7. Metallurgical Aspects of Welding

Welding is the commonly used method for joining of metals. The process consists of filling the gap between the two ends with liquid metal which when solidifies forms a strong bond. During the process the base metal attains high temperature at the liquid/solid interface and a temperature gradient on both sides of the weld is formed. In case of ferritic steels, martensite or bainite or pearlite is formed adjacent to the weld at different cooling rates, depending on TTT and CCT curves of the alloys. Martensite and bainite are hard, and on cooling cold cracking occurs due to hydrogen pick up during welding. Cold cracking is avoided by taking steps to prevent hydrogen pick up, reducing the rate of cooling, and tempering of brittle structure by post-weld heat treatment (PWHT). In case of austenitic stainless steels welding is easy as no phase transformation is involved. However, to avoid hot cracking small amounts of ferrite in the consumable is necessary for which WRC 1992 constitution diagram is used. Duplex stainless steel (DSS) alloys are required to follow a procedure which ensures weld deposit to have equal ferrite/austenite phase mixture that provides good corrosion resistance and ductility. In this chapter, various metallurgical aspects of welding of ferritic steels such as the phenomenon of cold cracking and its avoidance by controlled weld procedure, dehydrogenation of weld prior to cooling at room temperature, use of low-temperature intermediate PWHT and other stress relieving methods have been detailed. The methods to avoid PWHT such as temper bead welding, buttering technique, friction stitch and seam welding have been described. Welding of austenitic stainless steel along with the allied problem of HAZ cracking, welding of DSS steels, titanium and of dissimilar metals and outlines of underwater welding have been provided. Use of friction heat for joining, used in nuclear and offshore industry, has also been described.