2 Influence of heat treatment stress on quenching crack of steel
Factors that exist in different parts of the quenched part that can cause stress concentration (including metallurgical defects) have a promoting effect on the generation of quenched cracks, but only in the tensile stress field (especially under the maximum tensile stress) will be manifested, if there is no cracking promotion effect in the compressive stress field.
The quenching cooling rate is an important factor that can affect the quenching quality and determine the residual stress, and also a factor that can give an important and even decisive influence on the quenching crack. In order to achieve the purpose of quenching, it is usually necessary to accelerate the cooling rate of the parts in the high temperature section, and make it exceed the critical quenching cooling rate of steel in order to obtain martensitic structure.
As far as residual stress is concerned, this can reduce the tensile stress on the surface of the workpiece and achieve the purpose of restraining the longitudinal crack by increasing the thermal stress value that counteracts the effect of organizational stress. The effect will increase with the speed of high temperature cooling. Moreover, in the case of quenching, the larger the cross-section size of the workpiece, although the actual cooling rate is slower, the risk of cracking is greater. All this is due to the fact that the thermal stress of this kind of steel slows down with the increase of the size of the actual cooling rate, the thermal stress decreases, the organizational stress increases with the increase of the size, and finally the formation of the tensile stress based on the organizational stress on the surface of the workpiece.
For this type of steel, only longitudinal cracks can be formed in high-hardenability steel hardened under normal conditions. The reliable principle to avoid quenching cracking is to try to minimize the anischronicity of the martensite transition inside and outside the section. Slow cooling in the martensitic transition zone alone is not sufficient to prevent the formation of longitudinal cracks. Under normal circumstances, it can only produce arc cracks in non-hardenable parts, although the overall rapid cooling is the necessary formation conditions, but its real cause of formation is not in the rapid cooling (including martensitic transition zone) itself, but the local location of the quenching part (determined by the geometric structure), the cooling rate in the high temperature critical temperature zone is significantly slowed down, and therefore not caused by hardening.
The transverse break and longitudinal split produced in the large non-hardenable parts are caused by the residual tensile stress with thermal stress as the main component acting on the center of the quenched part, and at the center of the section of the quenched part at the end of the quenched part, the crack is first formed and expanded from the inside out. In order to avoid such cracks, the water-oil dual-liquid quenching process is often used. In this process, rapid cooling in the high temperature section is implemented only to ensure that the outer metal gets martensitic structure, which is harmful from the point of view of internal stress. Secondly, the purpose of slow cooling at the later stage of cooling is not to reduce the expansion rate and the stress value of the martensitic phase transition, but to minimize the temperature difference of the section and the shrinkage rate of the metal in the center of the section, so as to reduce the stress value and ultimately inhibit the quenching crack.
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