Metallurgy of welding stainless steels

Based primarily on microstructure, five stainless steel types are recognized: ferritic, martensitic, austenitic, duplex and precipitation-hardening. The major problem in ferritic stainless steels is the tendency to embrittlement, aggravated by various causes. During welding, control of heat input is essential and, in some cases, also a postweld heat treatment. The austenitic type is the easiest to weld, but two important issues are involved in the welding of these steels: hot cracking and formation of chromium carbide and other secondary phases on thermal exposure. The nature of the problems and remedial measures are discussed from a metallurgical perspective. Duplex stainless steels contain approximately equal proportions of austenite and ferrite. The article discusses the upset in phase balance during welding both in the weld metal and heat-affected zone and the formation of embrittling secondary phases during any thermal treatment. Martensitic stainless steels are susceptible to hydrogen-induced cracking. Welding thus involves many precautions to prevent it through proper preheat selection, postweld heat treatment, etc. In the welding of precipitation-hardening stainless steels, it is usually necessary to develop in the weld metal strength levels matching those of the base metal. This is achieved by applying a postweld heat treatment appropriate to each type of alloy.