Abstract: The die-casting mold made of 8407 steel was re-polished to remove the surface pits appeared in service, however, the pitting appeared again on the working surface in re-using of the die. The chemical composition, surface defect features and microstructure of failed die-casting mold parts were analyzed by means of metallographic microscope, scanning electron microscope and others. The results show that there are different degrees of cracking around and at the bottom of the surface pits, and slope-like indentations are formed on the edges of the pits. No chemical reactions such as oxidation and corrosion are found in the pits on the working surface of the mold, which implies that the pits are caused by extrusion deformation. The reason for the pitting on the working surface of the die-casting mold is that the polishing process is too strong, so that bright quenching structure forms in the surface layer of the mold working surface caused by high temperature and rapid cooling, and the resulted stress causes transverse crack of working face and form local longitudinal cracks. Under the influence of the heat from surface polishing, the secondary surface layer of the working surface is softened by annealing, thus forming a deformed fibrous structure. In the using process of the die, the longitudinal cracks on the working surface are crushed to form pits, which causes the surface macro-pits on the working surface of the mold, leading to the raised appearance defects on the surface of die-casting products of zinc alloy.

Key words: 8407 steel die-casting mold, quenching structure, transverse crack, pitting failure analysis