The factors (including metallurgical defects) that can cause stress concentration in different parts of the quenching parts, it can promote the quenching crack, but only in the tensile stress field, especially under the maximum tensile stress, it will be displayed, if there is no crack-inducing effect in the compressive stress field.
Quenching cooling speed is an important factor that can influence quenching quality and determine residual stress, and it is also an important factor that can influence quenching crack. In order to achieve the quenching purpose, it is usually necessary to accelerate the cooling rate of the parts at high temperature and make them exceed the critical quenching cooling speed of the steel to obtain the martensite tissue. In the case of residual stress, it can reduce the tensile stress on the workpiece surface and achieve the aim of restraining longitudinal crack because it can increase the thermal stress value of canceling the effect of tissue stress. The effect will increase with the speed of high temperature cooling. Moreover, in the case of quenching, the larger the section size workpiece, although the actual cooling speed is more slow, the risk of cracking is greater. All this is because the thermal stress of this kind of steel increases with the size, the actual cooling rate slows down, the thermal stress decreases, and the tissue stresses increase with the increase of the size, and finally the effect of the tensile stress on the workpiece surface is formed. And the traditional idea that the smaller the slower the cooling stress is, the worse. For such steel parts, longitudinal cracks can only be formed in high hardenability steels quenched under normal conditions. The reliable principle of avoiding quenching is to try to minimize the unequal time of martensite transformation in and out of the section. Only the slow cooling in the martensite transition zone is not sufficient to prevent the formation of longitudinal cracks. In general, only in the non hardenability parts of the arc crack, although the overall rapid cooling as a necessary form of conditions, but it is not the real cause of rapid cooling (including the martensitic transformation Zone) itself, but the quenching part of the local position (by the geometrical structure), in the high-temperature temperature zone, the cooling rate significantly slowed down, Thus no hardening is caused. The transverse break and longitudinal splitting in large non hardenability parts are caused by the residual tensile stress which is the main component of thermal stress in the center of the quenching part, and at the center of the section at the end of quenching, the crack is formed and expanded from inside to outside. In order to avoid this kind of crack to produce, often make water-oil double liquid quenching process. In this process, the rapid cooling in the high temperature section is carried out only to ensure that the outer metal is obtained from martensite, and that it is harmful to the heat from the point of view of internal stress. Secondly, the purpose of cooling in the later stage is not to reduce the velocity of martensite transformation and the value of tissue stress, but to minimize the temperature difference between the section and the central part of the metal shrinkage speed, so as to reduce the stress value and finally inhibit the quenching crack.
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