Effect of post-weld tempering pulse on microstructure and mechanical properties of resistance spot welding of Q&P1180 steel

Abstract Resistance spot welds (RSWs) of quench and partitioning (Q&P) steels exhibit poor mechanical properties due to the brittleness of the nugget microstructure. To solve this problem, the nugget microstructure in RSWs of Q&P1180 steel was modified by applying a post-weld tempering pulse with different protocols. Effects of the post-weld tempering pulse on microstructural evolution and mechanical properties of RSWs were investigated. The results indicate that a proper post-weld tempering pulse (cooling time of 1000 ms and post welding current of 4 kA) achieved a 70% improvement in the cross-tension peak load and a 6% improvement in the tensile-shear peak load. The nugget, consisting of tempered martensite and retained austenite with low hardness and high toughness, hindered the crack propagation towards the nugget during the cross-tension test and improved cross-tension property. In the tensile-shear testing, a new failure mechanism was discovered. Despite the decrease in nugget hardness, the improvement of toughness helped enlarge the effective area subjected to the shear load during the tensile-shear test, resulting in the improvement of the tensile-shear property.