What is the meaning of underheat in heat treatment

Heat treatment often presents the question of overheating or underheating, so what is overheating and what is underheating?

The critical point (or critical temperature) is defined as the temperature at which the transformation of steel is about to start or end when it is heated or cooled. Fe-Fe3C A1, A3 and ACM are all critical points of steel. It is worth noting that Fe-Fe3C is critical point of heating and cooling under equilibrium condition. Under the condition of actual cooling or heating, the speed can not be infinitely slow, and supercooling or overheating will occur. The actual phase transition temperature and equilibrium critical point on the phase diagram will produce the phenomenon of deviation, and the faster the heating or cooling speed is, the greater the deviation will be. In order to distinguish easily, AC1, AC3 and ACCM are used to represent the critical points in actual heating and AR1, Ar3 and ARCM are used to represent the critical points in cooling.

Heat treatment under heat refers to the insufficient heating temperature, there will be a lot of unmelted carbide, high carbon area is very few. Under heating can reduce the hardness of steel parts, which can be saved by reheating. When the steel is heated by austenitizing, if the heating temperature is too high or the heating time is too long, the austenite grains will grow and coarsen, and the martensite formed will also coarsen. This phenomenon is called overheating. Superheat makes the properties of the steel worse, especially the impact toughness is obviously reduced, and the workpiece is easy to be deformed or even cracked during quenching. The superheated steel can be normalized or annealed for refining structure first, and then re quenched according to normal specification.

Overburning refers to the over heating temperature, which not only causes the steel to form coarse austenite grains, but also causes oxidation at the austenite grain boundary.