The Origin, Significance and Prevention of Defects in Welds

A defect is the discontinuity which by nature renders the weldment unable to meet specifications or acceptance standards. Thermal cycle of welding process leads to melting, casting and heat treatment of similar or dissimilar materials which are joined together and thus the defects associated with melting, casting and heat treatment are also found in the welded structure. During the last two decades, development of high yield strength steels e g Micro alloyed HSLA, Maraging steel etc. with narrow ductility range and low toughness caused enormous problem for welding fabricator, designer and user. The steels for cryogenic purpose (9% nickel steel) and creep resistance steel need special attention so as to develop necessary sub-zero and creep resistance properites in the respective weld fabricated structure. Significance of the discontinuities with respect to service behaviour assessed with greater sophistication by using fracture mechanics techniques and 'Fitness for Purpose' concept. In this article the nature, origin, effect and prevention of major metallurgical defects encountered in fusion welding processes are discussed.