Mechanical Properties and Microstructure of Resistance Spot Welded Joints of AISI 409M Ferritic Stainless Steel

Resistance spot welding is widely used as a sheet metal joining process, especially in automotive and rail coach manufacturing industries. Ferritic stainless steel (FSS) is used for structural applications in these areas, owing to its superior corrosion resistance compared to conventional carbon steels. In this paper, mechanical properties and microstructure of resistance spot welded FSS joints to grade AISI 409M was investigated. Tensile shear test and micro hardness test were carried out to assess the mechanical properties and failure characteristics of weld joints. Microstructure was evaluated with optical microscopy. The effect of welding current on nugget growth, peak load, fusion zone (FZ) hardness, failure energy and failure mode were analyzed at various current ratings, keeping electrode force, weld cycle time and electrode tip diameter as constants. The results showed that, with the increment in welding current, tensile shear strength increased. Nugget diameter was found to be increasing with increase in current, in expulsion free welds. At low currents, failure mode was found to be interfacial and the pull out mode of failure occurred, at relatively higher current values. Average FZ hardness was found to be more than that of base metal (BM) but less than that of maximum hardness at heat affected zone. Grain enlargement was noticed at high temperature heat affected zone. FZ microstructure consists of predominantly columnar ferrite. Failure occurred at the BM region in all the samples welded above 10 KA current.