We know that overheating during heat treatment is likely to lead to austenite grain coarsening, which will degrade the mechanical properties of parts.

1. General overheating: heating temperature is too high or holding time is too long at high temperature, causing austenite grain coarsening. Coarse austenite grains will reduce the strength and toughness of steel, increase the brittle transition temperature and increase the tendency of deformation and cracking during quenching. The reason for overheating is that the furnace temperature meter is out of control or mixing. The superheated structure can be annealed, normalized or tempered several times at high temperature, and then re-austenitized under normal conditions to refine the grains.