Microstructure and Hardness Behavior of Friction Stir Welds of 2 GPa Strength Hot Press Forming Steel

Hot press forming (HPF) steels with a full martensite structure and 2.0 GPa strength (HPF2.0) were friction stir welded as bead-on-plate using a WC–Co12% tool. The stir zone was composed of recrystallized grains of various sizes and had comparable to or higher hardness than the base material. However, the intercritical heat-affected zone (ICHAZ) and subcritical HAZ experience softening. During the tensile test, fractures consistently occurred at the softened HAZ. The minimum hardness of the ICHAZ in the friction stir welds was 300 HV, which was similar to that in the arc welds with polygonal ferrite (328 HV) and lower than that in the laser welds with needle-like ferrite (400 HV). Remarkably, the softened width of the friction stir welds was narrower than half that of the arc welds. These results confirm the possibility of expanding the application of friction stir welding to next-generation HPF steels.