heat treatment stress
Heat treatment residual force refers to the residual stress of the workpiece after heat treatment, which has an extremely important impact on the shape, size and performance of the workpiece. When it exceeds the yield strength of the material, it causes the deformation of the workpiece. It will crack the workpiece when it exceeds the strength limit of the material. This is a harmful side of it. It should be reduced and eliminated. However, if the stress is controlled to distribute reasonably under certain conditions, the mechanical properties and service life of the parts can be improved, and the harmfulness can be turned into advantage. It is of far-reaching practical significance to analyze the distribution and variation of stress in steel during heat treatment so as to improve the quality of products. For example, the reasonable distribution of surface residual compressive stress on the service life of parts has aroused widespread attention.
(1) heat treatment stress of steel
In the process of heating and cooling, the temperature difference is formed due to the difference between the cooling speed and the time of the surface and the heart, which will lead to the stress of the volume expansion and the uneven contraction, that is, the thermal stress. Under the effect of thermal stress, because the surface temperature is lower than the heart and the contraction is greater than the heart, the heart is pulled. When the cooling end, the final cooling volume of the heart can not be carried out freely. That is to say, under the action of thermal stress, the surface of the workpiece is compressed and the heart is pulled. This phenomenon is affected by factors such as cooling rate, material composition and heat treatment process. The higher the cooling rate, the higher the carbon content and the alloy composition, the greater the uneven plastic deformation produced by the thermal stress in the cooling process, and the greater the final residual stress. On the other hand, when the microstructure changes in the process of heat treatment, when the austenite is transformed into martensite, the volume expansion of the workpiece will be accompanied by the increase of the specific volume, and the phase change of the parts of the workpiece successively, resulting in the uneven volume of the volume and the formation of the tissue stress. The ultimate result of tissue stress change is tensile stress on the surface and compressive stress at the center, just opposite to the thermal stress. The magnitude of the stress is related to the cooling rate, shape and chemical composition of the workpiece in the martensitic transformation zone.
Practice has proved that any workpiece in heat treatment process, as long as there is phase change, thermal stress and tissue stress will occur. Only the thermal stress has been produced before the transformation of the tissue, and the stress of the tissue is produced during the transformation of the tissue. In the whole process of cooling, the result of the combined effect of thermal stress and tissue stress is the actual stress in the workpiece. The result of the combined action of these two stresses is very complicated, which is affected by many factors, such as composition, shape, heat treatment process and so on. In the course of its development, there are only two types, that is, thermal stress and tissue stress, the two offsets when the action is opposite, and the two are overlapping when the direction of action is the same. No matter whether it is counteracting or overlapping each other, the two stresses should have a dominant factor. The result of the thermal stress in the dominant position is the tension of the workpiece's heart and the pressure of the surface. When the tissue stress is dominant, the result is that the surface of the workpiece is pulled under the compression surface.
(two) the effect of heat treatment stress on the quenching crack
The factors that can cause stress concentration on different parts of the quench (including metallurgical defects) can promote the production of quenching cracks, but only in the tensile stress field (especially under the maximum tensile stress) can be shown, if there is no cracking in the compressive stress field.
Quenching cooling rate is an important factor that can affect the quenching quality and determine the residual stress, and is also an important and decisive factor for the quenching crack. In order to achieve the purpose of quenching, it is usually necessary to speed up the cooling rate of the parts in the high temperature section, and make it more martensitic than the critical quenching speed of steel. As far as the residual stress is concerned, this can reduce the tensile stress on the surface of the workpiece and reduce the longitudinal crack because it can increase the thermal stress value that counteracts the stress of the tissue. The effect will increase with the increase of cooling rate at high temperature. Moreover, in the case of quenching, the larger the section size of the workpiece, although the actual cooling rate is slower, the greater the risk of cracking. All this is due to the increasing of the thermal stress of this kind of steel with the increase of the size, the decrease of the actual cooling rate, the decrease of the thermal stress, the increase of the microstructure stress with the increase of the size, and the result of the action characteristic of the tensile stress mainly on the workpiece surface. It is quite different from the traditional idea that the slower the cooling is, the smaller the stress. For such steel parts, only longitudinal cracks are formed in high hardened steel quenched under normal conditions. The reliable principle to avoid quenching is to try to minimize the unequal time martensitic transformation inside and outside the section. Slow cooling in the martensitic transformation zone is not enough to prevent the formation of longitudinal cracks. In general, the arc crack in the non hardenability part can only be produced, although the integral rapid cooling is the necessary forming condition, but the real cause of its formation is not in the rapid cooling (including the martensite transition zone) itself, but the local position of the quench (determined by the geometric structure) and the cooling rate in the critical temperature zone. Significantly slowed down and thus not hardened. The transverse and longitudinal splitting produced in large non hardenability parts is caused by the residual tensile stress of the thermal stress as the main component in the center of the quenching part, while the crack is first formed and expanded from the inside to the outside at the center of the quenching part. In order to avoid such cracks, water oil double fluid quenching process is often used. In this process, the rapid cooling in the high temperature section is carried out only in order to ensure the martensitic structure of the outer metal, and from the point of view of internal stress, fast cooling is harmful. Secondly, the purpose of cooling late cooling is not to reduce the expansion velocity of martensitic transformation and the value of microstructure stress, but to reduce the temperature difference of the cross section and the shrinkage velocity of the metal in the central part of the section as far as possible, thus reducing the stress value and finally restraining the quenching crack.
(three) the effect of residual compressive stress on the workpiece
Carburized surface strengthening is widely used as a method to improve fatigue strength of workpieces. On the one hand, it can effectively increase the strength and hardness of the workpiece surface and improve the wear resistance of the workpiece. On the other hand, carburizing can effectively improve the stress distribution of the workpiece, obtain large residual compressive stress on the surface layer of the workpiece, and improve the fatigue strength of the workpiece. If the austempering is carried out after carburizing, the surface residual compressive stress will be increased, and the fatigue strength will be further improved. 
