1. Characteristics of quenching cracks
During the quenching process, when the huge stress generated by quenching is greater than the strength of the material itself and exceeds the plastic deformation limit, cracks will occur. Quenching cracks often occur shortly after the martensitic transformation begins. The distribution of cracks has no certain rules, but they are generally easy to form at sharp corners and cross-section mutations of the workpiece.
The quenching cracking observed under the microscope may be intergranular cracking or transgranular cracking; some are radial, and some are separate lines or networks.
Quenching cracks caused by excessive cooling in the martensitic transformation zone are often distributed transgranularly, and the cracks are straight, with no branching small cracks around them.
Quenching cracks caused by excessive quenching heating temperature are distributed along the grain, with sharp crack tail ends, and show overheating characteristics: coarse acicular martensite can be observed in structural steel; eutectic or angular carbides can be observed in tool steels.
High carbon steel workpieces with surface decarburization are prone to form network cracks after quenching. This is because the volume expansion of the surface decarburized layer during quenching and cooling is smaller than that of the non-decarburized core, and the surface material is stretched into a network shape by the expansion of the core.
2. Characteristics of non-quenching cracks
Cracks that occur after quenching are not necessarily caused by quenching. They can be distinguished according to the following characteristics:
For cracks discovered after quenching, if there is oxidation and decarburization on both sides of the crack, it can be determined that the crack already existed before quenching. During the quenching and cooling process, cracks are likely to form only when the amount of martensite transformation reaches a certain amount. The corresponding temperature is approximately below 250°C. At such low temperatures, even if cracks occur, decarburization and obvious oxidation will not occur on both sides of the cracks. Therefore, cracks with oxidative decarburization are non-quenching cracks.
If the cracks already exist before quenching and are not connected to the surface, although such internal cracks will not cause oxidation and decarburization, the lines of the cracks will appear soft and the tail ends will be round and bald.

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