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/s
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Metallurgical Aspects of Welding
Abstract 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.
Keywords Cold cracking Welding stress Post-weld heat treatment bead welding Stainless steel welding Hot cracking
© Springer Nature Singapore Pte Ltd. 2017 A.K. Lahiri, Applied Metallurgy and Corrosion Control, Indian Institute of Metals Series, DOI 10.1007/978-981-10-4684-1_7
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7 Metallurgical Aspects of Welding
Introduction
Welding as a joining method for fabrication of pressure components replaced riveting in the early 1930s. In conventional fusion welding, joining is achieved by applying heat, with or without the addition of the filler meal, to produce a localized union through fusion. Weld metal has a cast structure against worked structure of the base metal. Ideally, the filler metal should match the composition of the base metal but a closer matching of mechanical properties (tensile, impact, creep, etc.), freedom from cracking and, in some cases, resistance to corrosion are of greater importance, which because of the structural differences are not always attained. The present chapter does not cover the topic
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