Effect of heat treatment residual stress on quenching
Factors (including metallurgical defects) that can cause stress concentration in different parts of the quenching parts can promote the formation of quenching cracks, but only in the tensile stress field (especially under the maximum tensile stress) can it be shown that there is no cracking promotion in the compressive stress field.
Quenching cooling rate is an important factor that can influence the quality of quenching and determine the residual stress. In order to achieve the purpose of quenching, the martensite structure must be obtained by accelerating the cooling speed of parts in the high temperature section and making it exceed the critical quenching speed of steel.
In terms of residual stress, it can reduce the tensile stress on the workpiece surface by increasing the value of thermal stress which can offset the stress on the structure. The effect will increase with the high temperature cooling rate. Moreover, in the case of quenching, the larger the section size of the workpiece, although the actual cooling speed is slower, the greater the risk of cracking. All these are caused by the fact that the thermal stress of this kind of steel decreases with the increase of the size, the actual cooling speed slows down, the thermal stress decreases, and the tissue stress increases with the increase of the size. Finally, the action characteristics of the tensile stress mainly acting on the workpiece surface are formed. And contrary to the traditional idea that the slower the cooling, the less stress. For this type of steel, only longitudinal cracks can be formed in hardened high - hardenable steel parts under normal conditions.
The principle to avoid cracking is to try to minimize the anisochrony of martensite transformation inside and outside the section. Slow cooling in martensite transition zone alone is not enough to prevent longitudinal fracture. Usually can only be produced in the hardenability of arc crack, although the necessary forming conditions of the rapid cooling as a whole, but it's really the reasons, but not in rapid cooling (including the martensitic transformation zone) itself, but a partial quenching position (determined by the geometrical structure), in the critical temperature of high temperature zone significantly slow cooling rate, thus no hardening. The transverse fracture and longitudinal cleavage in large non-quenchable parts are caused by the residual tensile stress with thermal stress as the main component acting on the center of the quenching part, while at the center of the section hardened at the end of the quenching part, cracks are first formed and expanded from the inside to the outside.
In order to avoid such cracks, water - oil quenching process is often used. In this process, rapid cooling is carried out in the high temperature section only to ensure that the outer metal gets martensite tissue. From the perspective of internal stress, rapid cooling is harmful and useless. Secondly, the purpose of slow cooling at the later stage of cooling is not to reduce the expansion speed and tissue stress value of martensite phase transition, but to minimize the temperature difference of the section and the shrinkage speed of metal at the center of the section, so as to reduce the stress value and ultimately inhibit the cracking.