ss lecture

8/12/2019 Ss Lecture

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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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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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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

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