Microstructure of Repair Welding Heat-Affected Zone of a Mo-Modified AISI H13 Hot-Work Tool Steel for Die-Casting Die

The heat-affected zone (HAZ) formed on welding-repaired die-casting die features a hardness distribution that can potentially initiate thermal shock cracks. This study aimed to analyze the microstructure of a repair welding HAZ formed on Molybdenum-modified AISI H13 steel (Mo-modified H13 steel) used for the die-casting die in industry to identify the characteristics causing this hardness distribution. In a model weld-repaired die, the HAZ from 0–2.8 mm from the boundary of the weld metal was hardened by up to 140 HV compared with that of the original substrate, whereas the region from 2.8–5.0 mm was softened by up to 200 HV. Microstructural and crystallographic analyses reveal that the hardened region had a martensite structure, which corresponded to the origin of the hardening. Contrastingly, the softened region exhibited a typical annealed structure with granular Mo and V carbides. This structure results in the loss of the secondary hardening effect of the Mo-modified H13 steel substrate, which decreases hardness.