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Prevent steel from quenching crack and select steel from hardenability

In general, steels with good hardenability tend to have large quenching crack. However, steels having the same hardenability have less quench cracks in steels having lower carbon content. The hardenability of steel is generally expressed by the ideal critical diameter DI, and the larger the DI, the better the hardenability. DI varies with C%, the type and content of alloying elements, and grain size. For example, 0.4% C, 0.25% Si, 0.70% Mn, 1.0% Cr, 0.3% Ni steel quenching when quenched in oil (grain grade 8), Calculate DI worth DI=68.58mm, as shown 0.4% C consideration, do not worry about quenching.


When it is necessary to use a steel with good hardenability, in order to avoid quenching cracks, a milder quenching medium can be used to achieve quench hardening and strengthening. For example, steels such as Cr12MoV and W18Cr4V can be hardened by oil quenching, salt bath quenching, and air quenching. Therefore, in order to avoid quenching and cracking, it is one of the parts design schemes to select a steel material with better hardenability and a moderate quenching medium. For example, if a part is made of No. 20 steel, it can only get hardness greater than HRC58 after quenching in water. The part is porous and thin, and it is easy to crack and scrap when quenched in water. It is now made of 20CrMnTi steel with better hardenability. After carburizing with oil, it can achieve the required hardness and reduce cracks.


When choosing steel, you should also pay attention to the level of martensite point (Ms). As mentioned above, in general, the lower the Ms point, the greater the tendency to quench cracking. When the Ms point is high, the martensite formed by the phase change may be self-tempered immediately, thereby digesting a part of the phase transformation stress and avoiding quenching. According to the experimental results, the Ms point is lowered by 8 ° C, and the quenching probability can be increased by 6 times. Therefore, steel with a slightly higher Ms point should be selected.


Although the selection of parts with good hardenability and the use of a milder cooling medium during quenching can reduce quenching and cracking, this will increase the material cost economically. Therefore, comprehensive consideration is needed to determine trade-offs.