Effects of post-weld heat treatment on mechanical properties and microstructure of resistance spot–welded lightweight steel

To improve the mechanical properties of resistance spot–welded 780-MPa-grade lightweight steel, post-weld heat treatment (PWHT) through the application of a second welding current and using a furnace was performed after resistance spot welding. It was not possible to obtain satisfactory mechanical properties of the weld through the application of a second welding current. In the case of the PWHT by a furnace, the required loads were satisfied when PWHT was applied at temperatures of less than 300 °C for more than 30 min, and at more than 300 °C for 15 min or more. Due to the softening of martensite by PWHT, the resistance to fracture progression during tensile test was increased, and mechanical properties could be improved.