Mechanical Seal Materials Metals

Mechanical Seal Materials Metals FSA Knowledge Series

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Abstract This Fluid Sealing Association Knowledge Series training presentation introduces the variety of metals used in mechanical seal construction.

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Contents

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▪

Atomic Structure of Metals

▪

Metallic Bushing Materials Copper Alloys

▪

Solidification and Microstructure

▪

Specific parts, Bellows, Springs, Screws

▪

Cast and Wrought Metals

▪

Alloy Designation Systems

▪

Stainless Steel •

Stainless Steels for Mechanical Seals

•

Austenitic Stainless Steel

•

Specialty Stainless Steel

•

Duplex Stainless Steel

•

Martensitic Stainless Steel

•

Precipitation Hardenable Stainless Steel

•

Superaustenitic Nickel Alloys

•

Nickel Based Super Alloys

Atomic Structure of Metals ▪

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The majority of common metals have either a Face Center Cubic structure or a Body Centered Cubic structure, although some have a Hexagonal Close Packed structure. The major differences between these structures is the arrangement of the atoms that form a "building block". The different building blocks lead to different physical properties of metals.

Atomic Structure of Metals A Face Center Cubic Structure consists of an atom at each cube corner and an atom in the center of each cube face. A hard sphere concept can be used to describe atomic packing in each building block. These atoms are closed packed, i.e. they cannot be packed any tighter, and each atom touches its neighbor in any direction.

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Atomic Structure of Metals A Body Center Cubic Structure consists of atoms at each cube corner and one atom is at the center of the cube. Compared to a Face Center Cubic structure, this structure has more space between the atoms.

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Atomic Structure of Metals A Hexagonal Close Packed Structure is similar to a Face Centered Cubic structure in that they atoms are packed close together, however the arrangements of the atoms differs slightly resulting in the structure being less ductile.

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Atomic Structure of Metals The building blocks affect the metals physical properties. For example, bending a metal requires the planes of atoms formed by a collection of many of these building blocks to slide past one another. The different structures require different amounts of energy (force) to move therefore affecting how malleable or ductile the metal is.

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Atomic Structure of Metals Examples of metals with different structures:

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▪

Face Center Cubic Austenitic stainless steels 300 series – e.g. AISI 302, AISI 316 Non-magnetic

▪

Body Center Cubic Ferritic stainless steels Carbon steel and some 400 series stainless steels Magnetic

▪

Hexagonal Close Packed Martensitic Stainless Steels

Solidification and Microstructure Metals and metal alloys are created by mixing and melting the ingredients together.

The molten mixture is then poured into a mold and allowed to cool. As the metal cools, solidifications starts occurring with the atoms arranging themselves into the basic building blocks (body or face centered cubic structures) which grow and get larger forming crystals (or grains). These grains keep growing until their growth is blocked by another grain and a boundary is formed.

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Solidification and Microstructure

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1. Numerous individual crystals start solidifying

2. Crystals grow as metal cools

3. Irregular grains form as crystals grow together

4. Solidified metal consisting of numerous grains and grain boundaries

Solidification and Microstructure The rate of cooling will affect the size and shape of the grains that are formed. Larger grains have fewer grain boundaries which can affect the resulting physical properties of the metal.

Decreasing cooling rate 13

Grain structure visible in a sectioned and etched ingot

Cast and Wrought Metals Casting Produced by pouring molten metal in a mold to produce a desired object at or near finished shape.

Wrought Produce a cast ingot, then mechanically work the ingot to produce sheet, plate, extruded bar, tube, etc.

This results in a smaller, more directional grain structure and improved physical properties.

Note: All wrought alloys can be cast but not all cast alloys can be produced as wrought 14

Cast and Wrought Metals Comparison of Wrought and Cast Alloys Wrought alloys are initially cast as ingots and then hot/cold worked mechanically into the desired form by rolling , extrusion, forging. The resulting difference in strength is due to the cold/hot working of wrought alloys to obtain the desired shapes since it creates a dislocation density that reduces slip of atomic planes in the grain structure.

Grain structure: Surface Finish: Strength:

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Wrought Alloys Fine Excellent Excellent

Cast Alloys Coarse Poor to good Good

Cast and Wrought Metals Comparison of Wrought and Cast Alloys Weld Test – ASTM A890 Gr. 5A to Wrought 2507 Duplex Stainless Steel

Cast

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Wrought

Alloy Designation Systems The Unified Numbering System The Unified Numbering System (UNS) is an alloy designation system widely accepted in North America. It consists of a prefix letter and five digits designating a material composition.

The UNS is managed jointly by the American Society for Testing and Materials (ASTM) and the Society of Automotive Engineers (SAE).

A UNS number alone does not constitute a full material specification because it establishes no requirements for material properties, heat treatment, form, or quality.

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Alloy Designation Systems Alloy Casting Institute (ACI) Managed by the Steel Founders’ Society of America, the ACI alloy designation system uses a series of letters and numbers to describe the composition of a cast alloy.

Example:

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CF8M C F 8 M

(Cast 316 stainless steel) Cast corrosion resistant alloy 19-23% Cr and 9-12% Ni 0.08% C maximum Alloy contains molybdenum

Alloy Designation Systems American Society for Testing and Materials (ASTM) ASTM’s designation system for metals consists of a letter (A for ferrous materials) followed by an arbitrary sequentially assigned number. These designations often apply to specific products. An ASTM standards created using rationalized SI units have a suffix letter M. Example - ASTM A 582/A 582M-95b (2000) A - Describes a ferrous metal, but does not sub classify it as cast iron, carbon steel, alloy steel, tool steel, or stainless steel 582 - Is a sequential number without any relationship to the metal’s properties M - Indicates that the standard is written in rationalized SI units, hence together 582/A582M includes both US customary and SI units 95 - Indicates the year of adoption or last revision and a letter b following the year indicates the third revision of the standard in 1995 (2000) - Indicates the year of last re-approval 19

Alloy Designation Systems American Iron and Steel Institute (AISI) AISI designation system for stainless steels consists of a 3 digit number which can be followed by a modifying letter indicating alloy variations of the original alloy.

Example:

AISI 316L

Austenitic - 2 Austenitic - 3 Martensitic and Ferritic - 4 Sequential number without any relationship to the metal’s properties

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Alloy Modifier L – Low carbon (<0.03%) H – High Carbon N – Nitrogen added Cu – Copper added

Example of Alternate Alloy Designations For 316SS

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Standards Organization

Designation

Standards Organization

Designation

EN steel number

1.4401 1.4436

DIN

X5CrNiMo17 12 2 X5CrNiMo17 13 3 X5CrNiMo 19 11 X5CrNiMo 18 11

EN steel name

X5CrNiMo17-12-2 X5CrNiMo18-14-3

BS 970

316S 29 316S 31 316S 33 En58J

SAE grade

316

UNI

X5CrNiMo17 12 X5CrNiMo17 13 X8CrNiMo17 13

UNS

S31600

JIS

SUS 316 SUS316TP

Alloy Designation Systems Trade Names Trade names are often used to identify propriety alloys produced by a manufacturer.

Examples include: Carpenter® 20cB3 Hastelloy® C-276 Inconel® 718 Ferralium® 255

(Carpenter Technology Corporation) (Haynes International Inc.) (Special Metals Corporation) (Meighs Ltd., Langley Alloys Division)

In some instances, these alloy are also available from other manufacturers under a generic name (e.g. Alloy 20, Alloy C-276,…)

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Stainless Steel What is a Stainless steel?

Definition: Iron based alloy with >12% Chromium Classes Austenitic Ferritic Duplex Martensitic Precipitation Hardenable

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Examples 302, 304, 316, 317, Alloy 20 Generally not used for Mechanical Seals CD4MCuN, 2205 410, 416, 440, CA6NM 17-4 PH, AM 350

Stainless Steel Why are stainless steels corrosion resistant?

Corrosion resistance is obtained by a passive oxide layer on metal surface (Chrome oxide). Alloying elements can improve physical properties and corrosion resistance:

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Stainless Steel Stainless Steel Alloying Elements and Their Purpose

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Chromium

Oxidation Resistance

Nickel

Austenite former - Increases resistance to mineral acids. Produces tightly adhering high temperature oxides

Molybdenum

Increases resistance to chlorides

Copper

Provides resistance to sulfuric acid. Precipitation hardener together with titanium and aluminum

Manganese

Austenite former - Combines with sulfur. Increases the solubility of nitrogen

Sulfur

Austenite former - Improves resistance to chlorides. Improves weldability of certain austenitic stainless steels. Improves the machinability of certain austenitic stainless steels

Titanium

Stabilizes carbides to prevent formation of chromium carbide. Precipitation hardener

Niobium

Carbide stabilizer - Precipitation hardener

Aluminum

Deoxidizer - Precipitation hardener

Carbon

Carbide former and strengthener

Stainless Steel Groups Type

Magnetic

Examples

Austenitic

No

304, 316, Alloy 20

Duplex

Yes

CD4M, Alloy 255, Alloy 2205

Martensitic

Yes

410, 416, 440

Precipitation Hardenable

Corrosion Resistance

Hardness

Strength

Sometimes 17-4PH, AM 350

Excellent Poor

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Stainless Steel for Mechanical Seals Stainless Steels for Mechanical Seals Typical materials used for the construction of mechanical seal hardware and internal components include:

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▪

Austenitic Stainless Steels

▪

Specialty Stainless Steels

▪

Duplex Stainless Steels

▪

Martensitic Stainless Steels

▪

Precipitation Hardenable Stainless Steels

▪

Superaustenitic Nickel Alloys

▪

Nickel Based Super Alloys

Austenitic Stainless Steel Microstructure 100% austenitic if wrought, Non-magnetic May contain approx. 15% ferrite if cast making them slightly magnetic Very ductile, work hardenable, lower strength, non-hardenable by heat treatment, easy to weld Corrosion resistance Good general resistance Improved resistant to organic acids, nitric acids, alcohols, non-halogenated hydrocarbons, hydroxides Inhibited by some oxidizing chemicals Poor pitting and crevice resistance Will chloride stress corrosion crack 28

Austenitic Stainless Steel Composition in weight %* C

Cr

Mo

Ni

Si

Fe

Cast CF8

0.08

18-21

---

8-11

2.0

Bal.

Wrought 304

0.08

18-20

---

8-10.50

1.0

Bal.

Cast CF3

0.03

17-21

---

8-11

2.0

Bal.

Wrought 304L

0.03

18-20

---

8-12.0

1.0

Bal.

Cast CF8M

0.08

18-21

2-3

9-12

2.00

Bal.

Wrought 316

0.08

16-18

2-3

10-14

1.00

Bal.

Cast CF3M

0.03

17-21

2-3

9-13

1.50

Bal.

Wrought 316L

0.03

16-18

2-3

10-14

1.00

Bal.

CG8M

0.08

18-21

3-4

9-13

1.50

---

317

0.08

18-20

3-4

11-15

1.0

---

*maximum unless otherwise indicated 29

Specialty Stainless Steel Alloy 20 Microstructure 100% austenitic cast or wrought Non-magnetic Hardness – 133 Brinell Corrosion resistance General resistance superior to 316, 317 and duplexes Resistant to Sulfuric acid, organic acids, hydroxides, non-halogenated organics Pitting and crevice corrosion resistance Inferior to super duplex stainless steels

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Specialty Stainless Steel Alloy 20 Specifications UNS 08020 ASTM B463, B473

Composition in weight %* C

Si

Cr

Ni

Mo

Cu

Cb + Ta

Cast CN7M

0.07 1.50

19-22

27.5-30.5

2-3

3-4

---

wrought 20 Cb-3

0.06 1.0

19-21

32-38

2-3

3-4

8 x %C 1.00 max.

*maximum unless otherwise indicated

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Duplex Stainless Steel Duplex Stainless Steels Microstructure 40-60% ferrite, balance austenite Corrosion resistance Between 317 and Alloy 20 Good pitting and crevice resistance Improved resistance to Acid-chloride, seawater Ferrite improves resistance to acid-chloride stress corrosion cracking Mechanical properties About 2x stronger than AISI 300 series (austenitic) stainless steel Increased hardness over austenitic stainless steel Non-hardenable by heat treatment, easy to weld 32

Duplex Stainless Steel Duplex Stainless Steels Composition in weight %* C

Si

Cr

Ni

CD4MCu

0.04

1.0

24.5-26.5

CD4MCuN

0.04

1.0

Alloy 255

0.04

Alloy 2205

0.03

Mo

Cu

4.75-6.0 ---

1.75-2.25

2.75-3.25

24.5-26.5

4.7-6

0.1-0.25

1.7-2.3

2.7-3.3

1.0

24-27

4.5-6.5

0.10-2.25

2.0-4.0

1.5-2.5

1.0

21-23

4.5-6.5

0.03-0.20

2.5-3.5

---

*maximum unless otherwise indicated

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N

Martensitic Stainless Steel Martensitic Stainless Steel Microstructure Martensite, Strongly Magnetic Heat treatable to high hardness levels Hard to impossible to weld As quenched structure is unstable (highly stressed) Requires a tempering heat treatment to relieve the stresses Corrosion resistance Generally poor compared to other stainless steels.

Equivalent to AISI 304

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Martensitic Stainless Steel Martensitic Stainless Steel Composition in weight %* C

Mn

Si

Cr

Ni

Mo

Cast CA6NM

0.06

1.00

1.00

11.5-14.0

3.5-4.5

0.40-1.0

Wrought 410

0.15

1.00

1.00

11.5-13.5

---

---

Cast CA15

0.15

1.00

1.50

11.5-14.0

1.0

0.5

Wrought 4161

0.15

1.25

1.0

12.0-14.0

---

---

*maximum unless otherwise indicated 1. Sulfur 0.15 min.

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Precipitation Hardenable Stainless Steel Precipitation Hardenable Stainless Steels Microstructure Typically martensitic, but some special grades are austenitic Magnetic Relatively soft and ductile in the solution-annealed state Extremely high strength after precipitation heat treatment Corrosion resistance Generally poor compared to other stainless steels.

Equivalent to AISI 304 Mechanical properties The strengthening mechanism comes from the formation of submicroscopic precipitates, which are compounds of aluminum, copper, titanium, or molybdenum.

These precipitates provide resistance to strain exerted on the structure 36

Precipitation Hardenable Stainless Steel Precipitation Hardenable Stainless Steels Composition in weight %* C

Mn

Si

Cr

Ni

Cu

Cb+Ta

Al

17-4PH

0.07

1.0

1.0

16.5

4.0

4.0

0.3

---

17-7PH

0.09

1.0

1.0

17.0

7.1

---

---

1.0

AM 350 1

0.10

0.8

0.25

16.5

4.3

---

---

---

*maximum unless otherwise indicated 1. Mo 2.8%, N 0.10%

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Superaustenitic Nickel Alloys Superaustenitic Nickel Alloys Microstructure Austenitic Non-magnetic Work hardenable, non-hardenable by heat treatment, easy to weld Corrosion resistance Excellent corrosion resistance to oxidizing environments, pitting and crevice corrosion.

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Superaustenitic Nickel Alloys Superaustenitic Nickel Alloys Composition in weight %* Cr

Mo

Fe

Ni+Ta

C

Al

Ti

Co

Alloy 625 1 20-23

8-10

5.0

3.15-4.15

0.1

0.4

0.4

1.0

CW6MC

20-23

8-10

5.0

---

0.06

0.4

0.4

---

Cr

Mo

Fe

W

C

Mn

Si

Co

16

16

5.0

4.0

0.01

1.0

0.08

2.5

4.5-7.5

3.72-5.25

0.12

1.0

0.08

---

Alloy C-276 3

CW12MW 4 15.5-17.5 16-18 *maximum unless otherwise indicated 1. Mn 0.5%, Si 0.5%, P 0.015%, S 0.015%

2. Mn 1.0%, Si 1.0%, P 0.015%, S 0.015%, Ni+Cb 3.15-4. 5% 3. V 0.35%, 39

4. V 0.2-0.4%, S 0.03%, P 0.04%

Nickel Based Super Alloys Nickel Based Super Alloys Microstructure Austenitic Non-magnetic Precipitation hardenable, Excellent weldability Exceptionally high yield, tensile and creep-rupture properties at elevated temperatures Corrosion resistance Excellent corrosion resistance to many inorganic and organic, other than strongly oxidizing, compounds throughout wide ranges of acidity and alkalinity. Good pitting and stress-corrosion cracking resistance.

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Nickel Based Super Alloys Nickel Based Super Alloys Composition in weight %* Ni

Cr

Mo

Ni+Ta

C

Al

Ti

Co

Inconel 718 1 50-55

17-21

2.8-3.3

4.75-5.5

0.08

0.2-0.8

0.65-1.15

1.0

CW6MC

8-10

5.0

---

0.06

0.4

0.4

---

20-23

*maximum unless otherwise indicated 1. Mn 0.35%, Si 0.35%, P 0.015%, S 0.015%, B 0.006%, Cu 0.3%, Fe bal

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Nickel Super Alloys: Alloy 718 19 Cr, 3 Mo, 53 Ni, Bal Fe

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▪

Specifications ▪ Cast – ▪ Wrought – ASTM B637, B670, NACE MR-01-75 ▪ UNS N07718

▪

Corrosion resistance ▪ Pitting and crevice resistance better than duplexes

▪

Excellent weldability ▪ No post weld heat treatment required

▪

Good high temperature oxidation resistance

Metallic Bushing Materials: Copper Alloys Four Primary Types ▪ Coppers – essentially pure

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▪

Cupronickels – copper alloyed with nickel

▪

Brasses – copper alloyed with zinc

▪

Bronzes – copper normally alloyed with tin * May also be alloyed with Al, Al and Ni, P and Si

Metallic Bushing Materials One of the functions of a bushing in a mechanical seal is to prevent contact between rotating parts of the seal with stationary parts of the seal which may create damage and the possibility of sparks.

Copper alloys that contain lead are the most popular bushing materials as the lead content helps with lubricity and reduces the tendency to gall when contact with a rotating part is made. Common materials include: • Leaded Tin Bronze • Leaded Red Brass

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Leaded Tin Bronze & Leaded Red Brass Leaded Tin Bronze C93200

Cu

Sn

Pb

Zn

Fe

Ni

Sb

S

81-85

6.3-7.5

6-8

2-4

0.2

1.00

0.35

0.08

Leaded Red Brass C83800

Cu

Sn

Pb

Zn

Fe

Ni

Sb

P

S

Al

82-83

3.3-4.2

5-7

5-8

.3

1

.25

1.5

.08

.005

Composition in weight %*

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Function Specific Materials Metal bellows material ▪ AM 350 ▪ ▪

▪

316 SS ▪ ▪

▪

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Excellent chemical resistance

Alloy 718 ▪

▪

Can be work hardened Broad corrosion and chemical resistance

Alloy C276 ▪

▪

Good corrosion resistance Moderate physical properties

Alloy 20 ▪ ▪

▪

Good physical properties Moderate corrosion resistance

High temperature strength

Alloy 42 – End fittings for high temperature service ▪

Low coefficient of thermal expansion

Function Specific Materials Springs Material ▪ 316 ▪

▪

Alloy C276 ▪

▪

Resistance to chloride stress corrosion

ElgiloyTM ▪ ▪

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Used in single coil spring designs

Cobalt, Chromium Nickel Alloy High strength

Function Specific Materials Set screws ▪

Hardened steel ▪ ▪

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Coated/uncoated Heat treatment

▪

Stainless steel (300, 400 series)

▪

Specialty (Monel/Alloy C276/Alloy 20)

▪

Duplex Stainless

▪

Forged Screws with rolled threads will exhibit better physical properties than machined ones

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