Overheating phenomenon of heat treatment

In order to make the metal workpiece with the required mechanical properties, physical properties and chemical properties, in addition to the rational selection of materials and various forming processes, heat treatment process is often essential. Iron and steel is a widely used material in the mechanical industry. The microstructure of iron and steel is complex. The thermal treatment plant can be controlled by heat treatment.

In the process of heat treatment, it is easy to cause the austenite grain to be coarse and the mechanical properties of the parts will be reduced. Then, what are the defects of overheating in the heat treatment process? Here's a look:

1. General overheating: excessive heating temperature or too long heat preservation time at high temperature, causing the austenite grain coarsening is called overheating. The large austenite grain will lead to the decrease of strength and toughness, the increase of brittle transition temperature, and the increase of deformation and cracking tendency during quenching. The cause of overheating is the furnace temperature instrument out of control or mixing (often do not understand the process). After annealing, normalizing or tempering, the superheated structure can be reaustenified under normal conditions to refine the grain.

2. Fracture inheritance: steel with superheated structure, after re-heating and quenching, although the austenite grain refinement, but sometimes still appear thick granular fracture. The theory of fracture inheritance is controversial. It is generally believed that the sundries such as MnS were dissolved into austenite and concentrated in the crystal interface due to the high heating temperature, and these inclusions would precipitate out along the crystal interface when cooled, and they were easy to break along the massive austenite grain boundary when impacted.

3. Inheritance of thick tissue: when steel parts with thick martensite, bainite and weistenite structures are austenitized, they are heated at a slow speed to the normal quenching temperature, or even lower, and their austenite grains are still thick. This phenomenon is called tissue heredity. To eliminate the heredity of thick tissue, intermediate annealing or multiple tempering treatments may be used.

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