, respectively.

tions come from at least 80 individual measurements for

Compression tests were carried out at temperatures

SiC and twin spacing. The mean values of secondary pha-

between room temperature and 300°C using an INSTRON

se particles and grain size were evaluated by stereological

testing machine, according to ASTM E9. Cylindrical speci-

methods (ASTM E112) and were obtained from a series of a

mens of 8mm in diameter and 12 mm long were deformed

minimum of 150 individual measurement per each experi-

at an initial strain rate of 2.8·10-4 s-1.

mental condition. Mean secondary phase particle diameter

The microstructure was inspected by optical microscopy

and SiC geometrical dimensions (length, size and thick-

(OM) and by transmission electron microscopy (TEM). Thin

ness) were evaluated by stereological means. In particu-

foils for TEM were mechanically thinned down to ~100 µm,

lar, particle thickness was measured directly on TEM by an

punched, and then dimpled to a thickness of ~20 µm at the

in-line stereo-features measurement facility.

center of the 3-mm disk. This was furthermore thinned to electron transparency using a precision ion polishing sy-

Results and discussion

stem (GatanTM PIPS) with an initial tilt angle of 8°, followed

Microstructure and mechanical properties

by a running angle of 3°, at voltage of 4.5 V and cooled by

The microstructure of the composite is reported in Figure

liquid nitrogen. A PhilipsTM CM20® working at 200kV and

1. The SiC particles are not uniformly distributed in the ma-

equipped with a double tilt specimen holder was used.

trix as they mostly tend to randomly cluster.

Detected secondary phase particles were identified by se-

Fig.1 - Microstructure of the as-produced, undeformed WE54-15%vol.SiC composite.

The compressive true stress-strain curves at strain rate of

grain size, Dg. It appeared that a linear relationship betwe-

2.78 x 10−4s−1 and different temperatures are shown in the

en the alloy strengthening and the grain size evolution with

Figure 2 inset. Figure 2 also shows the yield stress obtained

compression temperature was identified.

experimentally as a function of the measured alloy mean

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Scientific papers - Miscellaneous

Fig.2 - Yield strength, σ0.2 vs. mean grain size, Dg. Inset reports the true stress-strain curves at different compression test temperatures (room-temperature, 50, 100, 150, 200, 250, and 300°C) [25].

This was as: σ0.2 = 272 – 6.1·Dg, and thence in the present

started to decrease with the temperature. In other words,

case, a constant strengthening ratio reduction of ~6 MPa

the compression strength of the WE54 SiC reinforced al-

per micron of grain size increase could be established. The

loy did not changed from room temperature to 200°C. This

temperature dependences of the characteristic stresses,

ultimately means that, according to the 90/385/EEC (active

the yield stress σ0.2 and the ultimate stress (UTS) σU, are

implantable medical devices directive), the compression

shown in Figure 3. It appears a clear influence of the test

behaviour of the present alloy is stable for the human body

temperature on the resulting alloy strain hardening, at a test

temperatures and for the possible local mechanical pro-

temperature beyond 200°C, as the compressive flow stress

cesses of working heating and cooling prior implantation.

Fig.3 - Temperature dependences of the alloy strength, UTS (σmax), and YS (σ0.2), from room temperature (17°C) to the maximum test temperature of 300°C [25].

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Memorie scientifiche - Miscellanea This interesting alloy mechanical response was accounted

the present WE54-SiC alloy.

from a microstructure viewpoint by a systematic micro-

The microstructure inspections revealed the twin forma-

structure inspection carried out by electron microscopy

tion within the magnesium matrix. These were of nano-

(TEM).

metric scale and were found to be quite narrow and lying

Figure 4 reports representative TEM micrographs of the al-

parallel to each other (Figure 5). Anyhow, nano-twins were

loy microstructure at room temperature (Figure 4a), after

detected only at testing temperature above 200°C. The im-

compression test at 50°C (Figure 4b), 150° (Figure 4c), and

portant role of twinning in the deformation of hexagonal

300°C (Figure 4d). It appeared that the microstructure re-

closed-packed (hcp) lattice alloys is well known. On the

mained stable up to 200°C, being quite similar to the one

other hand, dislocation density plays important role in har-

observed at room temperature (that is in the as-produced

dening mechanism in composite. With addition of the rein-

condition). Yet, the microstructure after the compression

forcing phase, the geometrically necessary dislocations

tests at 300°C clearly revealed a significant grain coarse-

are generated to accommodate the mismatch of plastic

ning and secondary phase deterioration, that is coarsening

deformation in the matrix. New dislocations arise directly

of the reinforcing secondary phase particles. In addition,

in the production process due to the solidification process

at 300°C compression test, most of the coarse secondary

during preparation of the composite. A higher dislocation

phase particles tend to align almost continuously at grain

density in the composite material induces a higher level

boundaries. This indeed correspond to a weakening mor-

of internal stress. The detected higher matrix dislocation

phology for the intergranular particles that can led to me-

density as well as the reinforcement/matrix interfaces can

chanical failure by grain decohesion. This microstructure

provide high diffusivity paths in the composite alloy. Signi-

degradation with compression temperature is ultimately

ficant amount of dislocations were generated during com-

believed to drive the corresponding mechanical failure of

pressive deformation.

a)

b)

c)

d)

Fig.4 - Representative TEM micrographs showing the microstructure at room temperature (as-producued condition), a); after compression test at 50°C, b); 150°C, c); and at 300°C, d). Rectangular shaped secondary phase particles are Mg24Y5 hardening phases, according to [26].

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Scientific papers - Miscellaneous Based on the quantitative evaluation of all the strengthening

formation, secondary phase formation and eventual coar-

features appeared in the alloy microstructure a strengthe-

sening, composite model of strengthening given by the

ning model was proposed. The major microstructure fea-

SiC particles. This microstructure-based alloy strengthe-

tures contributing to the alloy strength are: grain-structure

ning model is reported in the following.

through Hall-Petch mechanism of strengthening, twinning a)

b)

c)

Fig.5 - Representative TEM micrographs showing the absence of nanotwins after compression at 100°C, a), the formation of nano-twins after compression test at 200°C, b), and after compression test at 300°C, c).

Microstructure-base alloy strengthening model

Hall–Petch relationship, which relates the alloy stress, ∆σHP,

a. Grain size strength (Hall-Petch)

to the mean grain size, Dg, by Eq. (1), [27]:

The strengthening contribution given by the grained structure of the magnesium matrix is expressed by the

(1)

where KHP = 0.28 MPa·m1/2 is the Hall-Petch constant [27].

med sample, and ∆σHP = 135 ± 15 MPa, ∆σHP = 110 ± 10 MPa,

sion test temperature as the mean grain size increased to

respectively. That is, a drastic reduction of the alloy stren-

This contribution varied significantly with the compres-

∆σHP = 50 ± 10 MPa, after compression at 50, 150, and 300°C,

one-order of magnitude at the maximum test temperature

gthening contribution, which more than halved, occurred

of 300°C. In fact, the mean grain size varied from the mini-

after compression at 300°C, respect to the strengthening

mum value obtained at the undeformed experimental con-

contribution yield by the grained structure after compres-

dition (as-prepared), where Dg = 2.8 ± 0.6 µm, up to Dg = 4.3

sion at 150°C.

at 50, 150, and 300°C, respectively. These measured mean

b. Twinning

grain sizes corresponded to a Hall-Petch grain strengthe-

Twin boundaries, σT, was calculated by a Hall-Petch type

± 0.7 µm, Dg = 6.9 ± 0.7 µm, Dg = 33 ± 1 µm, after compression

ning contribution of ∆σHP = 165 ± 15 MPa, for the undefor-

relationship as follows, Eq. (2) [28]:

(2)

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Memorie scientifiche - Miscellanea Where VvTwins is the measured volume fraction of the twins,

essentially irrelevant.

average twin boundary spacing. The detection of twin for-

c. Secondary phase particles

mation during compression at the different temperatures

The strength given by the secondary phase particles is mo-

was carried out by tilting the crystal as to be oriented with

delled by the Orowan mechanism. That is, the precipitation

respect to the beam direction to show {10-12} crystallo-

hardening is an effective strengthening factor not only at

graphic plane. This to highlight the twin boundary edge

room temperature but also at higher temperatures, even

lying direction <10-11> | | {10-12}.

although the secondary phase particles started to coarsen

Twins were detected essentially after compression at

with compressive temperature. As long as these particles

200°C, and not for lower testing temperatures nor in the

are within 50-80 nm in equivalent diameter these act as im-

as-prepared condition. Thus, this contribution only holds

penetrable particles for the sliding dislocations. Thence,

for the conditions of compression at 300°C (Figure 5).

gliding dislocations bows out between consecutive parti-

Anyhow, the volume fractions of the detected twins were

cles bypassing and leaving a loop around them to proceed

quite low, being VvTwins = 2-3 vol.%. The mean twin spacing

in their sliding motion. Thence, following the pioneering

was measured as λTB = 22-to-30 nm. Thus, the twin stren-

works by Scattergood and Bacon, this strengthening me-

gthening contribution accounted for a quite low value ran-

chanism was here modelled using Eq. (3) [29,30]:

kTB = 35 MPa·µm1/2 is a Hall-Petch type constant, λTB is the

ging ∆σT = 4-7 MPa. This make the twinning contribution

(3)

where B = 0.6 for screw dislocations and 0.7 for edge dislocations, T’ = 6.5 is the Taylor factor for Mg, E = 44.4 GPa is the magnesium Young’s modulus, b is the Burgers vec-

± 5 MPa, for the undeformed condition, and to ∆σOrowan = 60 ± 5 MPa, ∆σOrowan = 45 ± 5 MPa, and ∆σOrowan = 25 ± 5 MPa, after compression at 50, 150, and 300°C, respectively.

tor that for Mg is 0.32 nm, ѵ = 0.27 is the Poisson’s ratio, λ represents the average interparticle spacing, which based

d. SiC particle composite strength contribution

on ASTM E112 stereology methods λ = [1.25/(d/NV)-πd/4],

According to the shear-lag model proposed by Nardone

with NV the particle volume density and d the mean particle

and Prewo in [31], the composite particles do contribute

equivalent diameter [29,30]. This latter was quantitatively

to alloy reinforcement carrying a fraction of the load from

evaluated by TEM inspections. It resulted that the particle

the matrix. This alloy strengthening contribution strongly

mean diameter, d, increased significantly form the as-pro-

depends on the shape and morphology of the particles; it

duced, undeformed condition, where d = 50 ± 5 nm, to d =

specifically depends on the particle aspect ratio [32]. Thus,

90 ± 5 nm, after compression at 50°C, d = 125 ± 5 nm, after

the proposed relationship for a composite strengthened

compression at 150°C, and d = 240 ± 20 nm, after the maxi-

alloy is (Eq. (4a)), [31]:

mum test compression temperature of 300°C. This strengthening contribution accounted for ∆σOrowan = 70

(4a)

where σ0 is the unreinforced matrix yield stress, VvSiC the

particle aspect ratio. These were directly measured on

SiC particle volume fraction, L the particle size facing the

TEM. The SiC volume fraction, VvSiC, was determined using

load direction, t the mean particle thickness, A = L/t the

areal analysis (Aa) stereology method (ASTM E-112). For

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Scientific papers - Miscellaneous equiaxed particles, or alternatively particles with 2D-shape

to Eq. (4b):

close to circle, as in the present case, the Eq. (4a) reduces

(4b)

A further strengthening mechanism acting in the composi-

size, and on the effective temperature change. Upon hi-

te WE54 alloy refers to the different thermal expansion co-

gh-temperature compression, the relatively large thermal

efficients (CTE) between the SiC particles and the magne-

expansion coefficient between the matrix and the SiC parti-

sium matrix. This induces a dislocation density increment

cles creates a misfit strain at the SiC-Mg interface. Thermal

with the applied stress, yielding an additional strengthe-

stress can be partially released by the dislocation genera-

ning contribution to the alloy. The amount of the thermal

tion and accumulation in the surroundings of the reinfor-

stress induced by the presence of the reinforcement de-

cement surfaces. Thus, according to [33-35] the induced

pends upon the particle volume fraction, morphology, and

extra dislocation density can be calculated as Eq. (5a):

(5a) where C = 12 for equiaxed particles, ε = ∆α·∆T is the misfit

SiC particles. The thermally generated dislocation density

strain, ∆T is the temperature variation, ∆α = 21·10-6 K-1 is the

yields a strengthening contribution of (Eq. (5b)), [33,35]:

difference between matrix and SiC thermal expansion, VvSiC the particle volume fraction, and t’ the minimum size of the (5b) with α1 = 0.35, and G = 17480 MPa is the shear modulus of

to the metallic magnesium matrix induces geometrically

Mg.

necessary dislocations resulting in an additional strengthe-

Since the average residual stress generated by the thermal

ning contribution to the alloy. The resulting matrix-to-par-

expansion is of tension nature, it is actually a negative con-

ticle misfit depends on the reinforcement size and mor-

tribution to the strengthening to the magnesium composi-

phology [37]. The density of the geometrical necessary

te alloy [36].

dislocations is given by Eq. (6a) [35,36]:

The different nature of the ceramic SiC particles respect (6a) where εp = 0.28 is the plastic strain. The corresponding

strengthening contribution is thus (Eq. (6b)): (6b)

According to the statistical evaluations of all the meaningful

SiC strengthening contribution was evaluated as, Eq. (7):

microstructure features appearing in the Eqs. (4)-to-(6), the

(7)

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Memorie scientifiche - Miscellanea Thus, ∆σSiC = 57 ± 3 MPa, for the undeformed condition,

Hall-Petch) and the one generated by the existing secon-

∆σSiC = 63 ± 3 MPa, ∆σSiC = 115 ± 5 MPa, ∆σSiC = 25 ± 5 MPa,

dary phase particles (primarily the Mg24Y5 particles) steadily

after compression at 50, 150, and 300°C, respectively. That

reduce with compression temperature. On the other hand,

is, the SiC strengthening contribution tended to increase

the contribution yield by the presence of SiC reinforcing

steadily with the compression temperature up to 150°C, to

coarser particles slightly increased from compression

drastically reduce, and then degrading, at 300°C.

temperatures within 150°C, than eventually it drastically reduced from 150 to 300°C. Anyhow, the general trend

e. Strengthening term combination

with compression temperature of the overall strengthe-

Before considering the proper combination of all the here

ning contribution, as calculated by combining the single

detected strengthening terms, the individual impact on

contributions by using Eq. (8), reported in the following,

the composite alloy strengthening at the different com-

showed an almost continuous reduction up to 3-fold less

pression temperatures was addressed. To this purpose,

from room temperature to 300°C. Figure 6 allowed identi-

Figure 6 shows the trend with compression temperatu-

fying the ∆σSiC as the actual distinctive strengthening contri-

re of ∆σHP, ∆σOrowan, ∆σSiC. It resulted that the strengthening

bution with compression temperature.

contribution coming from the matrix (grain size, through

Fig.6 - Plot of the strengthening terms as calculated by Eqs. (1)-to-(6b) for ∆σHP, ∆σOrowan, ∆σSiC vs. compression temperature. The variation of σmodel as obtained by Eq.(8) with compression temperature is also reported for a direct comparison.

As for the overall combination of the individual stren-

strengthening ability, which act unevenly throughout the

gthening contribution, according to Lilholt [38] the stress

matrix, are most suitably combined as the square root of

contributions acting uniformly throughout the matrix are

the sum of the squares [39,40].

superimposed linearly, whereas mechanisms of similar

Thence the following model is here proposed, Eq. (8):

(8)

∆σSiC being the strengthening contribution coming from

The obtained agreement between the modelled yield

the Sic particles as determined by linearly adding Eqs.

strength and the experimentally measured compression

(4)-to-(6d).

strengths was of some 20% between the model and the

Table 1 reports the obtained results by applying the Eq. (8)

measured values at room temperature. The difference re-

to the as-prepared alloy, and the compression tested con-

duced to some 10-15% for compression at 50 and 150°C,

ditions at 50, 150, and 300°C.

but accounted of 30% at the maximum test temperature of

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Scientific papers - Miscellaneous 300°C. In this latter case, the model overestimation is likely

degradation occurred at 300°C came from the significant

to be attributed to a further Orowan-like strengthening

reduction of the secondary phase strengthening contribu-

contribution that was not here taken into consideration

tion, and to a lower extent from the SiC strength contribu-

due to a lack of experimental information. A further rea-

tion. The comparison between the values here obtained

son could be due to observed alloy mechanical degrada-

for the WE54-SiC composite and the ones obtained for a

tion occurred at compression temperature of 300°C, whi-

WE54 compressed for temperatures ranging 50-to-300°C

ch was due to an excess of grain size coarsening. This, in

[41] showed a significant strengthening due to the presen-

turns, resulted in a strength reduction of one-third respect

ce of SiC for test temperature up to 200°C. The mechanical

to the value obtained for the undeformed condition. A se-

degradation observed at 300°C were also observed in [41]

cond microstructure factor affecting the alloy mechanical

for a WE54 SiC-unreinforced alloy.

Tab.1 - Alloy strength as obtained by applying Eq. (8) to the as-prepared alloy, and the compression tested conditions from room-temperature to 300°C. Values coming from a WE54 alloy published by Beladi and Barnett [41] are also reported for comparison.

(1)

YS, σ0.2:

undeformed

compression at 50°C

compression at 100°C

compression at 150°C

compression at 200°C

compression at 300°C

σexp, MPa

250 ± 5

245 ± 5

235 ± 5

230 ± 5

230 ± 5

70 ± 5

σmodel, MPa

305 ± 15

265 ± 15

-

270 ± 20

-(1)

100 ± 20

σWE54, MPa [41]

220

-

205

-

180

100

: The strengthening obtained for compression at 200°C are not reported here as their evaluation was jeopardized by an excess of

experimental data scatter.

Conclusions

It also resulted that at 300°C grain size drastically increased

WE54 Mg-RE (RE=Y, Nd, Gd, Dy) alloy based composite

up to one-order of magnitude respect to the undeformed

was prepared by squeeze casting. Compression tests were

condition.

carried out in the temperature range from room tempera-

A mixed combination of the different strengthening terms

ture up to 300°C. The stress–strain curves showed a high

was used to meet the yield stress obtained mechanically.

degree of work hardening at the lower temperatures. The

Specifically, a linear sum of the evenly distributed stren-

most important contributions to the alloy stress were gi-

gthening terms in the matrix was quadratically combined

ven by the matrix small grain size and by the precipitation

to the contributions given by the presence of the SiC par-

hardening, prior of excessive coarsening upon testing at

ticles: σ0+((∆σHP+∆σT+∆σOrowan)2+∆σSiC2)0.5. This microstructu-

temperatures beyond 200°C. The reinforcing phase contri-

re-based model was able to describe the microstructure

butes to strengthening mainly through an increased dislo-

factors contributing to the alloy strength at the different

cation density, i.e. by thermal and geometrical mismatch.

testing temperatures.

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Memorie scientifiche - Miscellanea ACKNOWLEDGEMENTS

Author wish to thank Prof. Zuzanka TrojanovĂĄ for the fruitful discussion.

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Memorie scientifiche - Miscellanea

Knock (and pre-ignition) damage on engine components: case studies E. Balducci, S. Parisi, F. Boccia, M. Barichello, L. Ceschini

Due to the stringent regulations in terms of CO2 emissions, in order to increase engine efficiency, design strategies are oriented towards both increased spark advance and higher compression ratio, determining a higher probability to induce knock and abnormal combustions. Since slight knock does not lead to compromising engine damage, it should be tolerated in order to pursue the maximum efficiency. In the present study, knocking damages on several combustion chamber components after industrial durability tests are shown, aiming to underline the main effects of knock and to guide materials and coatings selection. Some hints are also made to the deleterious effects caused by persistent knocking combustions turning into pre-ignition. Piston Al alloys are usually more sensitive to knock induced thermo-mechanical stresses, but also cylinder heads, liners and spark plugs might be affected.

KEYWORDS: KNOCK - AUTOMOTIVE PISTON - COMBUSTION CHAMBER COMPONENTS - FAILURE ANALYSIS INTRODUCTION Knocking combustions are one of the main concern in the latest generation Spark-Ignition (SI) engines, since higher compression ratio and spark advance are desired to maximize engine efficiency, leading to higher pressure and thermal load inside the combustion chamber and thus to a higher probability to induce abnormal combustions [1,2].

This issue is further emphasized by the widely adopted turbochargers, whose aim is to regain a high output power, in spite of engine downsizing [3]. Differently from a normal combustion, which is triggered by the spark plug and it develops through a spherical flame front, knock consists in the spontaneous auto-ignition of the end-gas ahead of the propagating flame. This irregular combustion mode is characterized by a substantial increase of the rate of heat release and pressure oscillations, whose result is an enhan-

E. Balducci

Dip. di Ingegneria Industriale (DIN), Alma Mater Studiorum – Università di Bologna / Reparto Sperimentazione Motopropulsori – Ferrari Auto S.p.A, Maranello, Modena

Corresponding author e-mail: Eleonora.Balducci@Ferrari.com

S. Parisi, F. Boccia, M. Barichello

Reparto Sperimentazione Motopropulsori – Ferrari Auto S.p.A, Maranello, Modena

L. Ceschini

Dip. di Ingegneria Civile, Chimica, Ambientale e dei Materiali (DICAM), Alma Mater Studiorum – Università di Bologna

ced heat flux coupled with higher mechanical stresses [4,5].

Due to the need to push the limits of engine efficiency, knock should be today partially tolerated, thus it becomes necessary to accurately estimate possible knocking damages [6–8]. In the following paragraphs, knocking damages after typical industrial durability tests are shown, aiming to

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Scientific papers - Miscellaneous underline both that slight knock is not detrimental to engi-

Spectruma GDA 650). Aiming to prevent piston rings sti-

ne life [8,9] and that materials and coating selection plays a

cking, the 1st ring groove is hard anodized. Cylinder heads

key role in limiting the damage, besides a calibration strate-

are made of a gravity die cast A356 Al alloy, whose micro-

gy able to promptly switch off abnormal combustion over a

structural characteristics were deeply investigated in [10].

certain threshold.

The analyzed cylinder liners are Nikasil coated [11], while spark plugs consist of noble metals electrodes and a brittle,

EXPERIMENTAL METHODS

ceramic insulator nose enveloping the ground electrode.

The objects of the experimental activities have been the

The 1st level analysis of all components was carried out

combustion chamber components which are mainly af-

through a high-resolution Nikon D40 digital camera, a ste-

fected by knocking combustions: pistons, cylinder liners,

reo-microscope and a 3D digital microscope (Hirox KH-

cylinder heads, spark plugs. All analyzed pistons are forged

7700 equipment). More in depth analyses have been con-

and made of a nearly eutectic Al-Si alloy, T7 heat treated,

ducted through the Scanning Electron Microscope Zeiss

whose chemical composition (Table 1 [7,8]) was checked by

EVOÂŽ MA 50 (SEM), equipped with an energy dispersive

a Glow Discharge Optical Emission Spectroscope (GD-OES

X-Ray spectroscope (EDS).

Tab.1 - Results of GD-OES chemical analysis on pistons crown (average of 3 points of measurement)

El. [wt%]

Si

Cu

Mg

Ni

Fe

Al.

11.87

2.93

0.76

2.25

0.24

Bal.

RESULTS AND DISCUSSION

More interesting from a technical point of view and challen-

Pistons damage under low/middle knocking combu-

ging in terms of both materials research and engine control

stions.

strategies is a medium knocking level. The result of a persi-

As reported in the introduction, pressure oscillations as-

stent condition of medium knocking combustions is shown

sociated to knocking combustions induce increased ther-

in Fig.1: the occurrence of a sliding contact between cylin-

mo-mechanical stresses [4,5]. Due to the high sensitivity

der and piston valve relief is perceivable in the upper poli-

of Al alloys to thermal loads, pistons are mainly affected by

shed area, while the jagged edge of the valve relief shows a

knocking damage and deserve particular attention. Light

moderate erosion. An incipient erosion is also perceivable

knocking combustions usually produce almost no damage

at piston ring groove, highlighted by arrows at higher ma-

or typical slight erosion signs on Al pistons head, in parti-

gnitude: the ring groove is a crevice area, a potential site

cular at valve reliefs or at piston top land. A polished area

of knock triggering since it collects the end-gases furthest

at valve reliefs edge is frequently observed: due to the high

from the spark-plug [8,9,12], moreover the hard and britt-

surface/volume ratio of this area, local heating is conside-

le anodized layer is extremely sensitive to knock pressure

rably favored [8]; the consequent local thermal expansion

waves. It should be also highlighted that the anodized layer

produces a limited contact between cylinder liner and pi-

grows thanks to Al passivation, therefore it hardly covers

ston valve relief. In this case, none of the other components

eutectic Si or primary Si crystals, which form weak points

of the combustion chamber are involved, and typically the

[8,13]. SEM-EDS investigations have been carried out on

cylinder surface treatment is not affected. Many examples

the area highlighted in red in Fig.1, in order to depict the

of tiny damage under controlled knocking conditions for

first stages of the anodized layer damage. The results are

up to 15h bench tests are reported in [8]: it should be poin-

reported in Fig.2: small cracks are observed at ring groove

ted out that, to some extent, this is not a fatal engine dama-

edge, highlighted by the arrows, and EDS analyses in corre-

ge, since it simply produces debris inside the combustion

spondence of cracks show a higher Si content, confirming

chamber, without affecting piston structural resistance or

the presence of primary or eutectic Si crystals beneath.

engine compression ratio.

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Memorie scientifiche - Miscellanea

Fig.1 -Hirox image of an automotive piston intake valve relief. Erosion is perceivable both in the upper part of the valve relief (see jagged edge) and in the ring groove (white discoloration). Image from a turbocharged V8 engine.

Fig.2 -SEM micrographs of the 1st ring groove anodized layer, characterized by incipient erosion. EDS analyses revealed the presence of a higher amount of Si beneath the cracks. Image from a turbocharged V8 engine.

Even if no consequences are perceived at the initial stages

power loss and lower combustion efficiency. Typical images

the 1 ring functionality. A persistent erosion process mi-

of the anodized layer, several micro-welding spots can be

of the anodized layer erosion, this damage might obstruct st

ght produce a total wear of the anodized layer, leading to a direct contact between the 1 ring and piston Al alloy. Unst

der these conditions, the 1st ring is prone to sticking to the

lower flank of the ring groove due to the high pressure and temperature induced by combustion, inevitably resulting in

of ring sticking are reported in Fig.3. In case of heavy wear

observed (arrows in Fig.3a). At higher pressure/temperature,

the damage is rapidly intensified, leading to a complete ring sticking (Fig.3b); an Al transfer layer, typical of adhesive wear,

is also perceivable on piston ring (Fig.3c), and confirmed by SEM-EDS analyses (here not reported).

Fig.3 -Stereo-microscopy micrographs depicting 1st ring â&#x20AC;&#x201C; piston welding. (a) Micro-welding spots (pointed by arrows) and

Al plastic deformation. (b) Noticeable sticking area at the 1st ring groove. (c) Evidences of piston material deposited on piston ring.

Besides 1st ring sticking, one of the most critical knocking

of the valve relief (which is characterized by a reduced cross

ly checked through precision gauge blocks. Under severe

sional changes in the range 10-2 â&#x20AC;&#x201C; 10-1 mm, which in the worst

damage is piston ring groove deformation, which is usual-

knock erosion at piston head, in particular in correspondence

La Metallurgia Italiana - maggio 2020

section), the ring groove might undergo significant dimencases culminate in a complete piston groove closure and ring

pagina 22

Scientific papers - Miscellaneous locking. It is usually observed a correlation between plastic

piston top land towards the con-rod. It should be pointed out

shold. An example of that is reported in Fig.4: a tiny ring gro-

observed in combination with valve reliefs erosion, regardless

deformation and erosion of the valve relief over a certain threove deformation (order of magnitude 10-2 mm) corresponds

that the dimensional changes of the ring groove are frequently of 1st ring groove erosion: the peaks of eroded material in the

to a middle level erosion of valve relief (Fig.4a), while an in-

ring groove, locally reducing the clearance, provide a very

valve relief reported in (Fig.4b). Ring groove closure should

this is considered a fatal piston damage, since it compromises

creased deformation characterizes the heavily eroded exhaust

be mainly considered a consequence of knock-increased

thermal and mechanical loads, which induce deformation of

small contribution to ring locking. As well as micro-welding, the combustion gases sealing.

Fig.4 -Images of two pistons valve reliefs characterized by ring groove plastic deformation, V8 turbocharged engines. (a) Stereo-microscopy images at the intake valve relief, medium knocking erosion and tiny ring groove closure. (b) Macrograph at the exhaust valve relief, heavier knocking conditions: erosion is more pronounced, as well as ring groove closure.

A separate discussion should be made for naturally aspirated engines. In this case, a considerably lower pressure inside the combustion chamber is reached, leading to both:

â&#x20AC;˘ Lower probability to induce knock at the ring groove; anodized layer erosion is thus rarely observed.

â&#x20AC;˘ Slower wear rate of anodized layer due to lower contact pressure between piston ring and groove.

â&#x20AC;˘ Lower temperature inside the combustion chamber (since it is directly related to the combustion pressure [6,8]);

this leads to higher structural resistance (in particular of heat-sensitive Al components such as pistons) and lower

probability to incur in plastic deformation of the ring groove.

However, as well as pistons equipping turbocharged engi-

nes, knock-induced erosion is typically observed at piston valve reliefs. An example of middle-high level erosion is re-

ported in Fig. 5: it should be pointed out that no deformation of the ring groove is detected and no damage is observed at the anodized layer, even if both polishing and erosion are clearly perceivable, indicating the occurrence of knocking combustions.

Fig.5 - Knock induced erosion (middle level) in an automotive piston belonging to a naturally aspirated engine. Stereo-microscopy micrographs focus on intake valve relief and piston pin axis side; no damage of the anodized layer at the 1st ring groove is observed.

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Memorie scientifiche - Miscellanea Pistons damage under heavy knocking combustions

pre-ignition occurrence. A V8 turbocharged engine, wi-

(eventually turning into pre-ignition).

thout a calibrated knock control strategy, was involved.

As highlighted in the previous paragraphs, the major da-

The effects of this sequence on the 8 pistons are shown in

mages connected to knocking combustions are, in order

Fig.6b and here schematically described:

of importance: ring groove plastic deformation, complete

â&#x20AC;˘ At all Al pistons heads, melting signs are observed,

wear of anodized layer leading to micro-welding, erosion

often resulting in the formation of blow-by channels;

of the anodized layer, erosion at piston head (in particu-

under these conditions, also other combustion cham-

lar at valve reliefs). It should be however pointed out that

ber components are covered by molten Al spots, such

knock is an autocatalytic phenomenon: due to the induced

as injectors, spark-plugs, cylinder liners and heads.

temperature increase, a persistent knocking condition mi-

â&#x20AC;˘ The significant temperature increase and induced ther-

ght rapidly degenerate into more severe knocking cycles

mal expansion produce seizure marks, perceivable at

and finally into pre-ignition sequence (as confirmed by

all pistons lands, in particular at pin axis side for the ab-

[14]). In-cylinder pressure signals reporting a similar event

sence of piston skirt constraint.

are displayed in Fig.6a: pressure oscillations due to knock are rapidly intensified, then suddenly switched off due to

â&#x20AC;˘ In the severest cases, 1st ring is completely jammed or subjected to a substantial plastic deformation.

Fig.6 - Degeneration of severe knocking combustions into pre-ignition sequence. (a) In-cylinder pressure signal, filtered in order to show MAPO parameter (MAPO explanation in [6]). (b) Effects on pistons of severe knocking combustions + pre-ignition.

Damage of combustion chamber components under he-

typical knock damage of Al cylinder heads is limited to sur-

avy knock: cylinder heads, liners, spark plugs

face erosion, as reported in Fig.7: in this case, the intake side

In case of heavy pressure oscillations, besides pistons, other

is involved. Erosion at cylinder head does not usually com-

components of the combustion chamber are usually affected

promise engine functionality, until the integrity of its cooling

by knocking damage. Among them, as can be expected, Al

channels is preserved. This damage is however rarely obser-

cylinder heads are sensitive to knock-induced mechanical

ved and it is a marker of persistent high-level knocking com-

stresses, further emphasized by temperature increase. The

bustions, which should be avoided.

La Metallurgia Italiana - maggio 2020

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Scientific papers - Miscellaneous

Fig.7 -Macrographs reporting erosion at cylinder heads, V8 turbocharged engines.

Even if made by steel and Nikasil coated, cylinder liners as

melting of the liner is visible at the TDC, beneath the location

related to the knock-induced overheating, which leads to a

triggering knocking and pre-ignition combustions.

well might also be affected by knock. The damage is mainly reduced functionality of lubricants, possible breakdown of

the lubrication film and significant piston thermal expansion, though localized and limited to piston top land in case of ef-

ficient knock control strategy (see tiny seizure marks in the thrust and anti-thrust side in Fig.8a). In case of repeated knock and pre-ignition, the insufficient clearance between the ma-

tching surfaces and the resulting friction further intensify the

heat-induced expansion, finally resulting in complete piston seizure. Such destructive effects are reported in Fig.8b: several seizure marks are visible on cylinder liner, together with

transferred Al material. It is interesting to point out that also

of piston valve relief, which is supposed to be the hot spot Also spark-plugs might be affected by severe knock or pre-ignition and their damage inevitably compromises the com-

bustion process. In particular, partial rupture of the ceramic insulator due to thermal shock might be produced by increased thermo-mechanical stresses (Fig.9a); in this case, usually

spark leaks can be observed, indicating reduced spark plug

functionality. Al deposits coming from pistons are usually

observed, while melting of the electrodes and ceramic insulator is perceivable in the severest abnormal combustions, due to higher thermal loads.

Fig.8 - Macrographs reporting cylinder liners damage. (a) Tiny seizure marks (severe knock but efficient knock control). (b) Heavy seizure marks due to knock + pre-ignition; in addition, transfer of Al and partial melting under valve relief location are observed.

Fig.9 - Spark plugs damage due to knock or pre-ignition. (a) Breakage of ceramic insulator and spark leaks. (b) Electrodes melting.

La Metallurgia Italiana - May 2020

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Memorie scientifiche - Miscellanea Conclusions

leness of the anodized layer, which is substantially sensitive

and pre-ignition damage on several engine components after

sed brittleness and defects density, a thicker anodized layer

In the present paper, different case studies collecting knock industrial durability tests are reviewed, aiming to underline the critical issues to be taken into account for materials se-

lection in case of high output power engines, which are more likely to encounter abnormal combustions during their life.

The following damages are frequently observed at mid/high

knocking level on automotive pistons equipping turbocharged engines, and should be avoided in order not to compromise pistons functionality: (i) significant wear of the anodized layer at the piston ring groove (potentially turning into micro-welding), (ii) ring groove closure due to plastic de-

formation. Piston head and valve reliefs erosion is not compromising to some extent. Piston material selection plays a

decisive role in partially limiting these unfavorable effects, thus making it possible to accept light/mid knocking com-

bustions, which allow a higher efficiency engine operating

point. The understanding of the damage mechanisms offe-

red in the present paper suggests various insights for materials selection.

For example, it has been observed that the wear of the ring

groove anodized area is directly related to the intrinsic britt-

to knock pressure waves. It follows that, due to its increa-

cannot improve the ring groove response to knocking com-

bustions. It should be also stressed that the anodized layer hardly covers the primary Si particles of the Al-Si base alloy (here taken into account), and a more dense and uniform PEO

layer (plasma electrolytic oxidation) might help. Another so-

lution could be a co-forged steel ring carrier, but its higher cost makes it a preferential candidate for racing applications rather than for mass production pistons.

As regards ring groove closure, it is evident that the solution is continuing to work towards an increased alloy resistance to knock induced thermo-mechanical stresses, thus shifting the plastic deformation limit of the alloy.

Under heavier and persistent knocking combustions, often degenerating into pre-ignition, deleterious damages are witnessed, such as piston seizure and ring locking, coupled by cylinder heads, cylinder liners and spark plugs damage. These high-intensity abnormal combustions should be completely avoided through effective engine calibration strategies,

since materials selection is no more able to limit the effects on engine functionality.

Bibliography [1]

N. Cavina, E. Corti, G. Minelli, D. Moro, L. Solieri, Knock Indexes Normalization Methodologies, SAE Int. J. Engines. (2006).

[2]

G. Töpfer, J. Reissing, H. Weimar, U. Spicher, Optical Investigation of Knocking Location on S.I.-Engines with Direct-Injection, Sae Tech. Pap. Ser. (2000).

[3]

J. Mckenzie, W.K. Cheng, The Anatomy of Knock, (2016).

[4]

J. Mutzke, B. Scott, R. Stone, J. Williams, The Effect of Combustion Knock on the Instantaneous Heat Flux in Spark Ignition Engines, SAE Int. (2016).

[5]

B. Grandin, I. Denbratt, The Effect of Knock on Heat Transfer in SI Engines, Sae Tech. Pap. Ser. 2002-01-0238. (2002).

[6]

N. Cavina, N. Rojo, L. Ceschini, E. Balducci, L. Poggio, L. Calogero, R. Cevolani, Investigation of Knock Damage Mechanisms on a GDI TC Engine, SAE Int. (2017).

[7]

L. Ceschini, A. Morri, E. Balducci, N. Cavina, N. Rojo, L. Calogero, L. Poggio, Experimental observations of engine piston damage induced by knocking combustion, Mater. Des. 114 (2017) 312–325.

[8]

E. Balducci, L. Ceschini, N. Rojo, N. Cavina, R. Cevolani, M. Barichello, Knock induced erosion on Al pistons : Examination of damage morphology and its causes, Eng. Fail. Anal. 92 (2018) 12–31.

[9]

J. Fitton, R. Nates, Knock Erosion in Spark-Ignition Engines, SAE Tech. Pap. (1996) 11.

[10]

L. Ceschini, A. Morri, A. Morri, G. Pivetti, Predictive equations of the tensile properties based on alloy hardness and microstructure for an A356 gravity die cast cylinder head, Mater. Des. 32 (2011) 1367–1375.

[11]

H. Yamagata, The science and technology of materials in automotive engines, Woodhead, 2005.

[12]

R.J. Nates, Thermal Stresses Induced by Knocking Combustion in Spark-Ignition Engines, Sae Tech. Pap. Ser. (2000).

[13]

L.E. Fratila-Apachitei, F.D. Tichelaar, G.E. Thompson, H. Terryn, P. Skeldon, J. Duszczyk, L. Katgerman, A transmission electron microscopy study of hard anodic oxide layers on AlSi(Cu) alloys, Electrochim. Acta. 49 (2004) 3169–3177.

[14]

N. Cavina, N. Rojo, A. Businaro, L. Ceschini, E. Balducci, A. Cerofolini, Analysis of Pre-ignition Combustions Triggered by Heavy Knocking Events in a Turbocharged GDI Engine, Energy Procedia. 101 (2016) 893–900.

La Metallurgia Italiana - maggio 2020

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Scientific papers - Miscellaneous

Detection of Non-metallic Inclusions in 12Mn Steel Continuous Casting Round Billets X. Wang, J. Wei, S. Qiu

Automated particle analysis was carried out to explore non-metallic inclusions in 12Mn steel continuous casting round billets, which has the features of quick obtaining the morphology, size, original positions, and composition of inclusions in a certain area. Both morphology and composition of main inclusions were calculated by using thermodynamic calculation. Meanwhile, considering the all kinds of inclusions, the spatial distribution was identified, including the distribution of oxide and sulfide inclusions. The obtained information above could be adopted to explore the source of inclusions and modify ladle refining process.

KEYWORDS: NON-METALLIC INCLUSIONS; ASPEX SCANNING ELECTRON MICROSCOPE; OPA; SLIME; 12MN STEEL; CONTINUOUS CASTING; ROUND BILLETS INTRODUCTION Non-metallic inclusionsâ&#x20AC;&#x2122; characterization plays a crucial role in making clean steel. Inclusions in the final product are harmful to the quality of steel products [1, 2]. To obtain high-performance steel, the number of the inclusions have to be diminished. Generally speaking, non-metallic inclusions could be categorized as indigenous and exogenous. Indigenous inclusions stem from alloying elements which react with dissolved gas to generate solid inclusions during continuous casting. Some aspects can lead to inclusions, including deoxidation, reoxidation and solidification from reduced gas solubility. Sources outside of the liquid steel produce exogenous inclusions, such as slag

Xu Wang, Jun Wei, Shengtao Qiu

National Engineering Research Center of Continuous Casting Technology, Central Iron and Steel Research Institute, Beijing 100081, China

entrainment or refractory wear. The evaluation methods of non-metallic inclusions are important to metallurgists, which comprise the amount, size distribution, morphology, spatial distribution, and their chemical composition. [1, 3-12] More than twenty methods of evaluating inclusions impurities were summarized by Zhang. [1]. Metallographic microscope [13], one of these methods, is extensively used to explore size of inclusions and the two-dimensional morphology in steel. But, the chemical composition

La Metallurgia Italiana - May 2020

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Memorie scientifiche - Miscellanea of inclusions cannot be identified. Scanning electron mi-

Ultrasonic inspection [19-21] are more time-saving than

croscopy [14] allows exploration of morphology of inclu-

methods above, and could be applied for online analysis;

sions. Furthermore, scanning electron microscopy allows

yet the composition data and morphology are difficult to

identification of the elemental composition of inclusions

obtain. Nevertheless, other indirect methods, such as total

by exerting with energy-dispersive spectroscopy. But it is

oxygen (T.O) [23, 24] and continuous excitation of spark

too time-intensive to count the majority of inclusions by

spectrum, have the limitations of exploring both the com-

using this method. Although acid extraction method could

position [25, 26] and the distribution of inclusions.

explore the stereoscopic morphology of inclusions, it de-

However, as the above mentioned papers do not address

stroys sulfide inclusions to some extent. [15–18] Involving

specific steel grades, in the current study typical inclusions

the extraction using bromine-methanol, iron bromide dis-

in 12Mn steel round billets were investigated. Furthermo-

solves rapidly, so a chelate has to be added to dissolved

re, the size, spatial distribution, and composition of inclu-

iron in the electrolytic approach, which is conducive to

sions in 12Mn steel round billets were observed.

revealing the three-dimensional morphology and keeping composition of the inclusions. X-ray detection [22] and

INTRODUZIONE

Fig.1 - Flow chart of production process of 12Mn steel round billet

Fig.2 - Sample from 12Mn round billet

12Mn steel round billet samples were taken from conti-

sis pattern. To begin with, the certain area was classified

nuous casting round billet which stemmed from alumi-

to some fields by the microscope. The electron beam

num-killed low carbon steel in a six strand CC (Fig. 1). The

was set on a field, then the microscope implements qui-

schematic of billet is shown in Fig. 2. The composition in

ck search with search steps. The intensity in back-scatter

12Mn steel is illustrated in Table I. The casting speed was

electrons was transmitted into the computer. When par-

1.6 m/min to 1.8 m/min leading to 40 minutes of casting

ticle was obtained, the step size was dropped to measure

time per heat. Besides, round billet samples were achieved

the dimensions of the inclusions. The center of inclusion

at steady state. First, the samples were polished. Then, the

was illuminated, and the chord algorithm was identified to

2-dimensional morphologies and compositions of inclu-

explore the particle. Last, the beam was fixed in the center

sions in 12Mn steel samples were analyzed, assisted with

of inclusions to obtain the characteristic X-ray. Through

using an automated SEM–EDS—ASPEX 1020 system. [27]

combination of steps above in corresponding field, next

The frame-based analysis is the automated particle analy-

the beam was transformed to next field. [28, 29]

Tab.1 - Chemical composition of steel sample % Element

C

Si

Mn

P

S

Alt

T.O

12Mn

0.1

0.28

1.24

0.013

0.002

0.03

0.0025

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Scientific papers - Miscellaneous In the following experiment, the size distribution, rough

the two-dimensional morphologies and composition

morphology, and composition of inclusions were auto-

detected by using SEM-EDS. In order to obtain thre-

matically achieved by coping with ASPEX 1020 system.

e-dimensional morphologies of non-metallic inclusions

Furthermore, the ASPEX has the features of quick scan-

greater than 50 μ m in 12Mn steel round billet, inclusions

ning, as a result, most of the particles were tested in se-

were partially extracted from samples by the method of

veral hours, even though the image resolution of inclu-

Slime reported by Fang.[33] The scheme of the device

sions is not high. The sample detecting 10 x 15 mm 2 was

is illustrated in Fig. 5. The electrolyte contained 89%

measured. The beam scanned the area on the 10 x 15 mm 2

methanol + 5% + glycerine + 5%

sample, eventually 63 mm 2 scanned area was analyzed.

trithanolamine + 1% tetramethyl ammonium chloride.

μ m, implying

Argon atmosphere was implemented to prevent oxida-

that all the inclusions size measured by the ASPEX 1020

tion of 12Mn steel. The anode was cylindric with 5 mm

μ m. The phase diagrams of inclusion is

in diameter and the cathode was 100 mm high and 60

illustrated in Figure 3. Figure 4 represents the schematic

mm in diameter. After electrolysis, non-metallic inclu-

of scanning area by Original Position Analysis (OPA).

sions were partially extracted. At least 20 inclusions were

After the steel samples were polished, we could obtain

explored with SEM-EDS.

The minimum inclusion size was fixed at 2 was larger than 2

Fig.3 - Inclusion species phase diagram

Fig.4 - Scanning area of OPA

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Memorie scientifiche - Miscellanea

Fig 5 - Schematic of Slime to extract inclusions from 12Mn steel round billet

RESULT AND DISCUSSION

(Al 2O 3) shown in Figure 6. Most of alumina (Al 2O 3) inclu-

Figure 3 displays the composition of inclusions in 12Mn

sions originate from reoxidation. High magnesium oxide

round billet sample using ASPEX. The typical inclusions

inclusions stem from the reaction between magnesium

in the 12Mn steel round billets are alumina-based clu-

refractory and liquid steel, in particular tundish lining re-

sters, cluster inclusions or alumina magnesia (Al 2O 3-

fractory. Sulfide inclusions precipitate on the condition

MgO) spinel single, silicon oxide (SiO 2) bearing sphe-

of solidification of the steel. The resolution achieved by

rical inclusions, separate sulfide-based inclusions, and

ASPEX was fixed for providing enough data to obtain the

irregularly shaped slag inclusions containing potassium

size, area, and perimeter with available detecting time

oxide (MgO), calcium oxide (CaO), and sodium oxide

and storing space for a large number of inclusions.

Fig.6 - Inclusion morphology and composition (mass pct) obtained by SEM.

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Scientific papers - Miscellaneous

Fig.7 - Elemental mapping of typical MgO-Al2O3-CaO inclusions There existed a lot of MgO-Al 2O 3-CaO complex inclu-

racteristics were as follows: Al and Ca were mainly ele-

sions. The morphology and composition of MgO-Al 2O 3-

ments; little Mg content was located in the center region

CaO inclusions in molten 12Mn steel are shown in Figure

of MgO-Al 2O 3-CaO inclusions; and Fe content in MgO-

7. The size of these inclusions was approximately 10

Îź m.

The shapes of inclusions tended to be more spherical.

Al 2O 3-CaO inclusions was much lower than that in other matrix regions.

From elemental mapping (Figure 7), the common cha-

Fig.8 - Elemental mapping of typical SiO2-Al2O3-CaO inclusions

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Memorie scientifiche - Miscellanea The elemental mappings of typical SiO 2-Al 2O 3-CaO inclu-

are Al 2O 3-containing types and their average size is 3.3

sions in 12Mn steel are illustrated in Fig.8. The contents

Îź m calculated from Figure 9. Besides, in order to obtain

of Al and Ca are significantly higher than Si. The size of

the distribution of Al 2O 3-containing inclusions, we explo-

the main inclusions is about 10

Îź m. The area on the right

red the line distribution (Figure 10), two-dimensional di-

shows little amounts of Silicon.

stribution (Figure 11), and three-dimensional distribution

The method of OPA according to statistical analysis of

of Al (Figure 12). These results imply that Al 2O 3-contai-

more than ten thousands primary optical signals is ap-

ning inclusions were located in the center region of the

plied in this work. The main inclusions examined by OPA

round billet.

Fig.10 - Content-frequency distribution of Al

Fig.11 - Two-dimensional distribution of Al

Fig.12 - Three-dimensional distribution of Al La Metallurgia Italiana - maggio 2020

pagina 32

Scientific papers - Miscellaneous The inclusion in 12Mn steel round billets destroys the

significant. Moreover, the experimental results show that

uniform structure of metal, especially the large inclu-

the total average inclusions weight is 7.76 mg/10 kg for 4

μ m. In order to obtain non-metal inclusions with the size of greater than 50 μ m,

samples (Table 2), which is a common value. This is due

the inclusions in 12Mn steel round billet are extracted

LF and VD process), the large inclusions are fully floating

and separated by Slime method. The inclusions’ three

up to the surface of molten steel, resulting in fewer large

–dimensional morphologies are seen in Figure 13, from

inclusions in 12Mn steel.

sion with diameter greater than 50

to the fact that the refining time is sufficient (containing

above results, the numbers of larger inclusions are not Tab.2 - Analysis results of large oxide inclusions in 12Mn steel Electrolysis weight

Inclusions’ size level

Original weight

Remaining weight

kg

kg

kg

mg

mg/10kg

1

4.075

0.989

3.086

1.70

2

4.045

0.763

3.282

3

4.154

0.866

4

4.113

0.800

Sample

Total inclusions weight

<80

80~140

μm

140~300 μm

>300 μm

mg

mg

mg

mg

5.51

0.10

0.50

0.70

0.40

3.20

9.75

0.60

1.20

1.10

0.30

3.288

3.40

10.34

1.10

1.50

0.80

-

3.313

1.80

5.43

0.10

0.80

0.90

-

μm

Note: - represents there is no detected inclusion corresponding to size.

50~80μm

80~140μm

140~300μm

>300μm

Fig.13 - Large inclusions at 20 magnification

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Memorie scientifiche - Miscellanea As shown in Figure 14, the ternary diagram of oxides

diagram. The samples obtained from both steady sta-

obtained is depending on the chemical composition of

te (S1) and ladle (S2) indicate that most inclusions were

MgO, Al 2O 3 and CaO. Every dot in the ternary diagram

near the Al 2O 3 region. Exceeding MgO (greater than 30

means the chemical composition of one inclusion. So-

pct) could have a result of higher (over 2573 K (2300 °C))

lid inclusions could lead to clogging of 12Mn steel by

melting point of inclusions, and thus throughout the ca-

attaching to the nozzle during solidification. Inclusions

sting process these inclusions were solid. As plenty of

must be in the liquid state to avoid nozzle clogging. But

the inclusions were of high melting compositions, so the

the composition distribution means that the majority of

submerged entry nozzle was dramatically clogged during

inclusions were solid at casting temperature. There were

continuous casting (Figure 15), leading to premature ter-

only several inclusions in the liquid state, as shown by

mination of production.

the shaded area (temperature < 1500 °C) in the ternary

Fig.14 - Al2O3-CaO-MgO inclusions in 12Mn steel [34].

Fig.15 - Erosion in the submerged entry nozzle .

As shown in Fig 16, the computed diagram of Al-Ca-O

12Mn steel with composition (Table 1) was computed

system in 12Mn steel round billet at 2146 K (1873 °C) de-

with the help of the thermodynamic software FactSage

monstrates the stable areas of different inclusions for-

using ‘‘FACT53’’, ‘‘Ftoxid,’’ and ‘‘FSstel’’ databases[35]

med by [Al], [Ca], and [O]. The interaction activity coeffi-

and is illustrated in Figure 17. When [Ca] in 12Mn steel

cient and the logK for the formation in 12Mn molten steel

was 2 ppm, MgO•Al 2O 3 spinel inclusions cause the clog-

were different from others. [30–34] Thus, the activity of

ging of submerged entry nozzle, due to its high melting

oxide products is deemed to be unity. The numerical va-

point and high hardness. During the period of solidifica-

lue of the dotted circles represents the oxygen content.

tion, liquid inclusions were modified to CaMg 2Al 16O 27 and

The [Al] and [Ca] content in 12Mn steel must be set in the

CaS. After the temperature attained nearly 1573 K(1300

shaded region to prevent nozzle clogging in Figure 16.

°C), most of MnS and some of TiN secondary phase could

The Al 2O 3-3CaO and 7Al 2O 3-12CaO inclusions remain li-

precipitate. When [Ca] reaching from 5 to 10 ppm (Figu-

quid at 2146 K (1873 °C) among all Al-Ca-O inclusions.

res 17(b) and (c)), most of the spinel inclusions dimini-

The [Ca] and [Al] contents in 12Mn steel shown in Table 1

shed and finally disappeared, as a result, only liquid inclu-

were located in the Al 2O 3-rich area, implying that the cal-

sions remained in the molten steel. MnS inclusions were

cium treatment was effective and more Ca was required.

changed to CaS by coping with calcium treatment. When

The Equilibrium diagram of inclusions during casting of

adding more calcium (Figures 17(d) and (e)), CaS and li-

La Metallurgia Italiana - maggio 2020

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Scientific papers - Miscellaneous quid particles were obtained in molten steel, leading to

ce, controlling of the composition and refining are bene-

the result of clogging the submerged entry nozzle. Hen-

ficial to the last consequence on the casting process.

Fig.16 - Calculated stability diagram of Al-Ca-O system in the molten steel at 2146 K (1873°C [34]. (a)Ca=2ppm

(c)Ca=10ppm

(b)Ca=5ppm

(d)Ca=20ppm

(e)Ca=50ppm

Fig.17 - Equilibrium diagram of inclusions during solidification for 12Mn steel of composition: Fe-0.1C-0.28Si-1.24Mn0.002S-0.0006Mg-0.0025O-0.0045N-0.03Al-Ca in mass pct.

CONCLUSIONS

tial distribution and composition.

Automated particle analysis was widely used to explore

(2) The experimental results are in good agreement with

inclusions in 12Mn steel continuous casting round billet.

the thermodynamic calculation, indicating that most of

The conclusions were summarized:

the inclusions remained solid at casting temperature with

(1) Automated particle analysis was used to provide an

few in the liquid as inclusions are not changed by the re-

accurate and quick scanning in a certain area to obtain in-

fining process.

formation on inclusions, such as morphology, size, spa-

La Metallurgia Italiana - May 2020

pagina 35

Memorie scientifiche - Miscellanea ACKNOWLEDGMENTS

The authors are grateful for financial supporting and experi-

mental help from the Tianjin Iron and Steel Group Co., Ltd., Tianjin, China.

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Scientific papers - Miscellaneous

Study on Heat Transfer Characteristics during Solidification of 18-ton Steel Ingot with Large Ratio of Height to Diameter Z. Yu, H. Zhang, X. Wang, X. Wu

In order to investigate the heat transfer characteristics during solidification of steel ingot with large ratio of height to diameter, numerical simulations of solidification process of 18-ton steel ingot were carried out. The simulated results were verified by the temperatures measurement during solidification of steel ingot. In addition, in order to further certify the mathematical model, the solidification process of 5.3-ton ingot has been investigated, the simulated shape and length of the shrinkage pipe of steel ingot agree well with the experimental result observed by the sectioning.

It is found that the heat of molten steel is mainly transferred to the external environment through ingot body. With this context, the analytical method using the concept of thermal resistance is proposed to explore the heat transfer resistances of outer wall of mold, mold, ingot/mold interface and solidified shell. Consequently, the transient thermal resistances at different stages of steel ingot solidification were dug out. Based on the analyses of the heat transfer resistances at different stages of steel ingot solidification with different casting parameters, it is obtained that heat transfer of solidified shell is the main restrictive step of heat transfer in the whole solidification process for 18-ton steel ingot with large ratio of height to diameter.

KEYWORDS: KEYWORDS SOLIDIFICATION, STEEL INGOT, HEAT TRANSFER CONTROL, THERMAL RESISTANCE, NUMERICAL SIMULATION Introduction Large steel ingots are the basic raw materials for the equipment manufacturing industry. Its production capacity is directly related to the national industrial development level and economic lifeline. The steel ingot solidification process has an important influence on its quality and the property of final product. The solidification rate of steel ingot depends on the heat transfer from molten steel to external environment. As is known to all, it is difficult to measure heat transfer parameters directly during solidification of a steel ingot. However, with the rapid development of

Zhanyang Yu, Hui Zhang, Xuebing Wang Central Iron and Steel Research Institute,Beijing 100081,China

Xuan Wu

University of Science and Technology Beijing, Beijing 100083,China

computer technology, numerical simulation is being increasingly applied to study of solidification of steel ingot [1]-[4].The author [5] developed a two-dimensional numerical model of ingot solidification, and temperature distribution, distribution solid and liquid phase were calculated. Zhang et al. [6] studied the effect of casting pro-

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Memorie scientifiche - Miscellanea cess parameters on the shrinkage of ingot by numerical

been found to be the condition under

method. Wang [7] et al. studied that the best criterion has

(0)

(G and Rs are the temperature gradient and solidification

stigated. In addition, the effects of increasing the cooling

size of the simulated centerline shrinkage porosity agree

pouring temperature on the thermal resistances in diffe-

rate, respectively),with this criterion, the distribution and

well with the experimental results observed by the sectio-

ning and ultrasonic testing. Since Flemings and co-wor-

kers published their pioneer papers on macrosegrega-

intensity of the outer wall of the mold and the variation of rent stages of solidification process of steel ingot were further dug out.

tion[8]-[11], the mathematical model of macrosegregation

Numerical Simulation of Solidification of 18-ton Steel

ped various models to study the formation of macrosegre-

Establishment of Mathematical Model

has been developed continuously. Li et al.[12]13] develogation in ingots. Tu et al.[14] used a multicomponent multiphase model to study macrosegregation of 36-ton steel ingot. M.Wu et al.[15] introduced a four-phase mixed co-

lumnar-equiaxed solidification model to calculate the formation of shrinkage cavity and macrosegregation during

solidification of steel ingot. So far, scholars have focused their research efforts on the formation of defects in ingots,

such as shrinkage and macrosegregation. However, there

are few reports on heat transfer control during solidification of ingots with large ratio of height to diameter.

In this paper, through FEM (Finite Element Method) simulation in combination with the temperature measurement

and experimental sectioning investigation, the solidification process of 18-ton steel ingot with a large ratio of

height to diameter were carried out. An important result is obtained from the analysis of heat transfer characteristics

of ingot solidification, the heat of molten steel is mainly transferred to the external environment through ingot

body. With this context, the heat transfer restriction steps

of 18-ton ingot at different solidification stages were inve-

Ingot

Governing equation and geometry model

The solidification process of steel ingot was investigated by finite element method in ProCAST package. Assump-

tions were made: (1) the temperature and velocity of the molten steel poured into the mold from the top have an

even distribution; (2) the liquid metal was incompressible Newton fluid; (3) the convection was driven by thermal buoyancy; (4) the solutal convection was ignored; (5) the

heat transfer was coupled with the mold filling. A three-dimensional model is established based on the governing

equation, including the Navierâ&#x20AC;&#x201C;Stokes equation, the continuity equation, energy equation, Îş-Îľ equation, and VOF function.

The geometries and finite element mesh of the steel ingot,

the mold, the insulation brick and bottom pad are shown in Figure 1. The finite element mesh of the mold, the ingot, the insulation brick and bottom pad consisted of 71512 nodes and 334535 tetrahedral elements, which were selected based on several mesh refinements.

Fig.1 - Geometric models and finite element mesh of the mold, steel ingot, insulation brick and bottom pad, (a) Geometric models, (b)finite element mesh

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Scientific papers - Miscellaneous Thermophysical parameters

Insulation Brick are provided by the manufacturer, the va-

The thermophysical parameters of the Bottom Pad and

lues are shown in Table 1.

Tab.1 - The thermophysical parameters of Bottom Pad and Insulation Brick Material

Density(kg/m3)

Specific heat(J/gK)

Conductivity(W/(mK))

Bottom Pad

2900

1.5

4

Insulation Brick

500

1.08

0.78

The compositions of the steel ingot and mold are shown

and mold are calculated via database system in ProCAST

in Table 2. The thermophysical parameters of steel ingot

software, as shown in Figure 2.

Tab.2 - Composition of the ingot and mold(wt.%) Material

C

Si

Mn

P

S

Cr

Ni

Ingot

0.38-0.41

1.59-1.63

1.69-1.71

<0.005

<0.002

0.82-0.84

1.84-1.86

Mold (Ductile iron)

3.2-3.5

—

0.8-1.2

<0.005

<0.005

—

—

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Memorie scientifiche - Miscellanea

Fig.2 - Physical parameters of the ingot and mold:(a) density of the ingot;(b) thermal conductivity of the ingot;(c) enthalpy of the ingot;(d) density of the mold;(e) thermal conductivity of the mold;(f) enthalpy of the mold.

Boundary and initial conditions

transfer coefficient of metal-mold interface changes with

Because of the gap between ingot and mold, the heat

time[16][17].It is shown in equation 1.

(1)

where hc = 800(1-t/tgap), and tgap is average formation time of

20W·m-2·K-1. The heat transfer coefficient between steel and

the gap between the ingot and mold; εeff is effective emissi-

bottom pad was assigned as 100W·m-2·K-1.The heat transfer

vity in ingot/mold interface,0.7;σ is Stefan–Boltzman con-

coefficient between mold and bottom pad was assigned as

stant, 5.67x10 W·m ·K ;Ti is temperature of the mold in

200 ;The heat transfer coefficient between base and bottom

ingot/mold interface,Tmi is temperature of the ingot in in-

pad was assigned as 100 .

-8

-2

-4

got/mold interface ; tgap of 18 and 5.3 ton steel ingots were

The heat transfer coefficient of the outer wall of mold wall

assigned as 1200s and 80s[18].

was assigned according to the following Equation 2 in whi-

The heat transfer coefficients of both mold-insulation bri-

ch both radiation and convection heat transfer were taken

ck and steel-insulation brick interfaces were assigned as

into consideration[19].

(2)

where Tme is the temperature around mold wall; T∞ is the

The heat transfer between the top of the ingot and the ex-

equivalent temperature of surroundings; εme is emissivity

ternal environment is also treated according to equation 2.

of outer wall of the mold,0.85; and σ is the Boltzmann con-

The initial values for each material are shown in Table 3.

stant.

Tab.3 - Initial temperature values for each material Material

Steel

Mold

Insulation brick

Bottom pad

Initial value/°C

1560

80

40

200

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Scientific papers - Miscellaneous Verification of Mathematical Model

the mold during solidification of ingot was measured. Com-

Verification of temperature

parison between calculated results and measured values

In order to verify the accuracy of numerical simulation of

are shown in Figure 3. As can be seen that the calculated

ingot solidification, industrial experiments were carried out

values agree well with the measured ones.

in a steel plant. The temperature change of the outer wall of

Fig.3 - Position of measured point and comparison of calculated temperature with measured ones,(a) position of measured point,1#, 2# and 3# are the number of measuring points Verification of shrinkage pipe

(A)), photo of the 5.3-ton ingot (panel(B)), calculated result

In order to further certify the mathematical model, the soli-

(panel(C)), respectively. Figure 4 (A) and (C) confirmed that

dification process of 5.3-ton ingot with a height of 2820 mm

the simulated shape and length of the shrinkage pipe of the

and a diameter of 590 mm has been investigated. Figure 4

ingot are in good agreement with the experimental result

compiles the sectioned surface at top of the ingot (panel

observed by the sectioning.

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Memorie scientifiche - Miscellanea

Fig.4 - The sectioned surface at top of the ingot (A), photo of the 5.3-ton ingot (B), calculated result (C) Consequently, through FEM (Finite Element Method) simu-

tion front moves to the center of the ingot in a "deep U" sha-

and experimental sectioning investigation of the ingot, it is

rate of ingot body is faster than that of top and bottom of in-

lation in combination with the temperature measurement concluded that the mathematical model of solidification pro-

cess of ingot with large ratio of height to diameter in this paper is quite accurate.

Results and Discussion

Heat Transfer Characteristics of Steel Ingot Solidification

Fraction solid distribution at different time during ingot solidification is shown in Figure 5. As can be seen from figure that after the mold is filled with molten steel, the solidifica-

pe. The reason for this phenomenon is that the solidification got. In the final stage of solidification of the steel ingot, the

liquid steel in the center of ingot can solidify simultaneously and canâ&#x20AC;&#x2122;t be supplemented by the surrounding liquid metal, which easily leads to the defect formation of the shrinkage

porosity. At the end of ingot solidification, a deep shrinkage pipe was formed at the top of the ingot, which is one of the

characteristics of solidification of steel ingot with large ratio of height to diameter.

Fig.5- Fraction solid distribution at different time during ingot solidification

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Scientific papers - Miscellaneous Figure 6 shows evolution of heat storage of the mold during

ment, which leads to that the heat storage of mold increases

ge of solidification, the heat storage of mold is almost equal

quently, the mold is always in the state of heat storage during

solidification, as can be seen from figure that in the early stato the heat through the inner wall of mold. With the develop-

ment of solidification, the temperature on the outer wall of the mold is much higher, which accelerates the efficiency of

heat transfer from the outer wall of the mold to the environ-

slowly in the middle and late stages of solidification. Conse-

the whole solidification process of ingot, which is another characteristic of solidification of steel ingot with large ratio of height to diameter.

Fig.6 - Evolution of heat storage of the mold during solidification During the ingot solidification, the superheat and latent heat

ferent parts of ingot at the end of solidification is shown in

through those paths: 1) top of the ingot; 2) the side of the in-

mainly transferred to the external environment through in-

of molten steel are transferred to the external environment

sulation brick; 3) ingot body; 4) bottom of the ingot, as shown in Figure 7 (A). Proportion of total heat transfer quantity at dif-

Figure 7(B). As can be seen that the heat of molten steel is got body.

Fig.7 - Proportion of total heat transfer quantity at different parts of ingot at the end of solidification

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Memorie scientifiche - Miscellanea Thermal Resistance

with time. In order to control the solidification process of in-

tion process of ingot, the superheat and latent heat of mol-

at different stages of steel ingot solidification with different

The result is obtained from Figure 7 (B), during the solidifica-

ten steel are mainly transferred to the external environment through solidified shell, ingot/mold interface and mold in turn, as shown in Figure 8.

The heat transfer resistance of each section changes continuously with time. In different stages of the steel ingot solidification process, the restrictive step of heat transfer changes

gots, it is extremely important to identify the restrictive steps casting parameters. In the section, the heat transfer during

solidification of 18-ton ingot with a large ratio of height to diameter was studied. Then, the thermal resistance of each

heat transfer section in different solidification stages was found out.

Fig.8 - Schematic of heat transfer of steel ingot in the middle height during solidification, RM-0,thermal resistance of outer wall of mold; RM-1-0, thermal resistance of mold;

RAIR, thermal resistance of ingot/mold interface; RSHELL, thermal resistance of solidified shell In order to compare the heat transfer resistances of different

el ingot with large ratio of height to diameter were studied.

racteristics of heat transfer in the middle height of 18-ton ste-

resistance of each heat transfer section was found out.

heat transfer steps during solidification process, the cha-

Based on the concept of thermal resistance [ ], the thermal

Fig.9 - Evolutions of the transient thermal resistances of different heat transfer steps when the outer surface of the mold is cooled by air during ingot solidification

La Metallurgia Italiana - maggio 2020

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Scientific papers - Miscellaneous Figure 9 shows evolutions of the transient thermal resi-

Based on the analysis of the thermal resistances of diffe-

stances of different heat transfer steps when the outer wall

rent heat transfer steps under the above condition, heat

of the mold is cooled by air during solidification of ingot.

transfer of solidified shell is the main restrictive step of

As can be seen from the figure that the thermal resistances

heat transfer in the whole solidification process for 18-ton

of both the ingot/mold interface and the mold are relati-

steel ingot with large ratio of height to diameter.

vely small in the whole solidification process.

Although thermal resistance of the outer wall of the mold

Effect of Pouring Temperature on the Thermal Resi-

is larger than that of other heat transfer section in the early

stances

stage of solidification, the heat transfer of the outer wall of

The pouring temperature is a significant parameter in the

the mold is not the restrictive step of heat transfer. At the

solidification process of ingot, because it has a great in-

initial stage of solidification, the heat transfer rate of the in-

fluence on the defect formation of ingot such as cracks,

got is determined by both the heat transfer of ingot/mold

shrinkage porosity and macrosegregation.

interface and the heat storage of the mold. After the mold is

In order to explore the influence of different pouring tem-

filled with molten steel, the molten steel close to the mold

peratures on heat transfer resistances of steel ingot soli-

begins to form solidified shell of a certain thickness, heat is

dification, numerical simulations of solidification process

transferred to the mold through the ingot/mold interface,

of the 18-ton steel ingot with different pouring tempera-

which is the heat storage stage of the mold, as shown in

tures were carried out. Evolutions of the transient thermal

Figure 6. At the time, the heat transfer efficiency from the

resistances of different heat transfer steps with different

outer wall of the mold to the external environment is slow,

pouring temperatures are shown in Figure 10. As can be

therefore, the thermal resistance of the outer wall of the

seen from figure that at the initial stage of solidification, the

mold is greater than that of other heat transfer sections. In

change of pouring temperature has no effect on the ther-

the middle and later stages of solidification, the tempera-

mal resistances of different heat transfer sections. Howe-

ture on the outer wall of the mold is much higher, which

ver, in the middle and later stages of solidification, in-

accelerates the efficiency of heat transfer from the outer

fluence of variation of pouring temperature on the thermal

wall of the mold to the environment, the thermal resistan-

resistance of solidified shell is obvious. Steel ingot with a

ce of the outer wall of the mold decreases gradually.

high pouring temperature can lead to a small thermal re-

At the initial stage of solidification, the thermal resistance

sistance of solidified shell compared with that with a low

of solidified shell is small. With the development of soli-

pouring temperature.

dification, then solidified shell is getting thick, which re-

It can also be seen from the Figure 10 that at early stage

sults in the increase of corresponding thermal resistance.

of solidification, the thermal resistance of the outer wall

In the middle and late stages of solidification, the thermal

of the mold is the largest among all heat transfer sections.

resistance of the solidified shell is larger than that of other

In the middle and late stages of solidification, the thermal

heat transfer sections, especially near the final stage of so-

resistance of the solidified shell is larger than that of other

lidification, the thermal resistance of solidified shell is very

heat transfer section. In other words, the variation of pou-

large, therefore the heat transfer section of the solidified

ring temperature does not change the restrictive step of

shell is the restrictive step of heat transfer.

heat transfer in the process of steel ingot solidification.

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Memorie scientifiche - Miscellanea

Fig.10 - The transient thermal resistances of different heat transfer sections during steel ingot solidification with different pouring temperatures Conclusions

When the cooling mode of the outer wall of the mold is for-

el is mainly transferred to the external environment through

the transient thermal resistances of different heat transfer

In the process of ingot solidification, the heat of molten ste-

solidified shell, ingot/mold interface and mold in turn. The

heat transfer step that can be controlled artificially is the heat transfer from the outer wall of the mold to the environment.

Therefore, when the cooling mode of the outer wall of the

mold is forced air cooling, evolutions of the transient thermal resistances of different heat transfer steps were investigated.

When the outer wall of the mold is cooled by forced air, the

heat transfer coefficient between the outer wall of mold and the environment is 800W¡m ¡K [21].

-2

ced air cooling during solidification of ingot, evolutions of

steps are shown in Figure 11. It can be seen from the figure that the thermal resistance of the solidified shell is larger than that of other heat transfer section in the whole solidification process except in the initial stage of solidification.

The thermal resistance of the solidified shell is the inherent

characteristic of the ingot, which is not affected by external factors. Therefore, the solidification rate of the steel ingot is difficult to control.

-1

Fig.11 - Evolutions of the transient thermal resistances of different heat transfer steps when the cooling strength of the outer surface of the mold is forced air cooling during solidification of ingot La Metallurgia Italiana - maggio 2020

pagina 46

Scientific papers - Miscellaneous Based on the analyses of the thermal resistances of different heat transfer steps with different casting parameters, the thermal resistances of solidified shell are still dominant in the whole solidification process for 18-ton steel ingot with large ratio of height to diameter.

Effect of Cooling Intensity on the Thermal Resistance Numerical simulations of solidification process of 18-ton steel ingot with a large ratio of height to diameter were investigated and the simulated results were verified by the measured temperatures and experimental sectioning investigation.

The heat transfer characteristics during solidification of steel ingot with large ratio of height to diameter were dug out. The obtained results are summarized as follows: (1) The heat of molten steel is mainly transferred to the external environment through ingot body.

(2) In the early stage of solidification, the thermal resistance of outer wall of mold is the largest among all heat

transfer sections. In the middle and late stages of solidification, the thermal resistance of the solidified shell is larger than that of other heat transfer sections, therefore, the heat transfer section of the solidified shell is the restrictive step of heat transfer.

(3) The variation of pouring temperature does not change the restricted heat transfer section in the process of steel ingot solidification.

(4) The outer surface of the mold is cooled by forced air can lead to that the thermal resistance of the solidified shell is larger than that of other heat transfer section in the whole solidification process except in the initial stage of solidification.

(5) The heat transfer of solidified shell is the main restrictive step of heat transfer in the whole solidification process for 18-ton steel ingot with large ratio of height to diameter.

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La Metallurgia Italiana - May 2020

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Attualità industriale - Miscellanea

Il nuovo acciaio da utensile da lavorazione a caldo THERMODUR E40K SUPERCLEAN a cura di: E. Prati, A. Magistrelli - B.A Bonomi Acciai S.r.l

Al fine di prolungare la vita utile degli stampi di pressocolata e migliorare le proprietà di stabilità alle alte temperature unita all'elevata tenacità, Deutsche Edelstahlwerke ha sviluppato il nuovo acciaio per utensili a caldo Thermodur E 40 K Superclean (rifuso sotto elettro-scoria).

Thermodur E 40 K Superclean è stato sviluppato appositamente per gli stampi di pressocolata per soddisfare i più elevati requisiti di resistenza alla fatica termica, tenacità alle temperature elevate e resistenza all'usura a temperature elevate.

Rispetto ai gradi convenzionali di acciaio H11 e H13, Thermodur E 40 K Superclean mostra una straordinaria combinazione di stabilità alla temperatura e tenacità anche ad alte temperature. Queste proprietà rendono questo grado particolarmente adatto per stampi per pressofusione altamente sollecitati, stampi per estrusione e forgiatura.

Nel presente studio sono state studiate le proprietà selezionate (ad es. Tenacità Charpy-V a temperature elevate, resistenza agli shock termici e prova ad impatto con pendolo Charpy strumentato) del nostro nuovo acciaio per lavorazioni a caldo Thermodur E 40 K e sono state confrontate con i comuni gradi di acciaio H11 e H13.

KEYWORDS: ACCIAIO PER UTENSILI PER LAVORAZIONE A CALDO, THERMODUR E 40 K SUPERCLEAN, SPECIFICA NADCA#207, TENACITÀ, PROVA A IMPATTO STRUMENTATA, FATICA TERMICA, TRATTAMENTO TERMICO, COMPORTAMENTO ALLA TEMPRA, PROPRIETÀ FISICHE, PRESSOCOLATA, MICROSTRUTTURA Introduzione

La richiesta di materiali adatti per componenti di presso-

colata è ancora in aumento. Uno dei parametri essenziali

di un’attrezzatura di pressocolata è la sua durata utile, che descrive il numero di componenti prodotti con lo stampo

prima della sua avaria. La vita utile può essere incrementata grazie all’impiego di un materiale che esibisce buona sta-

Articolo presentato al NADCA di Indianapolis

Nell’80% dei casi, l’avaria degli stampi è causata dalla fatica

Deutsche Edelstahlwerke GmbH

bilità a caldo ed alta tenacità anche ad elevate temperature.

termica (Figura 1). La superficie del materiale di un componente di pressocolata subisce una permanente espan-

sione e contrazione durante l’esercizio, che è iniziato da

Dr.Till Schneiders

ed alla conferenza sugli utensili di Bratislava

Markus Gürcan

Deutsche Edelstahlwerke GmbH

un riscaldo e raffreddamento ciclico della superficie della

cavità durante l’iniezione della lega liquida. Ulteriori dettagli di questo meccanismo di avaria, descritti dalla co-

siddetta Teoria Kindbom, possono essere letti nella 1. La Figura 1 mostra i principali meccanismi di avaria di stampi di pressocolata 2,3.

La Metallurgia Italiana - maggio 2020

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Industry news -Miscellaneous

Fig.1 - Meccanismi di Avaria di uno stampo di Pressocolata Ulteriori possibilità di avaria sono le cricche da tensionamen-

per estrusione e matrici per forgiatura.

(4%). Per soddisfare i requisiti derivanti dalle esigenze ope-

SCHEMA DEGLI ESPERIMENTI

Thermodur E 40 K Superclean. Thermodur E 40 K Supercle-

le dimensioni del nostro nuovo acciaio per utensili a caldo

to (9%), la corrosione (7%) e il washout / metallizzazione

rative è stato sviluppato il nuovo acciaio per utensili a caldo an mostra un’eccezionale combinazione di stabilità a caldo e

tenacità anche a temperature elevate, particolarmente adatta per matrici per pressofusione altamente sollecitate, matrici

Vengono mostrate in Tabella 1 le composizioni chimiche e

Thermodur E 40 K Superclean a confronto rispetto ai comuni acciai per utensili a caldo H11, H13 e 2367.

Tab.1 - Composizione chimica e dimensioni dei quattro acciai per utensili a caldo studiati.

L’analisi chimica del nostro nuovo acciaio per utensili per

un contenuto di molibdeno dell’1,8% del nostro E 40 K è

Thermo-Calc. La Figura 2 mostra la precipitazione dei diversi

con le temperature di austenitizzazione tipiche di un ciclo di

lavorazione a caldo è stata sviluppata utilizzando il software carburi sull'asse del molibdeno nello stato di equilibrio. Con

possibile riportare in soluzione i carburi secondari precipitati tempra ottimale.

Fig.2 – Diagramma di equilibrio del Molibdeno

La Metallurgia Italiana - May 2020

pagina 49

Attualità industriale - Miscellanea Al fine di migliorare la tenacità, il Thermodur E 40 K Super-

alte temperature. Tutti gli elementi rivelano un contenuto

e -Cr (elementi formanti carburi) sono stati progettati per

tre, Thermodur E 40 K Superclean mostra un basso conte-

clean ha un basso contenuto di Si. I materiali base -Mo, -V

ottenere una qualità premium con proprietà superiori alle

costante su tutta la sezione trasversale (Figura 3 e 4). Inolnuto di elementi di lega secondari.

Fig.3 – Omogeneità Macroscopica

Fig.4 – Omogeneità Microscopica Il Thermodur E 40 K Superclean è rifuso sotto elettro-sco-

rie e trattato termicamente in accordo alla specifica NA-

DCA #207-2015. Tutti i campioni sono stati bonificati nell’intervallo di durezza di 45±1 HRC. La microstruttura, la micropulizia e la dimensione del grano sono state valutate secondo la specifica NADCA #207-2015.

TENACITÀ

I test di tenacità sono stati eseguiti a temperatura ambiente

secondo le specifiche europee con campioni non intagliati con una dimensione di 7 mm x 10 mm x 55 mm e secondo

la specifica NADCA #207-2015 con campioni con intaglio a

“V” con una dimensione di 10 mm x 10 mm x 55 mm. Tutti i campioni sono stati prelevati in direzione trasversale (nella sezione di transizione Q27 o nella sezione centrale Q17).

La tenacità a temperatura elevata è stata studiata tra 100 °C e 600 °C, con incrementi di 100 °C, utilizzando campioni Charpy-V.

La Metallurgia Italiana - maggio 2020

PROVA AD IMPATTO CON PENDOLO CHARPY STRUMENTATO

Le prove di impatto strumentate sono state eseguite se-

condo la norma DIN EN 14556 utilizzando una macchina per prove di impatto con un’energia di impatto massima di

300 J. Tutti i test sono stati eseguiti a temperatura ambiente

utilizzando campioni non intagliati con una dimensione di 5 mm x 10 mm x 55 mm. Per la determinazione dell’inne-

sco della cricca e della propagazione della cricca i risultati sono stati rappresentati in diagrammi forza-deviazione.

FATICA TERMICA

Il comportamento alla fatica termica è stato testato usando

la configurazione ciclica di riscaldamento e raffreddamen-

to in Figura 5. I campioni (50 mm x 55 mm x 10 mm) sono

stati prelevati dalla direzione trasversale dell’area di transizione del blocco. Al fine di evitare l’innesco di cricche a

causa di scanalature di rettifica, i campioni sono stati retti-

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Industry news -Miscellaneous ficati con una rugosità superficiale di max. 5 μm. I campioni

gas inerte (argon). I test sono stati impostati a 2000, 4000,

650 °C per induzione ad alta frequenza per quattro secondi

stati caratterizzati dal numero di cricche, dalla lunghezza

sono stati fissati su una ruota (Figura 5) e riscaldati fino a e successivamente raffreddati in una vasca acqua, che ave-

va una temperatura costante di 60 °C. Per evitare l’ossidazione, la struttura sperimentale chiusa è stata riempita con

6000 e 8000 cicli. I risultati dei test di fatica termica sono massima delle cricche e dalla lunghezza totale delle cricche. Ulteriori dettagli di questo test sono descritti in 4,5.

Fig.5 - Installazione sperimentale per testare il comportamento a fatica termica.

TRATTAMENTO TERMICO

La Figura 6 mostra il diagramma di trasformazione a raf-

freddamento continuo CCT del Thermodur E 40 K. Il com-

portamento è simile a H11, quindi è garantita la penetrazione di tempra di grandi sezioni trasversali.

Fig.6 - Diagramma di Trasformazione a raffreddamento continuo (CCT) COMPORTAMENTO AL RINVENIMENTO

perclean si verifica a 525 °C, la durezza ammonta a 53 HRC.

Thermodur E 40 K Superclean rispetto al comportamento

Superclean può essere paragonato al comportamento al

La Figura 7 mostra il comportamento al rinvenimento del al rinvenimento dei tre comuni acciai per utensili a caldo. Il

picco di indurimento secondario di Thermodur E 40 K Su-

La Metallurgia Italiana - May 2020

Il comportamento al rinvenimento del Thermodur E 40 K rinvenimento del H11 (Figura 7).

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Attualità industriale - Miscellanea

Fig.7 - Diagramma di Rinvenimento del Thermodur E 40 K Superclean.

RISULTATI E DISCUSSIONE

°C). A 20 °C la conducibilità termica ammonta a 28,3 W/m

Le proprietà fisiche della condizione bonificata (45±1 HRC)

valore massimo di 31 W/m K (300°C) e successivamente

PROPRIETÀ FISICHE

sono elencate in Tabella 2. Il coefficiente di dilatazione ter-

mica aumenta all’aumentare della temperatura da 11,5 K-1

(20 °C - 100 °C) a 13,2 K (20 °C- 600 °C) mentre la densi-1

tà diminuisce (da 7,79 kg/dm a 20 °C a 7,61 kg/dm a 600 3

3

K. Innanzitutto la conducibilità termica aumenta fino a un diminuisce a 27,8 W/m K (600°C). Per quanto riguarda que-

ste proprietà fisiche ci sono solo piccole differenze tra il

Thermodur E 40 K Superclean e i comuni acciai per utensili a caldo.

Tab.2 - Coefficiente di dilatazione termica, conducibilità termica e densità del Thermodur E 40 K Superclean a diverse temperature in condizione di bonificato.

MICROSTRUTTURA

Il Thermodur E 40 K Superclean presenta una microstruttura e macrostruttura omogenee sull’intera sezione tra-

sversale. La microstruttura ricotta è costituita da una matrice ferritica con una omogenea distribuzione di carburi

sferoidali. La microstruttura allo stato ricotto del Thermodur E 40 K Superclean è priva di bandosità eccessiva in conformità con la tabella di riferimento per la segregazio-

La Metallurgia Italiana - maggio 2020

ne delle bandosità NADCA per livelli di microbandosità e segregazione microchimica e può quindi essere classificata come accettabile. I requisiti per il materiale rifuso sotto

elettro-scorie, a livello di micropurezza, sono stati sod-

disfatti in conformità alla ASTM E45. Entrambe, struttura ricotta e struttura bonificata del Thermodur E 40 K Superclean, sono mostrate nella Figura 8.

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Industry news -Miscellaneous

Fig.8 - a) Microstruttura ricotta e b) Microstruttura bonificata di Thermodur E 40 K Superclean.

PROVA DI TRAZIONE

ratura ambiente Rm è pari a 1480 MPa e Rp0.2 è pari a 1270

mento Rp0.2 e la riduzione dell’area Z in funzione della tempe-

riduzione dell’area è dal 47% al 52% nella zona centrale e dal

La Figura 9 mostra la trazione Rm e la resistenza allo snerva-

ratura. Mentre Rm e Rp0.2 diminuiscono con l’aumentare della

temperatura, Z aumenta con temperature elevate. A tempe-

MPa indipendentemente dalla posizione del campione. La 56% al 58% nella zona esterna.

Fig.9 - Risultati delle prove di trazione di Thermodur E 40 K Superclean.

TENACITÀ

trale. Considerando i risultati del test di impatto strumentato

vorazione a caldo mostrano un comportamento di tenacità

dei quattro acciai per utensili a caldo esaminati nei diagram-

A temperatura ambiente, i quattro acciai per utensili da la-

paragonabile relativamente al test Charpy-V. Il Thermodur E 40 K Superclean mostra valori di circa 20 J nella sezione cen-

ci sono differenze significative. La Figura 10 mostra i risultati

mi forza-deviazione. Le energie massime di impatto relative sono riportate in alto a destra in ciascun diagramma.

Fig.10 - Diagrammi di forza-deviazione misurati in prove ad impatto con pendolo Charpy strumentato per a) Thermo-

dur E 40 K Superclean, b) H11, c) H13 e d) 2367. Tutti i campioni testati sono stati prelevati dal cuore (Q17). L’energia di impatto assorbita è mostrata in alto a destra in ogni diagramma.

La Metallurgia Italiana - May 2020

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Attualità industriale - Miscellanea Si può vedere che Thermodur E 40 K Superclean (152 J)

rapida crescita delle microcricche)6. Il Thermodur E 40 K

ha la massima tenacità seguita da H11 (136 J) > 2367 (112

Superclean ed il H11 mostrano una migliore resistenza alla

J) > H13 (92 J). Tutti gli acciai dissipano solo una piccola

propagazione delle cricche rispetto agli acciai H13 e 2367.

quantità dell’assorbimento di energia per l’innesco della

La Figura 11 mostra i risultati delle prove di tenacità a tem-

cricca (parte elastica lineare, parte elasto-plastica lineare e

perature elevate fino a 600 °C per gli acciai studiati. La tena-

parte plastica non lineare dei diagrammi), la frazione prin-

cità del Thermodur E 40 K Superclean (Figura 7a) aumenta

cipale dell’assorbimento di energia viene dissipata duran-

con l’aumentare della temperatura fino a 500 °C dove rag-

te la propagazione della cricca (area di caduta del carico e

giunge un livello di tenacità massimo di ≈82 J.

Fig.11 - Tenacità (Charpy-V) in funzione della temperatura per a) Thermodur E 40 K Superclean, b) H11, c) H13 e d) 2367.

RESISTENZA ALLA FATICA TERMICA

teriali studiati: Thermodur E 40 K Superclean, H11, H13 e

dal riscaldamento e dal raffreddamento nel test di fatica

modur E 40 K Superclean è meno suscettibile alla forma-

Il cambio periodico di tensione e compressione causato

termica porta a una rete di cricche. La Figura 12 mostra la comparsa di cricche a caldo dopo 4000 cicli per tutti i ma-

2367. Rispetto ai comuni acciai per utensili a caldo, il Therzione di cricche da fatica termica (Figura 12a).

Fig.12 - Microstruttura di a) Thermodur E 40 K Superclean, b) H11, c) H13 e d) 2367 dopo 4000 cicli nel test di fatica termica.

La resistenza al controllo del calore è stata studiata attra-

sistenza alla fatica termica. La Figura 13a mostra la massima

le lunghezze delle cricche dopo 4000 cicli. In questo con-

mero totale delle cricche in funzione del numero di cicli,

verso la massima lunghezza della cricca e sommando tutte testo: minore è la lunghezza della cricca, maggiore è la re-

La Metallurgia Italiana - maggio 2020

lunghezza della cricca e la Figura 13b la lunghezza del nuche è stato diversificato tra 2000, 4000, 6000 e 8000.

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Industry news -Miscellaneous

Fig.13 - a) Massima lunghezza della cricca e b) lunghezza totale delle cricche di Thermodur E 40 K Superclean, H11, H13 e 2367 dopo 2000, 4000, 6000 e 8000 cicli nel test di fatica termica.

Soprattutto dopo 6000 e più cicli termici, il nuovo acciaio per utensili a caldo Thermodur E 40 K Superclean mostra

una lunghezza della cricca totale inferiore rispetto agli acciai comuni (vedi Figura 13b). È ben noto che il comportamento alla fatica termica è influenzato tra l’altro da due effetti opposti. L’innesco della cricca ha avvio sotto un ca-

rico ciclico quando il limite di snervamento viene superato localmente. Le prime cricche compaiono, ad esempio, in

corrispondenza di inclusioni non metalliche o carburi eutettici . Questo effetto può essere ritardato da un’elevata 2,6

durezza, ma ciò è limitato dalla perdita di tenacità e dal rischio di rottura. Una volta innescata la cricca, è utile che l’utensile presenti una buona resistenza alla propagazione

della cricca. Poiché la fatica termica nella pressofusione è imprevedibile, è molto utile rallentare la propagazione delle cricche mediante una buona combinazione di stabi-

lità alle alte temperature e tenacità per utensili altamente

sollecitati. La maggiore resistenza di Thermodur E 40 K Superclean rispetto alle altre qualità contro le cricche da fatica termica deriva dalla maggiore tenacità alla durezza data di 45±1 HRC.

CONCLUSIONI E PROSPETTIVE

Al fine di aumentare la durata degli utensili da lavorazione a caldo, è consigliabile considerare che ci sono molte applicazioni diverse che richiedono diverse proprietà dell’ac-

ciaio. Gli acciai per utensili per stampi di pressocolata richiedono una buona resistenza alla fatica termica, il che

La Metallurgia Italiana - May 2020

può essere ottenuto grazie ad una buona combinazione di

stabilità alle alte temperature e tenacità per utensili altamente sollecitati.

Esistono diversi approcci per misurare la tenacità. Il valore ottenuto nel test Charpy-V è un valore integrale dell’inne-

sco della cricca e della propagazione della cricca, il volu-

me testato e la zona deformata plasticamente sono piuttosto grandi. Questo test fornisce informazioni non isolate

circa la propagazione della cricca e c'è solo un riflesso limitato del carico nella prassi industriale. La tenacità come

resistenza alla propagazione delle cricche può essere determinata mediante il test di impatto strumentato. I dia-

grammi forza-deviazione mostrano che Thermodur E 40 K Superclean mostra un’ottima tenacità e resistenza contro la propagazione delle cricche. Lo stesso vale per i risultati

del test di fatica termica. Una volta innescata la cricca, la crescita è determinata dalla capacità del materiale di resistere a questa crescita, ovvero il livello di tenacità. Contra-

riamente al test Charpy-V, il volume testato in prossimità

dell’estremità della cricca è molto piccolo, il che riflette le esigenze operative nel processo di pressofusione.

Thermodur E 40 K Superclean mostra la più corta lunghezza massima della cricca così come la minore lunghezza

totale delle cricche. Ciò approva i risultati della prova di impatto strumentata e sottolinea l’eccellente idoneità di

questo acciaio per applicazioni che richiedono una buona tenacità.

pagina 55

Attualità industriale - Miscellanea RIFERIMENTI [1]

Kindbom, L., Warmrissbildung bei der Temperaturwechselbeanspruchung von Warmarbeitswerkzeugen, Arch. Eisenhüttenwesen, vol. 35, n. 8 (1964).

[2]

Berns, H., Beispiele zur Schädigung von Warmarbeitswerkzeugen, Z. Werkst. Wärmebeh. Fertigung, vol. 59, n. 6 (2004).

[3]

Tang, C., Jahedi, M. Z., Brandt, M., Indagine sulla reazione di saldatura nella pressofusione di magnesio ad alta pressione, in: L’uso di acciai per utensili, esperienza e ricerca, Proc. 6th Int. Conf. Utensili Universität Karlstad, Schweden, pagg. 137-146 (2002).

[4]

Malm, S., Svensson, J., Tidlund, J., Controllo del calore negli acciai per lavori a caldo, 2 ° Int. Colloquio su acciai per utensili per lavorazione a caldo, cercle d’Etudes des Metaux, Saint-Etienne (1977).

[5]

Ehrhardt, R., Fuchs, K.-D., Miglioramento delle caratteristiche di controllo del calore di AISI H13 mediante modifica della composizione chimica, in: Die Casting of Heartland, 22th Internat. Die Casting Congress & Exposition, Indianapolis, Stati Uniti (15-18 settembre 2003).

[6]

Roberts, G., Krauss, G., Kennedy, R., Tool Steels, 5a edizione, ASM International (1998).

[7]

Wellinger, K., Pröger, M., Vasoukis, G., Kraft- und Wegmessungen als Grundlage für das Verständnis des Kerbschlagbiegeversuchs, Z. f. Werkstofftechnik / J. of Materials Technology, vol. 4, n. 1 (1973).

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Industry news -Miscellaneous

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Scenari - Miscellanea

LA METALLURGIA NEI GRANDI POEMI DELL’ANTICHITÁ a cura di: Giuseppe G. Casarini – Binasco (MI) La nascita della metallurgia: la lavorazione dei metalli e

come mette in evidenza Lucrezio i tempi cambiano :”/Ora

l’uso dei metalli

è trascurato il rame, l'oro è asceso al più alto onore./Così il

Il poeta latino Tito Lucrezio Caro ( 98-54 A.C.) nel suo De

volgere del tempo tramuta le stagioni delle cose:/ciò che

Rerum Natura (13), fedele al pensiero di Epicuro e par-

era in pregio, diventa alfine di nessun valore;/”…

tendo dall’analisi delle particelle minime ed indivisibili, gli atomi, ed analizzando i processi della conoscenza umana

Usura e corrosione dei metalli

ed i meccanismi che presiedono ai fenomeni naturali, ci

L’osservazione di Lucrezio sui metalli e sul loro decadi-

introduce, poeticamente nel Libro V alla nascita della me-

mento con specifico riferimento alla concezione atomisti-

tallurgia ed alla lavorazione dei metalli.:

ca delle cose, si fa ancora e più profonda ( Libro I) : qualsi-

”Comunque sia, quale che fosse la causa per cui l'ardore/

asi sia la natura del metallo o della lega: oro, ferro, bronzo,

delle fiamme aveva divorato con orrendo fragore le selve/

al pari delle pietre, tutto ciò, con l’impiego e nel tempo, si

dalle profonde radici e aveva cotto a fondo col fuoco la

usura e si corrode senza che noi ne possiamo conoscerne

terra,/colavano dalle vene bollenti confluendo nelle ca-

il perché:

vità della terra/rivoli d'argento e d'oro e anche di rame e

“Per di più, nel corso di molti anni solari l'anello,/a forza

di piombo./E quando gli uomini li vedevano poi rappresi/

d'essere portato, si assottiglia dalla parte che tocca il dito;/

risplendere sul suolo di lucido colore,/li raccoglievano,

lo stillicidio, cadendo sulla pietra, la incava; il ferreo vome-

avvinti dalla nitida e levigata bellezza,/e vedevano che era-

re/adunco dell'aratro occultamente si logora nei campi;/e

no foggiati in forma simile a quella/che aveva l'impronta

le strade lastricate con pietre, le vediamo consunte/dai

dell'incavo di ognuno./Allora in essi entrava il pensiero che

piedi della folla; e poi, presso le porte, le statue/di bronzo

questi, liquefatti al calore,/potessero colando plasmarsi in

mostrano che le loro mani destre si assottigliano/al tocco

qualsiasi forma e aspetto di oggetti,/e che martellandoli si

di quelli che spesso salutano e passano oltre./Che queste

potesse forgiarli in punte di pugnali/quanto mai si volesse

cose dunque diminuiscano, noi lo vediamo,/perché son

acute e sottili,/sì da procurarsi armi e poter tagliare selve/

consunte. Ma quali particelle si stacchino in ogni/momen-

ed asciare il legname e piallare e levigare travi/ed anche

to, l'invidiosa natura della vista ci precluse di vederlo./“

trapanare e trafiggere e perforare/.

Riciclaggio Le proprietà dei metalli

Virgilio, nel Libro VII dell’Eneide, ci offre un saggio poeti-

Di seguito e sempre nel Libro V, Lucrezio mette in eviden-

co sui riciclaggi del ferro e dell’acciaio: il nemico incombe

za come, dopo la scoperta della metallurgia, gli uomini

e bisogna difendersi : attrezzi agricoli e mezzi per disso-

abbiano imparato a conoscerne subito le caratteristiche e

dare il terreno vengono rifusi e trasformati sotto forma di

l’utilità:

armi e di corazze:

“E dapprima s'apprestavano a far queste cose con l'argen-

“Cinque grosse città con mille incudi/ a fabbricare, a risar-

to e l'oro/non meno che con la forza violenta del possente

cir si dànno/ d'ogni sorte armi: la possente Atina,/ Ardea

rame,/ma invano, poiché la tempra di quelli vinta cedeva,/

l'antica, Tivoli il superbo,/ e Crustumerio, e la torrita An-

né potevano sopportare ugualmente il duro sforzo./Difatti

tenna./ Qui si vede cavar elmi e celate;/ là torcere e covrir

‹il rame› era più pregiato e l'oro era trascurato/per l'inu-

targhe e pavesi:/per tutto riforbire, aüzzar ferri,/ annestar

tilità, perché si smussava con la punta rintuzzata./” ma,

maglie, rinterzar corazze,/ e per fregiar piú nobili armatu-

La Metallurgia Italiana - maggio 2020

pagina 58

Experts’ Corner - Miscellaneous

re,/ tirar lame d'acciar, fila d'argento./ Ogni bosco fa lance,

e leghe/ in piú guise si fan d'ogni metallo./ Di sette falde

ogni fucina/ disfà vomeri e marre, e spiedi e spade/ si for-

in sette doppi unite,/ ricotte al foco e ribattute e salde,/ si

man dai bidenti e da le falci.”/

forma un saldo e smisurato scudo,/ da poter solo incontro a l'armi tutte/ star de' Latini. Il fremito del vento /che

Sfolgoranti descrizioni

spira da' gran mantici, e le strida/ che ne' laghi attuffati, e

Omero (IX sec. A.C.), nell’Iliade come nell’Odissea e pa-

ne l'incudi/ battuti, fanno i ferri, in un sol tuono/ ne l'antro

rimenti Virgilio, nell’Eneide, quasi gareggiando tra di loro,

uniti, di tenore in guisa /corrispondono a' colpi de' Ciclo-

ci offrono a profusione, “forgiando” indimenticabili versi,

pi,/ ch'al moto de le braccia or alte or basse/ con le tena-

una sfolgorante descrizione di metalli in varie forme e dal-

glie e co' martelli a tempo fan concerto, armonia, numero

le fogge e decorazioni le più diverse: armi, scudi, cocchi

e metro/”

divini, vasellame, suppellettili, abitazioni, strumenti musicali; per brevità ci si dovrà limitare solo ad alcuni rimandi: al lettore diligente la voglia ed il compito di dar seguito

Una profusione di oggetti metallici

a personali approfondimenti.

Poi in un crescendo di citazioni, sia in Omero che in Virgilio, appaiono magnifiche descrizioni di: cocchi divini, va-

Gli scudi di Achille e di Enea

sellame, suppellettili, abitazioni, strumenti musicali:

Di seguito sono riportati i versi che descrivono il lavoro di Efesto-Vulcano nell’atto di forgiare, su richiesta di Teti,

Iliade

la madre di Achille, il nuovo scudo del Pelide dopo che

Nel bel mezzo della battaglia tra Achei e Troiani, ecco in-

quello indossato in sua vece da Patroclo era stato preda di

tervenire in aiuto dei due schieramenti, alcune divinità ar-

Ettore a seguito dell’uccisione del fraterno amico.

mate di tutto punto (Iliade-Libro V):

”Eran venti che dentro la fornace/per venti bocche ne

“Immantinente al cocchio Ebe le curve/ruote innesta. Un

venìan soffiando,/e al fiato, che mettean dal cavo seno,/or

ventaglio apre ciascuna/d'otto raggi di bronzo, e si rivolve/

gagliardo or leggier, come il bisogno/chiedea dell'opra e

sovra l'asse di ferro. Il giro è tutto/d'incorruttibil oro, ma

di Vulcano il senno,/sibilando prendea spirto la fiamma./In

di bronzo/le salde lame de' lor cerchi estremi./Maraviglia

un commisti allor gittò nel fuoco/argento ed auro prezïoso

a veder! Son puro argento/i rotondi lor mozzi, e vergola-

e stagno/ed indomito rame. Indi sul toppo/locò la dura ri-

te/d'argento e d'ôr del cocchio anco le cinghie/con am-

sonante incude,/di pesante martello armò la dritta,/di ta-

bedue dell'orbe i semicerchi,/a cui sospese consegnar le

naglie la manca; e primamente/un saldo ei fece smisurato

guide./Si dispicca da questo e scorre avanti/pur d'argento

scudo/di dèdalo rilievo, e d'auro intorno/tre ben fulgidi

il timone, in cima a cui/Ebe attacca il bel giogo e le leg-

cerchi vi condusse,/poi d'argento al di fuor mise la soga./

giadre/pettiere; e queste parimenti e quello/d'auro sono

Cinque dell'ampio scudo eran le zone,/ (14 )

contesti. Desïosa/Giuno di zuffe e del rumor di guerra,/gli

Non da meno è l’abilità poetica di Virgilio, nell’VIII libro

alipedi veloci al giogo adduce./Né Minerva s'indugia. Ella

dell’Eneide, nel descrivere il lavoro dei Ciclopi, intenti nel-

diffuso/il suo peplo immortal sul pavimento/delle sale pa-

le nere fucine etnee del dio Vulcano, a forgiare , su richie-

terne, effigïato/peplo, stupendo di sua man lavoro,/e ve-

sta di Pallade-Atena, le armi di Enea:

stita di Giove la corazza/di tutto punto al lagrimoso ballo/

“Tosto che giunse: «Via, - disse a' Ciclopi -/ sgombrate-

armasi. Intorno agli omeri divini/pon la ricca di fiocchi Egi-

vi davanti ogni lavoro,/ e qui meco guarnir d'arme atten-

da orrenda,/che il Terror d'ogn'intorno incoronava/”

dete/ un gran campione. E s'unqua fu mestiero/ d'arte, di sperïenza e di prestezza,/ è questa volta. Or v'accingete

Odissea

a l'opra/ senz'altro indugio». E fu ciò detto a pena,/ che,

Oro, argento, rame: questa l’offerta, segno dell’opulenza

divise le veci e i magisteri,/ a fondere, a bollire, a martel-

delle case di Ilio, di un prigioniero troiano onde aver salva

lare/ chi qua chi là si diede. Il bronzo e l'oro /corrono a

la vita come descritto nel libro VII:

rivi; s'ammassiccia il ferro,/ si raffina l'acciaio; e tempre

“L'aggiungono anelanti i due guerrieri,/l'afferrano alle

La Metallurgia Italiana - May 2020

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Scenari - Miscellanea

mani, ed ei piangendo/grida: Salvate questa vita, ed io/ri-

mide coverto,/Sovra un distinto d'argentini chiovi/Seggio

scatterolla. Ho gran ricchezza in casa/d'oro, di rame e la-

a grand'arte fatto, e vago assai,/Mi pose: lo sgabello i piè

vorato ferro./Di questi il padre mio, se nelle navi/vivo mi

reggea/.E un'altra ninfa da bel vaso d'oro/Purissim'acqua

sappia degli Achei, faravvi/per la mia libertà dono infinito.”

nel bacil d'argento/“

Sempre nello stesso libro:“Palagio chiara, qual di sole

o luna,/Mandava luce. Dalla prima soglia Sino al fondo

Eneide

correan due di massiccio/Rame pareti risplendenti, e un

E non da meno, come descrizioni di opulenza e di splen-

fregioDi ceruleo metal girava intorno./Porte d'ôr tutte la

dori metallici, risultano questi vrsi tratti dal libro II dell’E-

inconcussa casaChiudean: s'ergean dal limitar di bronzo/

neide:

Saldi stìpiti argentei, ed un argenteo Sosteneano architra-

“Poscia che ciò come profeta disse,/ comandò come ami-

ve, e anello d'oro/Le porte ornava; d'ambo i lati a cui,Sta-

co ch'a le navi/ gli portassero i doni, opre e lavori/ ch'avea

van d'argento e d'ôr vigili cani:/Fattura di Vulcan, che in lor

d'oro e d'avorio apparecchiati/, e gran masse d'argento

ripose” … “Canto arricchillo. Il banditor nel mezzo/Sedia

e gran vaselli /di dodonèo metallo: una lorica/ di forbite

d'argento borchiettata a lui/Pose, e l'affisse ad una gran

azzimine; e rinterrate/ maglie, dentro d'acciaro e 'ntorno

colonna:/Poi la cetra vocale a un aureo chiodo/Gli appese

d'oro/, una targa, un cimiero, una celata,/ ond'era a pompa

sovra il capo, ed insegnagli/,Come a staccar con mano indi

ed a difesa armato/ Nëottòlemo altero”.

l'avesse.”

Ecco, nel libro X dello stesso poema, la munificenza di oro, argento, bronzo, che arreda le maritali stanze della maga Circe dove Ulisse riprende le vigorose forze:

“Bei tappeti di porpora, cui sotto/Bei tappeti mettea di bianco lino:/L'altra mense d'argento innanzi ai seggi/Spiegava, e d'oro v'imponea canestri:/Mescea la terza nell'argentee brocche/Soavissimi vini, e d'auree tazze/Coprìa le mense: ma la quarta il fresco/Fonte recava, e raccendea gran fuoco/Sotto il vasto treppié, che l'onda cape./Già fervea questa nel cavato bronzo,/E me la ninfa guidò al bagno, e l'onda/Pel capo mollemente e per le spalle/Spargermi non cessò, ch'io mi sentii/Di vigor nuovo rifiorir le

CONCLUSIONI La letteratura della classicità greco-latina, come messo in evidenza, offre un immenso tesoro di riferimenti alla metallurgia ed alla lavorazione dei metalli: metalli come simbolismo tra dei, miti e leggende, suggestivi versi sull’origine della metallurgia, sull’impiego e l’uso dei metalli, le loro proprietà, l’usura, la corrosione, il riciclaggio, nonché sfolgoranti descrizioni e una profusione di oggetti metallici. Ai poemi cavallereschi della letteratura italiana, se ci sarà, il prossimo appuntamento a cominciare dal Tasso e dall’Ariosto.

membra./Lavato ed unto di licor d'oliva,/E di tunica e cla-

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Experts’ Corner - Miscellaneous BIBLIOGRAFIA [1]

G. Casarini :” Riferimenti ad arti e mestieri alchemici metallurgici nella Divina Commedia: Fabbri e Ferraioli”-28° Convegno Nazionale A.I.M.-Milano Novembre 2000-Atti-Vol.2-pagg 635-541

[2]

G.Casarini:” Metallurgia e scienza nei gironi danteschi”- Civiltà degli Inossidabili - Ediz. Trafilerie Bedini-Dic.1992

[3]

G. Casarini:” Dante Alighieri e la Metallurgia”- Pianeta Inossidabili-Ediz. Acciaierie Valbruna-Giu.1995

[4]

G. Cozzo:” Le origini della metallurgia-I metalli e gli dei”-Editore G.Biardi-1945 Roma

[5]

E. Crivelli:” La metallurgia degli antichi”-Supplem. Ann. Enciclopedia della Chimica-Unione Tipografica Editrice- 1913 Torino

[6]

I. Guareschi :”Storia della Chimica-I colori degli antichi”- ”-Supplem. Ann. Enciclopedia della Chimica-Unione Tipografica Editrice- 1905 Torino

[7]

A. Uccelli-G.Somigli:”Dall’alchimia alla chimica-Storia della Metallurgia e delle lavorazioni meccaniche nel medio-evo”-Enciclopedia storica delle scienze e loro applicazioni”-U. Hoepli Editore-Milano

[8]

Esiodo: “ Le opere e i giorni”-Trad. G. Arrighetti-Ediz.Garzanti-1985

[9]

Ovidio:” Metamorfosi”-Ediz.varie

[10]

Tibullo: “Elegie”_Ediz.varie

[11] Virgilio:”Eneide”-Trad.A.Caro-Ediz.varie [12]

Virgilio: “Bucoliche”-Trad. L.Canali-Fabbri Editori

[13]

Lucrezio: “De Rerum Natura”

[14]

Omero: “Iliade”-Trad. V.Monti-Ediz.varie

[15]

Omero: Odissea”-Trad.I.Pindemonte-Ediz.varie

[16]

T. Tasso: “ La Gerusaleme Liberata”-Ediz. varie

[17]

L. Ariosto: “ Orlando Furioso”-Ediz.varie

La Metallurgia Italiana - May 2020

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Pubbliredazionale

BERGAMO . ITALY . 3-4 DECEMBER 2020

international

meeting

RAW

MATERIALS & RECYCLING

circular economy

sustainability metallurgical industry SCOPE

TOPICS

The environmental and economical sustainability of the metallurgical industry depends on the possibility to recover and recycle the metal products after their use. On the other hand, a new challenge is a zero-waste metallurgical process.

Such an approach has to be faced from perspective involving commercial, economic and technical issues. Although recycling is an intrinsic feature of all the metallurgical processes, an efficient circular approach needs to take into account a correct selection and cleaning of the metallic and glassy-ceramic materials in order to allow their use in the process avoiding or limiting the exploitation and the consumption of the natural resources.

Such a view imposes a multidisciplinary approach where the application of complex mechanical system, innovative chemical plants, electromagnetic devices, biotechnological techniques and managing algorithm even ruled by artificial intelligence have to be applied.

An efficient action inspired to the circular economy has to take into account a comprehensive vision where the recycling is associated to an efficient material and energy exploitation that implies an overall decrease of the polluting elements and greenhouse gases.

Complete informartion and forms are available at: www.aimnet.it/rawmat.htm

The Meeting will be performed through invited presentations and on the basis of the selection of the abstracts sent by authors and companies interested in sharing their updated knowledge on the following topics.

The deadline for the abstract undergoing the selection is on June 5, 2020, in order to fulfill the final program.

• • • • • • • • • • • • • • •

Scrap Scrap conditioning and management Recycling Tramp elements Slag recycling Slag valorization Recovery of valuable elements from wastes Recovery and recycling of the refractories Management of the oxide scales Treatment of dust Sludges Biometallurgy Biotechnological system for wate treatment Heat harvesting for material recovery CO2 capture and use

Organised by

In co-operation with

Patronized by

La Metallurgia Italiana - maggio 2020

pagina 62

Atti e notizie - AIM news

Eventi AIM / AIM events FaReTra (Fair Remote Training) - FORMAZIONE A DISTANZA ON DEMAND - lezioni registrate Corso ACCIAI AD ALTO CARBONIO Giornata di Studio TECNOLOGIA ED INNOVAZIONE NEI FORNI AD ARCO SIDERURGIA IN PILLOLE FAILURE ANALYSIS IN PILLOLE – percorso di avvicinamento alla Failure Analysis

www.aimnet.it

TECNOLOGIE ADDITIVE IN PILLOLE TECNOLOGIE PRESS & SINTER NON CONVENZIONALI

Per ulteriori informazioni rivolgersi alla Segreteria AIM, e-mail: info@aimnet.it, oppure visitare il sito internet www.aimnet.it

WEBINAR Giornata di Studio CORROSIONE SOTTO SFORZO E INFRAGILIMENTO DA IDROGENO NELLE APPLICAZIONI PETROLCHIMICHE > 26 giugno 2020 Giornata di Studio RIVESTIMENTI DECORATIVI AL SERVIZIO DELL'ESTETICA DEL PRODOTTO Corso METALLURGIA DI BASE PROPEDEUTICO AI TRATTAMENTI TERMICI > ottobre FORMAZIONE E AGGIORNAMENTO IN AULA (*) Giornata di Studio UTILIZZO LEGHE DI NICHEL IN SALDATURA > Milano, 7 ottobre Corso ANALISI CHIMICHE > Milano, 14-15 ottobre Giornata di Studio STAMPAGGIO: PROPRIETÀ MECCANICHE, TRATTAMENTO TERMICO E MECCANISMO DI DANNEGGIAMENTI > Ivrea c/o Confindustria Canavese, 22ottobre Corso GLI ACCIAI INOSSIDABILI > Milano, 28-29 ottobre e 4-5-10-11-18-19 novembre

L’elenco completo delle iniziative è disponibile sul sito: www.aimnet.it

(*) In caso non sia possibile svolgere la manifestazione in presenza, la stessa verrà erogata a distanza in modalità webinar

La Metallurgia Italiana - May 2020

pagina 63

save the date

High Tech Die Casting INTERNATIONAL C O N F E R E N C E VICENZA - ITALY 23-25 JUNE 2021

Organised by

Patronised by

will be held once in Vicenza, where the 1st HTDC Conference took place in 2002. Through the years, the HTDC Conference has grown and become a key-event for scientists and researchers from the foundry field of Aluminium, Magnesium and other non-ferrous alloys as well as for experts from the industry. The HTDC Conference series has been steadily growing in quality, impact and participation in the following conferences, organised in 2004, 2006, 2008, 2012 and 2016.

The HTDC Conference is the meeting point for experts involved in the research activities and development of foundry technologies, processes or materials, as well as those employed by the foundry companies of light and non-ferrous alloys, and who are prepared to share some of their current experiences.

Today, Die Casting production is facing new challenges: from eco-sustainability and efficient energy usage to optimisation of product properties and finishing, from tailored alloys’ properties to design of lightweight larger structural components. Only an “open-minded” approach, resulting in a high capability of being innovation-driven, integration-oriented and implementation-ready, will make Die Casting foundries successful in an international market dominated by strong competition.

To sustain the competitiveness in Die Casting production, HTDC Conference will cover these key features: • innovations, from alloys to processes, from design to applications, which are available to foundries, suppliers, end-users; • multi-disciplinary integration of concepts, methods and processes; • implementation, into the industrial context, of new and modern solutions for quality, efficiency, high performance.

HTDC Conference again

Exhibition & Sponsorship Sponsored by

The HTDC Organising Committee has prepared a selection of sponsorship opportunities designed to increase the visibility and to optimize the return of investment for sponsors participating in the HTDC Conference. Complete informartion and forms are available at: www.aimnet.it/htdc.htm

Conference organisers Associazione Italiana di Metallurgia Via Filippo Turati 8 - 20121 Milano, Italy E-mail: info@aimnet.it www.aimnet.it/htdc.htm

Atti e notizie - AIM news

Comitati tecnici / Study groups CT CORROSIONE (C) (riunione 14 febbraio 2020)

Iniziative future • Per la prossima edizione delle “Giornate Nazionali sulla Corrosione e Protezione” (Torino, giugno-luglio 2021) si discute della possibile collaborazione con altre associazioni e dei possibili patrocini. Viene nominato il comitato scientifico, mentre la Segreteria provvederà a interpellare sia le persone che sono ancora nel CT Corrosione ma non partecipano da tempo che altri gruppi di ricerca in differenti sedi universitarie che si occupano di corrosione.

• GdS “Corrosione sotto sforzo e infragilimento da idrogeno nelle applicazioni petrolchimiche”: questa manifestazione sarà organizzata in collaborazione con NACE Italia ed è già pronta una bozza di programma. La data, inizialmente ipotizzata per giugno 2020, sarà ridiscussa in funzione delle possibilità di organizzare riunioni.

• Il CT “Controllo e caratterizzazione dei prodotti” aveva avanzato la richiesta di co-organizzare una giornata sul tema “Corrosione per non corrosionisti”. La proposta sarà valutata con maggiore attenzione da un comitato ristretto.

Stato dell’arte e notizie • È stato invitato alla riunione un nuovo membro, poi ammesso al CT. • È stata confermata la volontà di dare un premio intitolato al prof. Francesco Mazza, da consegnare durante le Giornate Nazionali della Corrosione. L’ACI Italian Chapter ha assegnato un premio alla memoria del prof. Luca Bertolini, con il patrocinio di AIM.

CT METALLURGIA FISICA E SCIENZA DEI MATERIALI (MFM) (riunione telematica del 27 febbraio 2020)

Manifestazioni in corso di organizzazione • Dopo lunga discussione, il CT decide di organizzare una GdS dal titolo: “Resistenza a fatica dei materiali metallici: aspetti metallurgici ed approcci innovativi”; il previsto rifacimento del corso “Tenacità a fatica dei materiali metallici” viene così rimandato al 2021. La GdS viene fissata per il giorno 24 novembre 2020 a Milano. Il coordinatore Donnini preparerà a breve la locandina sulla base degli interventi già pianificati. Lo scopo è quello di approfondire le tematiche più critiche sull’argomento e aggiornare la situazione circa gli approcci sperimentali e di processo di più recente sviluppo. Nella mattinata saranno discussi gli aspetti ambientali e i fattori tecnologici determinanti per la valutazione e il miglioramento della resistenza a fatica di un materiale metallico.

Nel pomeriggio si discuterà delle più rilevanti caratteristiche, opportunità e criticità di alcune leghe metalliche particolarmente utilizzate in ambito industriale, sottolineandone le funzionalità più utili ed innovative.

Iniziative future • Il coordinatore Richetta aggiorna il CT circa l’organizzazione della GdS “Ingegnerizzazione delle superfici: design e caratterizzazione” in programma per fine giugno. Il presidente Cerri chiede di definire meglio i topics dell’evento perché le partecipazioni possano essere mirate al meglio. Bassani raccomanda di evitare sovrapposizioni di argomenti con GdS organizzate da altri CT.

CT METALLURGIA DELLE POLVERI E TECNOLOGIE ADDITIVE (MP) (riunione telematica del 01 aprile 2020)

Manifestazioni in corso di organizzazione • La GdS “Leghe per alte temperatura prodotte con tecnologia additive”, inizialmente programmata per il 17 marzo a Firenze, è stata spostata al 9 giugno, ma anche questa data sembra difficile da rispettare per via dell’inevitabile affollamento di persone che si verificherebbe. D’altra parte, il CT valuta un notevole punto di forza lo svolgimento di questa ed altre manifestazioni presso

aziende disposte ad ospitare l’evento. I coordinatori Biffi, Casati, Molinari, Ruggiero, insieme alla Segreteria AIM, verificheranno la possibilità di spostare la Giornata di Studio in autunno.

Iniziative future • Il Seminario in due giornate “Alfabetizzazione Additive e TT post-Additive”, in collaborazione con il CT Trattamenti Termici e Me-

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Atti e notizie - AIM news

Comitati tecnici / Study groups

•

tallografia” ha per coordinatori Vicario (da parte del CT MP) e Pellizzari (da parte del CT TTM). I rappresentanti delle ditte ospitanti concordano sull’opportunità di spostare l’evento da luglio 2020 a gennaio 2021: il programma dettagliato sarà quindi discusso nelle prossime riunioni.

La GdS “Materiali innovativi per additive manufacturing” si potrà tenere presso CRF a Mirafiori spostando la data alla primavera 2021. I coordinatori Biffi e Casati sono incaricati di definire i dettagli del programma.

Stato dell’arte e notizie • Il CT approva l’ingresso di quattro nuovi membri, tutti provenienti dal mondo industriale.

CT METALLI E TECNOLOGIE APPLICATIVE (MTA) (riunione telematica del 21 aprile 2020)

Iniziative future • La GdS “Utilizzo leghe di nichel in saldatura” – coordinatori Colombari e Rossetti – è prevista per il 7 ottobre a Milano. L’organizzazione è a buon punto, ma la conferma della fattibilità della giornata si potrà dare solo dopo la prossima riunione del CT a giugno.

• La GdS “Trattamenti per la rimozione del piombo per il riciclo degli ottoni” - coordinatore Loconsolo – è anch’essa a buon punto con la preparazione. La data resta fissata per il 4 novembre 2020, salvo conferma. La sede potrebbe essere Brescia, come inizialmente ipotizzato, ma sono aperte altre possibilità per diverse location.

Stato dell’arte e notizie • Durante la riunione è stata rinnovata la carica di presidente: al posto di Ezio Debernardi è stato eletto all’unanimità Vincenzo Loconsolo. Il mandato per la carica di segretario è stato riconfermato a Riccardo Gerosa, mentre il vicepresidente sarà eletto nella prossima riunione.

• Un nuovo membro ha chiesto di partecipare al CT MTA ed è stato accettato.

CT CONTROLLO E CARATTERIZZAZIONE PRODOTTI (CCP) (riunione telematica del 29 aprile 2020)

Manifestazioni in corso di organizzazione • Il corso “Failure Analysis” (coordinatori Ferrari, Bisaglia, Rolli, Donnini), già previsto a Milano per maggio 2020, è stato rimandato ai primi mesi del 2021. Nel frattempo, per una infarinatura di base sulle tematiche della failure analysis, è stato organizzato un corso da fruire esclusivamente tramite lezioni registrate. Questo corso si chiama “Failure Analysis in pillole – percorso di avvicinamento alla failure analysis” ed è proposto in 4 moduli di lezioni registrate da 15 minuti aventi per tema gli argomenti principali della failure. La locandina è già disponibile sul sito AIM e al momento ci sono 31 richieste di adesione. Al termine di ogni modulo ci sarà un questionario per valutare l’apprendimento dei singoli partecipanti. Probabilmente, in base alle richieste dei partecipanti, si potrà fissare un incontro telematico con i singoli docenti per discussioni ed approfondimenti.

• Il corso “Analisi chimiche” si svolgerà a Milano il 14 e 15 ottobre. Bassani mostra la locandina preparata dal coordinatore del corso Stella, che viene discussa ed approvata dai presenti.

Iniziative future • La GdS “Corrosione per non corrosionisti” – coordinatore Farina - sarà organizzata solo da CT CCP. La bozza di scaletta sarà fatta circolare tra un gruppo ristretto di membri per arrivare alla prossima riunione con una proposta condivisa.

• Il corso “Prove Meccaniche” si terrà nell’autunno 2021 con una formula rinnovata: due giornate per le prove di base e due giornate per le prove meno comuni e più complesse. Viene selezionato un comitato ristretto per definire il programma e i docenti.

• Viene proposta una manifestazione, molto probabilmente una GdS, dal titolo: “Caratterizzazione dei materiali da Additive Manufacturing”, da organizzare con la collaborazione dei CT Metallurgia delle Polveri e tecnologie additive” e CT “Metallurgia fisica”.

Il focus degli interventi dovrebbe essere principalmente centrato sulla caratterizzazione delle polveri.

Stato dell’arte e notizie • Il CT approva l’ingresso di un nuovo membro, che in passato ha già collaborato in lezioni del corso di Prove Meccaniche.

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Atti e notizie - AIM news

Comitati tecnici / Study groups CT TRATTAMENTI TERMICI E METALLOGRAFIA (TTM) (riunione telematica del 06 maggio 2020)

Manifestazioni in corso di organizzazione • La GdS “Il Mondo Industrial – aspetti metallurgici e metodologie di controllo” presso CNH Industrial Village a Torino è stata rimandata: il coordinatore Massa conferma il programma ma fa presente che la capienza delle sale e del museo CNH Industrial sarà ridotta a causa delle restrizioni sanitarie, e che bisogna trovare un’altra data accettabile per l’azienda ospitante. Anche un paio di relatori provenienti dall’estero potrebbero avere limitazioni. Probabilmente un rinvio a inizio 2021 ci eviterebbe questi problemi, sperando di essere tornati alla normalità per allora.

• Il “27° Convegno Nazionale Trattamenti Termici 2020” è stato rimandato al 18—19 novembre, sempre nella stessa sede di Genova – Magazzini del Cotone – Porto Antico. Ora bisogna attendere l’evoluzione dei contagi e delle restrizioni per confermare definitivamente queste date e riuscire ad organizzare un Convegno sentito dal pubblico.

•

•

•

La GdS “Stampaggio: proprietà dell’acciaio, trattamento termico e meccanismo di danneggiamento”, prevista presso la sede di Confindustria Canavese, sarà tenuta in autunno. Bassani suggerisce di tenere la giornata in modalità remota, dal momento che non ci sono visite a impianti. In tal caso la giornata si potrebbe separare in 2-3 differenti sessioni, per evitare ai partecipanti una giornata intera davanti al PC. Petta è favorevole perché in questo modo si riesce a dare continuità all’azione formativa AIM. La terza edizione del corso “Metallurgia di base propedeutico ai trattamenti termici” è previsto nel 2020 per la normale alternanza con i corsi di trattamenti termici e di metallografia. A causa della difficoltà di organizzare questo corso in presenza entro il 2020, si valuta l’opzione del corso a distanza: di discute della possibile durata delle lezioni, e del corso stesso che dovrebbe essere spalmato su più giornate non potendo tenere l’audience al computer per più di 2 ore o 2 ore e mezza. Si deve prevedere uno spazio per domande e chiarimenti, in cui ogni singolo docente abbia tempo di interfacciarsi con gli studenti evitando di spezzettare la lezione. I docenti saranno interpellati per verificare la loro disponibilità a rimodellare le lezioni.

Il seminario congiunto con il CT “Metallurgia delle polveri e additive manufacturing” è stato spostato a gennaio 2021 per poter fare la manifestazione di presenza: sono infatti previste visite alle due aziende ospitanti.

Iniziative future • Vista la situazione di incertezza, le manifestazioni del 2021 saranno definite nelle prossime riunioni. Si è già discusso di una GdS “Trattamenti termici e modellazione” – coordinatore Molinari – e di una GdS presso Getrag nella zona di Bari.

Stato dell’arte e notizie • Viene presentato ed accettato un nuovo membro del CT, proveniente dall’area industriale.

CT AIM / ASSOFOND – FONDERIA (F) (riunione telematica del 11 maggio 2020)

Manifestazioni in corso di organizzazione • 35° Congresso di Fonderia: il presidente Caironi segnala che è stato deciso di procedere con l’organizzazione per mantenere le date previste del 12 e 13 novembre 2020 a S. Eufemia (BS). Per la raccolta di memorie, che attualmente non sono ancora arrivate, si procederà con una ulteriore richiesta. Tutti sono invitati a contattare altri possibili relatori.

Stato dell’arte e notizie • I premi di studio Assofond saranno estesi agli studenti con lauree triennali, valutando la modalità di consegna del premio durante il Congresso.

• “Guida per i requisiti di qualità e di controllo dei getti di acciaio, ghisa e alluminio”: i membri del CT hanno ricevuto copia di una analoga guida realizzata nel 2003 da CAEF. Al momento CAEF non ha espresso intenzione di aggiornare od estendere questo documento, ma il CT ritiene che il lavoro possa aiutare nella regolarizzazione dei rapporti con le committenze e lo sviluppo di mercati basati sulla qualità dei getti, in modo da superare la concorrenza dei paesi che producono getti di scarsa qualità e a basso costo. La guida potrebbe essere indirizzata ai progettisti, con indicazioni sulla provenienza dei materiali utilizzati, la determinazione delle discontinuità ritenute accettabili, i mezzi adeguati alla loro determinazione ed un’indicizzazione sull’incremento dei costi per l’esecuzione di tali verifiche. A fine lavori si cercherà di dare ampia visibilità alla guida con l’organizzazione di corsi o giornate di formazione

La Metallurgia Italiana - May 2020

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10

c c

10th european conference on continuous casting 2020

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21-23 October 2020 Bari . Italy

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www.aimnet.it/eccc2020

member of MEMBER OF

c c Organised by

We would like to inform you that we agreed with Siderweb to organise ECCC 2020 in the new date of: 21-23 October 2020. The conference venue remains unchanged: The Nicolaus Hotel in Bari.

In case it wonâ&#x20AC;&#x2122;t be possible to guarantee the health and safety of our guests, our highest priority, we will decide and promptly communicate a new postponement.

We thank you for your understanding and we are looking forward to a successful ECCC Conference!

The ECCC is a unique forum for the European continuous casting community to exchange views on the status and the future development of the continuous casting process. The Conference program is abreast of the latest developments in control and automation, advanced continuous casting technologies, application of electromagnetic technologies and mechanical devices to improve the core microstructure, the lubrication issues for improving the surface qualities.

Steel metallurgical issues will be addressed as well as their physical and numerical simulation. The exchange of experience in operational practice, maintenance and first results from the recently commissioned plants will integrate the program. The Conference aims at promoting the dialogue among the delegates with industrial and academic background and among the participants in former Conferences and new members of the continuous casting community.

Programme and Registration The advance programme and the Conference registration fees are available on the Conference website: www.aimnet.it/eccc2020/

Exhibition and Sponsoring As an integral element of the event, a technical exhibition will be held during the event.

Companies have the opportunity to reinforce their participation and enhance their corporate identification by taking advantage of the benefits offered to them as sponsors of the event.

The detailed sponsorship packages are available on the Conference website: www.aimnet.it/eccc2020 Companies interested in taking part in the Exhibition or sponsoring the event may contact: e-mail: commerciale@siderweb.com THE ITALIAN STEEL COMMUNITY tel. +39 030 2540006

siderweb

Contacts ECCC 2020 Organising Secretariat AIM - Associazione Italiana di Metallurgia Via Filippo Turati 8, 20121 Milan - Italy aim@aimnet.it - www.aimnet.it/eccc2020