An experimental study on effects of post- heating parameters on resistance spot welding of SAPH440 steel A. R. Jahandideh 1 , M. Hamedi* 2 , S. A. Mansourzadeh 3 and A. Rahi 1 The fracture mode of spot welded joints, made of SAPH440 steel sheets, is investigated. It was found that the weldment failure in the peel test of the joints occurred through the weld nugget. This is called an interfacial failure and is not acceptable because it is a sign of insufficient mechanical strength. Investigation showed that this kind of fracture is attributed to the brittleness of the nugget zone, caused by its martensitic microstructure due to the high cooling rate in the welding. For eliminating this defect, resistance spot welding procedures were augmented with post- heating stage. This approach is intended to reduce the cooling rate after welding and also to temper the weld nugget, generating a more ductile microstructure in the weld zone. The results of this research can be used for planning spot welding process and provides a guideline for analysing the results of hardness and peel test. Keywords: Resistance spot welding, SAPH440 steel, Interfacial fracture, Post-heat Introduction Resistance spot welding (RSW) remains the primary process for joining body sheet metal in the automotive industry. The embedded infrastructure coupled with its superior surface finish makes RSW an economical process and aesthetically very desirable as well. During the RSW process, two continuously cooled electrodes clamp down on two or more worksheets; alternating or direct current is then passed through the electrodes at low voltage, which results in fusion at the faying surfaces of the worksheets. Each welding schedule typically involves four sequential steps: 1 (i) squeeze time (ii) weld time (iii) hold time (iv) off time. The latter is generally applied where the welding cycle is repetitive. High strength steels (HSS) have gained popularity due to their inherent strength and ductility characteristics. A JIS G3113 SAPH440 steel is used in the Asian automobile industries for body structures due to its formability and strength. A common concern about SAPH440 is its weld- ability problem, where interfacial fractures of its welds are reported under peel testing. Similar problems have been observed in HSS and advanced HSS. Spot weld failure mode is a qualitative measure of the weld quality which is of two types: interfacial and pullout. When the failure occurs through the weld nugget, it is called an interfacial failure, while in pullout mode, failure occurs by complete (or partial) withdrawal of the nugget from one sheet. Load carrying capacity and energy absorption capability for those welds that fail under interfacial mode is low compared to the pullout mode; 2,3 this is why the pullout mode is more desired. Gould et al. 4,5 tried to find the reasons for hold time sensitivity (HTS) of HSS. They concluded that HTS was only observed in the transformation hardened steel through a combination of disadvantageous stress states (undersized welds) and a crack susceptible microstructure. Khan et al. 6 studied the effect of weld microstructures on static and impact properties of advanced HSS spot welded steels. They stated that interfacial failure occurred when testing dual phase 600 (DP600), trans- formation induced plasticity 780 (TRIP780) and high strength low alloy steels. In addition, the weld test results by Sun et al. 7 show that, under lap shear loading, the weld fusion zone size is a critical factor in its static performance in terms of failure mode, peak load and energy absorption of DP800 and TRIP800 spot welds. The work of Yang et al. 8 showed that the weld length, specimen width and sheet thickness had great effect on the failure modes of the tensile testing specimen at DP600 steels. Recently, Pouranvari and Marashi 9 stated that there is no direct relationship between the tendency for interfacial failure and ultimate tensile strengths of base metals of DP and TRIP steels. Chuko and Gould 10 and Marya and Gayden 11 studied the idea of in situ post- weld heat treatment on TRIP and DP steels. It has been shown that tempering is an effective way of reducing HTS in hardenable HSS. Gould et al. 12 also predicted 1 University of Industries and Mines, Tehran, Iran 2 University of Tehran, Tehran, Iran 3 Sazeh Gostar Company, Tehran, Iran *Corresponding author, email mhamedi@ut.ac.ir ß 2011 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 8 May 2011; accepted 16 June 2011 DOI 10.1179/1362171811Y.0000000053 Science and Technology of Welding and Joining 2011 VOL 16 NO 8 669