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Welding of Martensitic Stainless Steel
Martensitic stainless steels are susceptible to cold cracking, in which diffusible hydrogen can play a contributing role. Procedures which have proven successful in welding martensitic stainless steels without cracking include:
(1) Maintenance of high preheat and interpass temperature— 400°F may be necessary for Type 410, and 600°F may be necessary for Type 420 stainless steel.
(2) Use of very low hydrogen filler metals, fluxes, shielding gas, etc.
(3) Use of high heat input welding procedures to slow weld cooling.
(4) Use of insulation and/or supplemental heating to slow the cooling of the weldment.
(5) Application of postweld heat treatment as soon as the weld cools sufficiently that martensite transformation is nearly complete [typically this temperature is about 200°F (90°C)]. Then the PWHT temperature must be chosen to temper martensite and remove hydrogen without re-austenitizing the metal; isothermal transformation diagrams
for the base metal and weld metal should be consulted concerning the temperature at which austenite will start to form for a particular base metal and filler metal.
Welding of Ferritic Stainless Steel
Ferritic stainless steels are susceptible to embrittlement from grain growth during welding, and from precipitation of intermetallic compounds during postweld heat treatment or service at temperatures from as low as 600°F (315°C) to as high as 1700°F (930°C). Due to the grain growth phenomenon, ferritic stainless steels are normally provided only in thin sections.
Procedures which have proven successful in joining stainless steels without serious embrittlement include:
(1) Use of low heat input single-pass welding procedures.
(2) Skip welding to avoid heat buildup in one area.
(3) Certain ferritic stainless steels which contain considerable carbon, such as Type 430, may benefit from a short-time PWHT which permits carbon to diffuse out of the ferrite to small martensite islands which are softened by the same PWHT—the provider of the base metal should be consulted for exact recommendations for such PWHT.
(4) Where such use does not cause adverse corrosion effects, use of an austenitic filler metal which includes some ferrite, instead of a ferritic filler metal, may provide a more forgiving weldment.