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Failure Analysis of Billet Caster Tundish Tilter Bolts

J N Mohapatra1, Rama Rao2,

Maruti Prasad

3 and D.

Satish Kumar4

1Seniror Manager, R&D and SS, JSW Steel Ltd., Toranagallu, Bellary 583275 Karnataka, India

2Junior Manager, R&D and SS, JSW Steel Ltd., Toranagallu, Bellary 583275 Karnataka, India

3Deputy General Manager, SMS-II, JSW Steel Ltd., Toranagallu, Bellary 583275 Karnataka, India

4 General Manager, R&D and SS, JSW Steel Ltd., Toranagallu, Bellary 583275 Karnataka, India ***

Abstract - Adetailedfailureanalysiswascarriedoutonprematurelyfailed60mmdiameterboltsofbilletcastertundishtilter. The analysis involves visual examination, chemical composition, microstructure, inclusion, bulk hardness, charpy impact toughness,fractographyofthefailedandnewbolt. Theresultshowthattheboltswereof34CrNiMo6gradewithtempered martensiticmicrostructure,havingMnSandaluminainclusions.Alltheboltshasfailedatthreadgrooveswiththerivermark on the failed surface indicating fast fracture of the bolts. The bolts were found to be heat treated at a wrong tempering temperatureleadingtopoorimpacttoughnessandhencewhiletilting;thebotswerefailedprematurelyduetoimpactload fromthejerk.Amixmodeoffailurewithnumeruscracksareseenonthefracturesurfaceindicatingpoortoughnessofthebolts conformedbycharpytest.Itisrecommendedtocorrecttheheattreatmentasperthespecificationtopreventsuchpremature failures.

Key Words: Tundish tilter bolts, wrong tempering temperature, high hardness, poor impact toughness, premature failure

1. INTRODUCTION

Therearemanyreasonsforwhichboltsandfastenerfail suchasoverloadingfailureduetoexcessloading,fatiguefailure resultingfrompresenceoflargeinclusions,dents,corrosionpits,highroughness,highcyclicload,improperalignment,loose connection,corrosion fatigue, embrittlementsuchastemper martensite embrittlement,temper embrittlement,hydrogen embrittlement,stresscorrosioncrackingetc.[1-8].

34CrNiMo6steelisamedium-carbonlow-alloyhigh-strengthquenchedandtemperedsteelwidelyusedinvariousindustrial fields. The steels are having excellent fatigue and wear properties in addition to high strength, high toughness and good hardenability[9].Itisalsoreportedthatthefastenersbeingstrongerarenotnecessarilybebetterashydrogencanhaveinthe highstrengthfastenerstogivetheadverseeffect[10].

Thespecificationfor34CrNiMo6steelisforgingorhotrollinginthetemperaturerangeof1100to850oC,normalisinginthe temperaturerangeof850to880oCfollowedbyaircooling,softannealinginthetemperatureof650to680oCfollowedby furnacecooling,hardeninginthetemperaturerangeof830-860oCfollowedbyoilquench,temperinginthetemperaturerange of540to660oCfollowedbyaircooling.Withthequenchandtemperedconditionforthethicknessrangeof40to100mm diameter, thepropertiesshouldbe minimumof 800MPa yieldstrength,1000to1200MPaultimatetensilestrength,11% elongationwith50%reductioninareaandnotchimpactenergyofminimum45Joule[11,12].Inthepresentinvestigation, 60mmdiameter34CrNiMo6boltsoftundishtilterstandwereanalyzedtofindouttherootcauseoffailure.

2. HISTORY OF FAILURE

ThebilletcasteratSMS-IIofJSWVijayanagarWorks,haveapproximateweightofemptytundish35ton(Skull7-10ton),with skullweightoftundish45tonandinemergencyconditiontundishweight70ton.Thetundishtilterboltsof60mmdiameter weregettingrepeatedfailureinapproximate6monthofservice.Theboltsexperiencetemperatureintherangeof100-150oC inservice.Inthepresentcase,allthefourboltswerefailedwhiletiltingleadingtofallofmaterial.Itissuspectedthatduetounbalancingofthetiltertheaccidentoccurred.Adetailedfailureanalysishasbeencarriedouttoinvestigatetherootcauseofthe boltfailure.Digitalimageforthevisualexamination,dimensionalmeasurement,SPECTROmakeOESforcomposition,optical microscopyusinganOlympusmakeopto-digitalmicroscope,Hitachimakescanningelectronmicroscope(SEM),Bulkhardness (Brinellhardness)measurementthroughanWelspunmakeUniversalhardnesstesterandZwick/Roellmakeimpacttester wereusedforthefailureinvestigationtosortouttherootcauseoffailureofthebolts.

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056 Volume: 10 Issue: 08 | Aug 2023 www.irjet.net p-ISSN: 2395-0072 © 2023, IRJET | Impact Factor value: 8.226 | ISO 9001:2008 Certified Journal | Page334

3. CHARACTERIZATION OF FAILED BOLTS

3.1 Visual Examination

Fig.1showsdigitalimageof(a)emptytundish,(b)skullfromthetundish(c)tundishtilter(d)

(f)boltsontundishtilterwhich werebeingfailed.Fig.2(a)-(d)showscloseviewoftheboltswiththeirfracturesurface.Itcanbeseenthattheboltsarefailed atthethreadregionswherethestressconcentrationissupposedtobehigh.Therivermarksonthefracturesurfaceindicates fastfractureofthebolts[13].

3.2 Chemical Composition

The chemical composition of a typical failed bolt was carried out using SPECTRO make OES as shown in Table-1. The composition matched with the specification of 34CrNiMo6 steel grade. This is a high alloy steel with through hardening characteristicswithultra-highstrengthrange.Thesteelhas0.017%Aland0.06%S,whichmeansitisanAldeoxidizedgrade thatisre-sulfurizedtoimprovethemachinabilityofthebolt.Thesefastenersmaintainstrength1000to1300MParetaining hightoughnessandfatiguelife.Theboltalsogetsheatedwhentundishisfilledwithmoltenmetal,ahighalloysteelwithstands temperaturepreventingthemicrostructuraldeteriorationinsuchenvironment.

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Fig-1:(a)EmptyTundish(b)Skull(c)TundishTilter(d)-(f)TundishTilterBolts Fig-2: (a)Failedtundishtilterbolts,(b)–(d)fracturesurfaceofthethreetundishtilterbolts.

3.3 Inclusion Analysis

TheopticalmicrostructureofthesteelshowinclusionasshowninFig.3(a)and(b)obtainedfromnewboltandfailedbolt respectivelytoevaluatetheworstfieldinclusionratingsasperASTME-45asinTable2.Theinclusionratinginthefailedand thenewlymadeboltisgiven.Thebolthashighlevelofinclusion,butinservicethehighlevelofinclusionhaspotential to initiatefatiguecracks.

3.4 Microstructure

The optical microstructure in Fig. 4 (a) and (b) shows the new bolts at the edge and center region whereas (c) & (d) correspondstothemicrostructureofthefailedbolts.Themicrostructureofthenewandfailedboltswerefoundtobetempered martensiticinnature.Nosignificantvariationinmicrostructurewasseenfromthesurfacetothecenteroftheboltsindicating nosurfacehardeningtreatmentismadeonthebolts.

TheSEMmicrostructureofthenewboltattheedgeregionatlowandhighmagnificationisshowninFig.5(a)&(b)andthatof thefailedboltin Fig. 5 (c)&(d)respectively.ThecoremicrostructureofthenewboltsisshowninFig.6(a)&(b)andthatof thefailedboltsisshowninFig.6(c)&(d)respectively.Thetemperedmartensiticstructureisseeninthemicrographsofboth the new and old bolts. However, the martensitic laths found in the microstructure of the new bolts indicating its low temperaturetempering.

Sample C Mn S P Si Al Cr Ni Cu Nb V Ti Mo B Sn 34CrNiMo6 0.300.38 0.500.58 0.035 0.025 0.100.40 1.301.70 1.301.70 0.40 max 0.150.30 NewBolt 0.352 0.60 0.053 0.045 0.272 1.155 1.455 0.140 0.005 0.015 0.002 0.215 0.0015 0.005 FailedBolt 0.355 0.61 0.056 0.042 0.276 0.017 1.151 1.450 0.139 0.007 0.015 0.002 0.212 0.0014 0.005

Table-1:Chemicalcompositionoftheboltscomparedwiththespecificationof34CrNiMo6

Bolts A-SulfideType B-AluminaType C-SilicaType D-GlobularOxideType Thin Thick Thin Thick Thin Thick Thin Thick New 2.0 - 1.5 - - - 1.5Old 3.0 - 1.5 - - - 1.5 -

Table-2: InclusionAnalysis–WorstFieldMethod(ASTME-45). Fig-3: Opticalimageoftheinclusioninnewbolt(a)andfailedbolt(b).

Fig-4: Opticalmicrostructure(a)edgeand(b)centerofthenewboltwith(c)and(d)aretheircorresponding microstructureofthefailedbolt.

NumerousinclusionsareseenintheSEMmicrographasshowninFig.7(a).TheEDSanalysiswasdoneontheinclusion1and2 andonthematrix3shownin(b)withthespectrumoftheEDSin(c).Thechemicalcompositionoftheinclusionsalongwiththe matrixisshowninTable-3,showsthattheinclusionsasMnS.Suchlargeinclusionsandtheirpopulationsarenotdesirablefor dynamicapplicationofanyengineeringcomponents. Inclusionsarethepointsofstressconcentrationregionsforfatiguecrack initiation,whichpropagatesforfinalfractureoftheengineeringcomponents.

Table-3: SEM-EDSoftheinclusions(1&2oninclusionsand3onthematrix)

Points O Na Mg Al Si P S Mn Cu Fe 1 0.11 0.25 0.22 0.15 0.13 0.10 32.50 60.35 1.43 4.77 2 0.23 0.28 0.24 0.16 0.15 0.14 32.42 60.27 1.52 4.59 3 0.64 0.73 0.26 0.18 0.52 0.06 0.28 0 1.85 95.47

Fig-5:SEMmicrograph(a)&(b)lowandhighmagnificationofthenewboltattheedgeregionwith(c)&(d) correspondingmicrographofthefailedbolt. Fig-6:SEMmicrograph(a)&(b)lowandhighmagnificationofthenewboltatthecenterregionwith (c)&(d) correspondingmicrographsofthefailedbolt.

3.5 Bulk Hardness and Impact Toughness

Theaveragebulkhardness(Brinellhardness)wasfoundtobe370BHNforthefailedboltand380BHNforthenewbolt.The hardnessandimpacttoughnessdatawithtemperingtemperaturewerefoundfromtheliteratureforthe34CrNiMo6gradeand plotted in Fig. 8. It can be seen that the hardness for both the new and failed bolts are corresponding to the tempering temperaturearound500oCwhichcorrespondstoanimpactenergyof45Joule.Hence,toconformtheexactvalueofimpact toughnessstandardsamples ofCharpyimpactspecimen weremadeandtestedina Zwick/Roell makeimpacttester.The resultswerefoundtobe19Joule,correspondingto425oCtemperingtemperature.Hence,itisinferredthatthelowimpact energyobtainedinthebolts,duetolowtemperaturetempering.

Fig-7:(a)SEMmicrographofinclusions(b)SEM-EDSoftheinclusions1&2with3atmatrix(c)SEM-EDSspectrum conforming MnSinclusion. Fig- 8:Bulkhardnessandimpactenergyofthe34NiCrMo6gradewithtestedvalueoffailedbolts.

4. FRACTOGRAPHY

SEMFractographofthebolt-1andbolt-2atvariousmagnificationsareshownin Fig. 9 (a)-(c)and(d)-(f)respectively.The radiatingcracksareindicativeoffatiguefailureofthebolts.Therivermarksareindicativeoflowcyclefatigueoftheboltsalso failureinimpactload.Thecracksinthesamplesindicatespoortoughnessofthesteel.Thefractographyofthefailedboltafter impacttestisshownin Fig. 10.Mixedmodeoffailurewithbothcleavagesanddimplesareseeninthefracturesurfaceofthe bolt.

Fig-9: (a)-(c)lowtohighmagnificationfractographicimageofthebolt-1with(d)-(f)correspondingmicrographsofthe bolt-2. Fig-10: Fractographyoftheboltafterimpacttest.

5. CONCLUSION

The root cause of failure is selection of wrong tempering temperature (425oC) instead of following the actual tempering temperature (540-660oC according to specification) leading to poor impact toughness of the bolts Stress concentrations increasedatthethreadgroovesduetoheavyload,duetojerktheboltsfailedduetopoorimpacttoughness.Therivermarkson thefracturesurfaceoftheboltsareindicativeoffastfractureoftheboltsduetotheimpactload/jerkload.

6. REMEDIAL ACTION

Poortoughnessofthebolts(19Joule)duetolowtemperaturetemperingisthemajorcauseoffailureoftheboltsandhenceitis suggested to temper the steel as per specification of 540 to 660oC to get very good impact toughness in the range of 70110Jouleorminimumof45Joule.Thematerialsurfacehardeningcanfurtherimprovethefatiguelifeofthebolts.Useofclean steelcangivebetterlifeofthesteelindynamicloadingconditionbyminimizingthestressconcentrationfrominclusions As theboltsareexperiencinghightemperature(100-200oC)thereisachanceoflooseningoftheboltsandhenceneedtobe tighteningperiodicallytoavoidfatiguefailureofthebolts.

Acknowledgement

TheauthorswouldliketoacknowledgetheSMS-IIdepartmentmaintenanceandR&Dcharacterizationteamfortheirsupport tocarryoutthefailureanalysis.

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BIOGRAPHIES

Dr JNMohapatraiscurrentlyworkingasaSeniorManageratR&DandSSofJSWSteelLtd.Heishaving 19 year of R&D experience with more than 13 years post doctorate experience in the field of NDT, FailureanalysisandAHSS.DrMohapatraisarecipientofJSPSPostdoctoralFellowintheyear2010, YoungScientistfromMRSIKolkataintheyear2008,NationalYoungNDTScientistbyISNTintheyear 2013.Hehasmorethan30SCIJournalpublications,05non-SCIjournalpublicationsandparticipated andpresentedhisworkinseveralnationalandinternationalconferences.