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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
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