Surface quenching cracks refer to the cracks produced during the quenching process or during the room temperature storage process after quenching, the latter is also called ageing cracks. In the quenching process, when the stress generated by quenching is greater than the strength of the material itself and exceeds the plastic deformation limit, it will cause cracks.
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Quenching cracks are often generated shortly after the start of martensite transformation. The distribution of cracks does not have a certain rule, but they are generally easy to form at sharp corners and sudden cross-sections of the workpiece. Quenching cracks caused by too fast cooling in the martensite transformation zone are often distributed across the grain, and the cracks are straight, and there are no small branch cracks around.
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The quenching cracks caused by the excessively high quenching heating temperature are distributed along the crystal, the crack tail is sharp, and shows the characteristics of overheating. Thick acicular martensite can be observed in structural steel, and eutectic can be observed in tool steel. Or horny carbide. High-carbon steel workpieces with surface decarburization are more likely to form network cracks after quenching. This is because the volume expansion of the surface decarburization layer during quenching and cooling is smaller than that of the undecarburized core, and the surface material is affected by the expansion of the core. It was pulled apart into a mesh. The quenching cracks on the surface will cause the bolt to break suddenly, and the fracture source of this type of fracture is on the surface.