Effect of heat treatment on quenching crack

In the process of quenching, when the huge stress produced by quenching is greater than the strength of the material itself, it will lead to cracks. Quenching cracks are often produced shortly after the martensitic transformation begins. The distribution of cracks is not regular, but it is easy to form in the sharp corner notch and section of the workpiece.

Effect of heat treatment on quenching crack

Heat treatment exists in different parts of quenched parts, which can cause stress concentration. It can promote the generation of quenched cracks, but only in the tensile stress field. If there is no crack promoting effect in the compressive stress field, here is a specific introduction to the effect of thermal stress on quenched cracks:

Quenching cooling rate is an important factor that can affect the quenching quality and determine the residual stress. It is also an important and even decisive factor for quenching cracks. In order to achieve the purpose of quenching, it is usually necessary to speed up the cooling rate of parts in the high temperature section, and make it exceed the critical quenching cooling rate of steel to obtain martensitic structure.

In the case of hardenability, the larger the section size, the slower the actual cooling speed, but the greater the risk of cracking. All of these are due to the fact that the thermal stress of this kind of steel slows down with the increase of the size, the thermal stress decreases, and the structural stress increases with the increase of the size. Finally, the tensile stress mainly composed of the structural stress acts on the surface of the workpiece. For this kind of steel, the longitudinal crack can only be formed in the high hardenability steel quenched under normal conditions. The reliable principle to avoid quenching crack is to try to minimize the unequal time of martensitic transformation inside and outside the section. The slow cooling in martensitic transformation zone is not enough to prevent the formation of longitudinal crack.

In general, cracks can only occur in non hardenable parts. Although the integral rapid cooling is the necessary forming condition, the real reason for its formation is not the rapid cooling itself, but the local position of the quenched parts. The cooling speed in the high temperature critical temperature zone is significantly reduced, so it is not caused by hardening. The transverse and longitudinal splits produced in the large non hardenable parts are caused by the residual tensile stress with the thermal stress as the main component acting on the center of the quenched part, while at the center of the section of the quenched part which is finally hardened, the cracks first form and propagate from inside to outside. In order to avoid this kind of crack, water oil quenching process is often used.

Quenching crack refers to the crack produced in the process of heat treatment or in the process of placing at room temperature after quenching. There are many reasons for quenching crack, one of which is heat treatment. It can be seen that Yantai heat treatment has certain influence on quenching crack.

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