Why are there quenching cracks?

1 Quenching crack

The quenching process is mainly used for steel parts, which is to heat the steel to a temperature above the critical temperature Ac3 (hypoeutectoid steel) or Ac1 (hypereutectoid steel), hold it for a period of time to make it fully or partially austenitized, and then The cooling rate of the critical cooling rate is quickly cooled to below Ms (martensitic transformation starting temperature) (or isothermal near Ms) to perform a martensite (or bainite) transformation heat treatment process.

Quenching cracking refers to the cracks generated during quenching or during room temperature after quenching. The latter is also called aging cracks. The distribution of cracks does not have a certain rule, but it is generally easy to form at the sharp corners and abrupt cross-sections of the workpiece. The root cause of quenching cracking is that the tensile stress exceeds the fracture strength of the material, or although the fracture strength of the material is not exceeded, the material will also crack due to internal defects. There are many specific reasons for quenching cracking, and the analysis should be based on the characteristics of the crack.

2 Causes of quenching cracks

The essential brittleness of martensite is the internal cause of quenching cracks, and the crystal structure, chemical composition, metallurgical defects, etc. of martensite are the influencing factors of the essential brittleness of martensite; the macroscopic internal stress caused by various process conditions, part sizes and shapes, etc. The size, direction, distribution, etc. are the external causes of quenching cracks. The quenching cracks of steel parts will be analyzed from micro to macro, from inside to outside.