Defects and Control in the Heat Treatment Process

Steel generally undergoes hot working and heat treatment to achieve higher strength and toughness or other special properties.

However, if the heating temperature is too high, it can instead lead to the deterioration of the mechanical properties of steel and even result in material scrapping.

Overheating Phenomenon

General Overheating: When the heating temperature is too high or the holding time at high temperature is too long, causing the austenite grains to coarsen, this is called overheating.

Coarse austenite grains can reduce the strength and toughness of steel, raise the brittle transition temperature, and increase the tendency for deformation and cracking during quenching.

The causes of overheating are usually furnace temperature instrument failure or improper mixing (often due to lack of process knowledge). Overheated structures can be refined by annealing, normalizing, or multiple high-temperature temperings, and under normal circumstances, re-austenitization can refine the grains.

Fracture Inheritance: Steel with overheated structures, even after reheating and quenching, can have austenite grains refined, but large granular fractures may still appear.

There is considerable theoretical debate about the origin of fracture inheritance. It is generally believed that excessive heating dissolves impurities such as MnS into austenite and causes them to concentrate at grain boundaries.

During cooling, these inclusions precipitate along the grain boundaries, making it easy for fractures to occur along coarse austenite grain boundaries under impact.

Coarse Structure Inheritance: For steel parts with coarse martensite, bainite, or pearlite structures, when re-austenitized and slowly heated to the conventional quenching temperature or even slightly lower, the austenite grains still remain coarse. This phenomenon is called structural inheritance.

To eliminate the inheritance of coarse structures, intermediate annealing or multiple high-temperature tempering treatments can be used.