MODE OF FAILURE OF RESISTANCE SPOT WELDED MARTENSTIC STAINLESS STEEL- PART II

Martenstic Stainless Steels are potential material for automobile industry. Martenstic stainless steel can be resistance spot welded either in annealed or in hardened tempered condition. Regardless of the condition, welding produces a hardened martenstic zone adjacent to weld. Hardness of this zone, though controlled to an extent by welding procedure, depends mainly on carbon content. As the hardness of metal in the heat affected zone increases, its toughness decreases and its susceptibility to cracking increases. Martenstic stainless steels having maximum carbon content of 0.15%, such as AISI grades 410 and 414, often produce satisfactory welds without post weld heat treatment. Steel with higher carbon content such as AISI grades 420 and 440 generally required post weld heat treatment mentioned by (Udin et al., 1954; ASM handbook, 1983).In the present investigation, 0.5 mm thin AISI 420 grade (0.29 % C) was selected for study. Cracking tendency ofthe material affects the mode of failure. Failure may occur either from interface (interface failure) or from nugget (Nugget pullout). Mode of failure was investigated by hand peel test, tensile shear test and cross tension test. Nugget pullout mode reveals the nugget has sufficient ductility and toughness and it is desired mode of failure. SEM images show that mode of failure is partially nugget pullout and partially interfacial. Insight from these studies will throw light on the mechanism of failure of resistance spot welds in martenstic stainless steel. Result indicates the faster cooling rate, higher carbon content, absence of post heating are responsible for cracking in the weld and subsequently interfacial failure