In the tempering process, after the screw is slightly damaged by the accumulation of hydrogen along the grain boundaries of the surface micro-regions, slight hydrogen embrittlement phenomena such as claw lines will occur, or it may be grinding cracks. The mesh cracks should be directly connected to the surface. Decarburization is related. Solving surface decarburization means solving crack problems.
The following four situations may cause network cracking:
1. There is the possibility of grinding cracks;
2. It may also be the crack caused by quenching;
3. It may be a tempering crack that transforms from Paralympic to martensite during the tempering process;
4. It may also be caused by hot cracking caused by excessive heating speed during the quenching process;
In the quenching process, due to the difference in surface carbon content, the surface structure transformation occurs first during the cooling process, and then the core structure transformation occurs, which causes surface tensile stress and cracks. In the cooling process, the surface first occurs due to The organization transforms, and then the heart organization transforms, which causes cracks on the surface of the material. The raw material has cracks, and there is decarburization on both sides of the crack after quenching. After quenching, the crack propagates and there is no decarburization on both sides of the expansion part. The surface of the raw material after decarburization and quenching depends on the size of the part, the severity of decarburization, and the process Quenching method and other factors, generally decarburization, the possibility of decarburization at high temperature after quenching is very small, after quenching decarburization and raw material decarburization are superimposed together, decarburization will increase the surface hardness, and so on.
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