What are overheating and underheating in heat treatment?

　　During heat treatment, overheating or underheating often occur. So, what exactly are overheating and underheating?

　　The microstructure after quenching, showing signs of overheating, can be observed at the rough edges of bearing components’ bearing parts. However, to accurately determine the extent of overheating, it is necessary to examine the microstructure in detail. If coarse needle-like martensite appears in the quenched microstructure of GCr15 steel, this indicates an over-quenched microstructure. The cause may be either complete overheating resulting from excessively high quenching heating temperatures or excessively long holding times during heating; alternatively, it could be partial overheating caused by severe banded carbides in the original microstructure, leading to the formation of coarsely developed martensitic needles in the low-carbon regions between the bands. In an over-heated microstructure, residual austenite increases, and dimensional stability deteriorates. Due to the coarse crystal structure resulting from overheating during quenching, the toughness and impact resistance of the part are reduced, thereby shortening the service life of the bearing. In cases of severe overheating, quenching cracks may even develop.

　　If the quenching temperature is too low or cooling is inadequate, a troostite structure exceeding the specified standards will form in the microstructure, known as underheated structure. This results in reduced hardness, a sharp decline in wear resistance, and a shortened service life of roller bearing components.