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Welding Metallurgy of
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Family of Stainless Steels
Stainless steels:A group of ferrous alloys that contain at least11% Cr, providing extraordinary corrosionresistance.
Categories of Stainless Steels:
Ferritic Stainless Steels
Martensitic Stainless Steels
Austenitic Stainless Steels
Precipitation-Hardening (PH) Stainless Steels
Duplex Stainless Steels
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Phase Diagram
(a) The effect of 17%chromium on theiron-carbon phasediagram. At low-carbon contents,ferrite is stable at all
temperatures.
(b) A section of the iron-chromium-nickel-carbon phase
diagram at a constant18% Cr-8% Ni. Atlow-carbon contents,austenite is stable atroom temperatures.
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(a) Martensitic stainless steel containing large primarycarbides and small carbides formed during tempering (
350).(b) Austenitic stainless steel ( 500).
(FromASM Handbook, Vols. 7 and 8, (1972, 1973), ASMInternational, Materials Park, OH 44073.)
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Role of Composition
Oxidation of Cr. Other elements like Si, Mn, Nb, Ti and Mo
also form oxides
Affinity of Cr for C could lead to carburisation of weldment
(Flux covering on SS Arc Welding electrodes do not contain
cellulose)
Nitrification: Cr, Ti, Zr, Nb, Al are strong nitride formers
Ni, Mn do not have any strong oxide forming tendency, and
are austenite stabilisers
Si is an oxide former, the oxide having a lower melting
point. For greater fluidity of slag, electrode filler material
with higher Si is used (0.25- 0.75%)
Mo improves corrosion resistance
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Role of Residual Elements
C, S and H are a few residuals which can be introduced in
weld metal by a surprising number of sources
Refractory-like slag are associated with oxides of elements
like Al, Ca, Mg, Ti and Zr
H could lead to embrittlement, blow holes and porosity (1 to12 ppm is within solubility limits)
N could enhance the propensity for fusion line porosity,
especially in materials with nitride formers
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Martensitic Stainless Steels
Used as corrosion resistant and creep resisting materials
Types 403, 410, 414, 416, 420, 431, 440,
Type 403, 410 have
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Welding cycle for X20CrMoV12 1
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Ferritic Stainless Steels As a group, not considered attractive from weldability angle.
Types are 405 (13%CR, 0.20% Al), 409 (0.08% C, 11% Cr), 430,
446. Cr levels 17-27%
Weldability better for lower Cr steels as it produces a small
amount of martensite in the ferritic structure, and this helps in
inhibiting grain growth
Type 405, 409 are fully ferritic and are employed where post weldannealing is not possible
The weld metal as lower notch toughness, to be borne in mind
during design
Preheating is done to primarily reduce shrinkage stresses
Any PWHT above 900 C develops coarser grain size. 400 C-
550C induces embrittlement. Sigmatisation is a problem in the
intermediate range
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Austenitic Stainless Steels
Cr-Ni type stainless steels
Used for resistance against corrosion, low magnetic
permeability, good high temperature strength, excellent
toughness at low / sub zero temperatures
Types 302, 303, 304, 309, 310, 316, 317, 321, 347, .
Considered the most weldable of high alloy steels
Excellent toughness in the as welded condition
Coefficient of liner expansion is 50% greater than plainsteel. Warpage and distortion issues need to be addressed
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Austenitic Stainless Steels
Weldability Issues to address:
Sensitisation and intergranular corrosion
Extra low C, Stabilised Grades, Solution HT
Stress Corrosion Cracking
Hot cracks manifestations as crater cracks, star cracks,
microfissuring and root cracks
Effect of delta ferrite and two phase - advantage
C, Mn, Npositive effect
Residual elements B, P, S, Se, Si, Nb, Taadverse
effect
No preheating
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PH Stainless Steels
Types 17-4 PH (4% Cu); 17-7 PH (1% Al), 17-10 PH (P-
0.25%), ..
Cu, Mo, Al, P, Ti, Nb are some of the intentional additions
Grade ISingle treatment (Martensite like) steels undergo
Austenite to Martensite transformation on cooling to roomtemperature. Amenable for PH because hardening elements
or compounds are soluble in austenite and relatively
insoluble in martensitic structure. Reheating produces these
precipitates. Ms around 150 C
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PH Stainless Steels
Group II (Double Treatment Steel (Semi-austenitic)
Gr I was not soft enough in annealed condition, so Gr II
was developed
As annealed austenitic. Ms below Room Temperature
Hardening by:
Reheating to 650-850 C precipitate carbides
Refrigeration below Ms (below70 C)
Cold working the material to produce martensite like
structure
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PH Stainless Steels
Group III (Austenitic PH steels)
Austenitic after annealing
Heat to 650-850 C for precipitation hardening
Hardness and Strength not as high as in I and II as
austenite is stable
While Grade I and II are weldable; Grade III is not
weldable. P causes hot shortness at welding temperatures