Stainless Steels[1]

8/22/2019 Stainless Steels[1]

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Stainless Steels


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Background

1913: Brearley, Sheffield

>10% Cr steel did not corrode in the industrial

atmosphere of Sheffield.

a consequence ofpassivation - the formation of

a protective oxide layer.

Passive layers form very rapidly, but are thin,

defect free and firmly attached to the surface ofthe steel.


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Important Elements

Chromium (Cr) for resistance to oxidising

environments, and a ferrite former.

Nickel (Ni) for resistance to reducing

environments, and an austenite stabiliser.

Molybdenum (Mo) improves the pitting

resistance.

And others ...


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Phase Diagrams

Temperature

1400 1200

12% Cr

+

60% Ni


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Pitting Resistance Equivalent

P.R.E. =

%Cr

+ 3(%Mo)

+ 16(%N)

a useful guide, but empirical. Elements

must be in solution.


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Some Alloys

Cr Ni Mo PREN

ferritic: 12 - - 12

304: 18 10 - 18 316: 18 10 3 27

duplex: 22 5.5 3.5 32.5

super-austenitic: 27 31 3.5 37.5


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Ferritic

AISI 430

0.1% C, 17% Cr

100% ferrite (body centred cubic)

low strength

poor toughness

good ductility


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Martensitic

AISI 410

0.15% max C, 13% Cr.

various amounts of martensite

Stronger

better toughness

production tubulars


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Austenitic

AISI 304

0.1% C, 18% Cr, 8% Ni.

0.03% C, 18% Cr, 10% Ni.

almost 100% austenite.

Excellent toughness (cryogenic).

low yield strength.

cladding, etc.


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Composition/Microstructure

chromium equivalent

Cr + 2Si + 1.5Mo + 5V + 5.5Al + 1.75Nb +

1.5Ti + 0.75W.

nickel equivalent

Ni + Co + 0.5Mn + 0.3Cu + 25N + 30C.

See Schaeffler diagram.


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Schaeffler Diagram

Chromium equivalent 40%

30%

Martensite

Austenite

Ferrite

A + F

M + F

F + M

A+F+M


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Duplex

25% Cr, 5% Ni.

flexibility for:

high P.R.E.

strength

combination of these

and resistance to:

sensitisation

stress corrosion cracking


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Precipitation-hardened austenitic

eg ARMCO: 17-10PH

0.1%C, 17%Cr, 11%Ni, 0.3%P

Proof Strengths

316 300Nmm-2

17-10P 680Nmm-2 (1120oC, wq + 24h 648oC)

Other p.h. elements: Al, Cu.

Can harden martensitic steels this way.


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High nitrogen austenitic

ARMCO Nitronic 32

0.1%C, 12%Mn, 18%Cr, 1.6%Ni, 0.35%N.

Increases proof strength by 40%.


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Pitting Corrosion

Pitting corrosion (eg 10mm in six months in

room temp seawater).

Related to localised breakdown of passive

layer.

Consequent rapid pit formation.


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Weld Decay

Sensitisation

Heat treatment (500 - 700oC) causes

chromium depletion at, or near grain

boundaries

hence intergranular corrosion


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Sensitisation

Cr rich

Cr poor

Chromium carbides


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Temperature, Time, Sensitisation

Temperat

ure

1000

400

log(time)

Grain-boundary

precipitation

Intergranular

Corrosion

~5 minutes


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Accelerated Sensitisation Test

For example

Grade 304

Solution treated at 1050oC

Boiling H2SO4, CuSO4

Susceptible at 700oC after 1 hour


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Prevention

low C

316 0.10%C max

316L 0.03%C max

stabilise

347 0.08%C max, Ti(4 x C) min

heat treat.


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Sigma Phase

sigma phase formation during heat

treatment (esp duplex).

depletion of ferrite if Cr and Mo present.


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Cost of Corrosion

NBS/Batelle Review of 1980

$70 billion is total cost (estimates vary between

$8 billion and $126 billion).

$10 billion could be saved - if best practice

were to be applied.

(about 3% of GDP)

Corresponding figures for UK (HoarReport, c1970)


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Costs of Corrosion

Replacement of lost material

Prevention costs

Energy costs of extraction of lost material Maintenance and repair costs

Lost production costs

Loss of business? Health and Safety costs

Stainless Steels[1]

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Stainless Steels[1]