The degree of the HAZ so ftening depends on the
martensite volume fraction of the BM. Xia et al. [16]studied the HAZ softening behavior of DP steels during laser welding. They found that the amount of HAZ softening is directly proportional to the martensite volume fraction. Higher DP grades exhibit greater potential for HAZ softening due to larger volume fraction of martensite which experiences post-weld tempering. It should be noted that the degree of HAZ softening is also depends on the welding process and heat input rate. The higher the heat input, the higher the degree of HAZ softening is. For example, HAZ softening of DP600 during RSW is not reported in the literature, due to rapid thermal cycle of RSW and short
dwell time at the peak temperatures, as well as it s martensite low volume fraction; while, some softening of HAZ-DP600 is reported during arc welding [22] and laser welding [16].