08 Stainless

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Stainless & heat resisting steels

EF402 Lecture 8


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Stainless steels

Alloys of iron with >11.5% chromium, plus Ni, Mn,Mo, N etcClasses of stainless steel

q Ferritic stainless steels (cheap, resistant to Cl -)q Martensitic stainless steels (strong, resistant to Cl -)q Austenitic stainless steels (ductile, wide applications)q Duplex (strong, ductile, resistant to Cl -)q Precipitation hardening (very strong)


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Effects of chromium on steel

Increases oxidation &corrosion resistanceIncreases hardenability

and hardnessStabilises ferrite

q above 12% no austenite atany temperature

Forms stable carbides

Wtloss

Cr percent0 2 4 6 8 10 12 14 16 18

Corrosion rate


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Corrosion of stainless steels

General corrosion not significantPitting corrosion

Crevice corrosionIntergranular corrosionStress corrosion crackingGalvanic corrosion


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Handling stainless steels

Avoid embedding surface with carbon steelq Causes local rust spotsq Scraping with steel tools or surfacesq Grinding debrisq Contamination can be removed by pickling

Done using oxidising acid solutions or pickling paste Passivating is necessary to restore oxide film


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General welding precautions

Avoid surface oxides (excessive heat tint) whichcan lead to corrosion in aggressive environments

q Argon gas backing for pipe weldsq Acid pickling to remove scaleq Glass bead blasting

Ensure slag is removed

Joint designs should avoid crevices


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Iron-chromium phase diagram

Fe Cr12%

Liquid

+ +

910C

1500C

T e m p e r a

t u r e

Compositions with over12% Cr never becomeaustenitic

Addition of austenitestabilising elementswidens gamma loop

q Carbon, nickel, nitrogen

Sigma stable at high Crlevels

q Slow transformation


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Effect of nickel additions

Stabilises austeniteMore than 8% in austenitic

stainless steels4 to 7% in duplexaustenite-ferrite steels

0 2 4 6 8

200 C

400 C

1000 C

% nickel

Ferrite ormartensite

Austenite

Fe-Cr-Ni equilibrium diagram(schematic)


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Iron-Chromium-Nickel

Fe

Cr

Ni10 20 30 40 50 60 70 80 90

90

80

70

60

50

40

30

20

1010

20

30

40

50

60

70

80

90

+

At 1100CAusteniticFerritic

Duplex


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Unwanted secondary phases

Impair corrosion performance and/or embrittleCarbides if carbon content is high (>0.03%)Intermetallic phases at higher levels of Cr or Mo

q sigma, chi, laves, alpha prime, R and tau

Nitrides


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Secondary phase formation

Depends on alloy composition, temperature andtime at temperature

q High temperatures tend to dissolve unwanted phases

May occur during hot fabrication processes or inhigh temperature service

q High temperature short times during welding causessigma in 254SMO and 2205

q Extended heating during heat treatment, hot work or inservice causes sigma and chi in 316L


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Weld decay

Heating between 480 and 820 C causes

carbide precipitation at grain boundarieswhich lowers Cr level in adjacent matrix.Corrosion occurs at the denuded areas ofsensitised steels if they are in aggressiveenvironments.


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Time-temperature sensitisation curve

10 sec 1 min 10 min 1 h 10 h 100 h 1,000 h 10,000 h400 C

500 C

600 C

700 C

800 C

900 C

0.019% C

0.06% C

Increasing carbon


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Avoiding weld decay

Use low carbon content steels (maximum 0.03%)q The L grades of stainless eg 304L

Stabilise carbon with Ti or Nb (Cb) which formcarbides that are stable at high temperature

q Grades 347 or 321 are stabilised versions of 304

Solution treat after weldingq Heat to 1100 C and water quench rapidly through the

sensitisation range


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Avoiding intermetallic phases

Avoid temperatures for the timescale at whichthey formSolution treat

q Typically 1100C for 1 hour and rapidly coolq Temperature and cooling depends on alloy

In some cases, removal may be almostimpossible


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Ferritic stainless

10.5 to 30% chromiumLow carbon, nickel, nitrogenVery resistant to chlorides, but tends to be brittle

q Grain growth during fabricationq Alpha prime ( ) at 475Cq Sigma at higher Cr levels


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Ferritic stainless overview

High Cr types solidify as ferrite and never becomeausteniticGrain growth during fabrication causes loss oftoughnessLow Cr types can become austenitic, and maytransform to martensite (cracking likely duringwelding)


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Fabrication of ferritic grades

Reasonable hot workabilityLimited amount of cold work before annealing

q High yield strength and work hardening rate

Welding is complexq Only thin sections weldedq Cool quickly to avoid embrittlement


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Austenitic stainless steel

Widespread uses, piping, process vesselsMinimum Ni or Ni + Mn to ensure structure iscompletely austeniticSome quenched from ~1100C to retain 100%austenite


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Fabrication of austenitic SS

Good hot ductility over a limited temperaturerange

q Embrittled by S, O and P

Better cold ductility than ferritic, martensitic orduplex grades, but limitedGood weldability when reasonable care isexercised


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Weld solidification cracking

Depending on compositionLow susceptibility if there is some ferrite duringsolidification

q Weld metal with 5% ferrite at room temperature(coincidence)

Predicted from Schaeffler, DeLong or WRC 1992diagrams


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Weld metal composition diagrams

First was the Schaeffler diagram - 1949q Still used because it covers wide range of compositions

Next was the DeLong - 1956q Nitrogen term added

WRC 1988 & 1992 diagramsq Kotecki and Siewert. Most accurate predictions

All cover weld metal only and are NOTequilibrium diagrams!


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Nickel and chromium equivalents

Some elements are similar to nickel & stabiliseaustenite in steels

q Carbon, manganese (now found to have little effect)

and nitrogenSome elements stabilise ferrite like chromium

q Molybdenum, niobium


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0

30

0 40

Martensitic alloys

Schaeffler diagram

Martensite

Austenite

A + M

M + FFerrite

A + F + M

Chromium Equivalent = Cr + Mo + 1.5 Si + 0.5 Nb

N i c k e

l e q u

i v a

l e n

t =

N i +

3 0 C +

0 . 5

M n

A + F

Duplex alloys

Austenitic alloys

Ferritic alloys


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WRC 1992 Diagram

0 2 48

1014 20

2430

100

40

5060

70

8090

AAF

FA

F

17 18 19 20 21 22 23 24 25 26 27 28 29 30 319

10

11

12

13

14

15

16

17

18

N i c k e

l E q u

i v a

l e n

t =

N i + 3

5 C +

2 0 N +

0 . 2

5 C u

Chromium equivalent = Cr + Mo + 0.7 Nb


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Super austenitic 6Mo stainless

Excellent resistance to pitting in chlorides (seawater)Serious segregation on solidificationSigma phases form in weld metal

q Use nickel-based filler metalsq Solution anneal after welding

1080C and water quench (impractical)


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Martensitic stainless

Fe-Cr-C alloysChromium and carbon balanced so thattransformation to austenite occurs on heating

q More than 18Cr always ferriticHigh hardenability - austenite to martensitetransformation under almost all cooling rates

q Air hardening steels


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Applications

12 Cr low carbon engineering grades 403, 410,414, 416, 420

q Well head Christmas trees, valves, pumps

High carbon grades 431q Cutlery, tools


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Weldable 13 Cr martensitic

Sufficient resistance to corrosion in H 2S (sour) &CO 2 contaminated petrochemicals

Newly applied to pipelinesVery low carbon, oxygen and nitrogen

q Typically 0.02% maximum

Weld preheat and PWHT is not requiredq

Duplex stainless steel filler used, which mayundermatch strength


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Duplex stainless steels

50/50 mix of ferrite and austeniteq Low Ni with 22-26Cr

High strength with good ductility and toughnessMore economical than austenitic stainless steelBetter resistance to chlorides than austeniticstainless steel


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Fabricating duplex stainless

More demanding than austenitic gradesq Avoid secondary phasesq Balance ferrite and austenite proportions

Hot work at high temperaturesq Higher ferrite levels

Cold work limitedq Work hardens rapidly


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Welding duplex stainless steel

Sensitive to arc energyq Solidify as ferrite, austenite forms during coolingq Low arc energy causes high ferrite levels

Embrittlement & loss of corrosion performanceq High arc energy causes sigma phase to form

Embrittlement & loss of corrosion performanceq Arc energy ranges

0.5 to 2.0 kJ/mm for 2205 0.5 to 1.5 kJ/mm for 2507


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Finishing stainless fabrication

Required for aggressive environments, food orpharmaceutical industriesAvoid or remove embedded iron

q Test surface for contamination Ferroxyl test is immediate, alternatively wet surface & leave for 24

hoursq Pickling with acid or paste

Remove heat tint by pickling

Remove surface contaminants such as slag, dirt andpaint, which are a crevice corrosion risk

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08 Stainless