Heat treatment residual force refers to the residual stress of the workpiece after heat treatment, which has an extremely important influence on the shape, size and performance of the workpiece. When it exceeds the yield strength of the material, it will cause deformation of the workpiece, and when it exceeds the strength limit of the material, it will crack the workpiece. This is its harmful side and should be reduced and eliminated.
But under certain conditions, controlling the stress to make it reasonably distributed can improve the mechanical performance and service life of the parts, and turn harm into profit. Analyzing the distribution and change law of stress in steel during heat treatment, and making it reasonable distribution has far-reaching practical significance for improving product quality. For example, the influence of the reasonable distribution of surface residual compressive stress on the service life of parts has attracted people’s attention.
1. Heat treatment stress of steel
In the process of heating and cooling the workpiece, due to the inconsistency of the cooling rate and time between the surface and the core, a temperature difference is formed, which will cause uneven volume expansion and contraction to produce stress, that is, thermal stress. Under the action of thermal stress, because the surface temperature is lower than the core part, and the contraction is greater than the core part, the core part is stretched. When the cooling is over, the final cooling volume shrinkage of the core part cannot proceed freely and the surface layer is compressed. Stretched. That is, under the action of thermal stress, the surface of the workpiece is compressed and the core is pulled.
This phenomenon is affected by factors such as cooling rate, material composition and heat treatment process. When the cooling rate is faster, the carbon content and alloy composition are higher, and the uneven plastic deformation under the action of thermal stress during the cooling process is larger, and the residual stress formed finally becomes larger. On the other hand, due to the change of the structure of the steel during the heat treatment process, that is, the transformation of austenite to martensite, the increase in specific volume will be accompanied by the expansion of the volume of the workpiece, and the phases of each part of the workpiece will change successively, resulting in inconsistent volume growth and structure. stress. The final result of the change in tissue stress is the tensile stress on the surface and the compressive stress on the core, which is just the opposite of the thermal stress. The size of the structure stress is related to the cooling rate of the workpiece in the martensite transformation zone, the shape, and the chemical composition of the material.
Practice has proved that as long as there is phase change in any workpiece in the heat treatment process, thermal stress and structural stress will occur. It’s just that the thermal stress has been generated before the transformation of the structure, and the structure stress is generated during the transformation of the structure. During the entire cooling process, the result of the combined effect of the thermal stress and the structure stress is the actual stress in the workpiece.
The result of the combined action of these two stresses is very complex and is affected by many factors, such as composition, shape, and heat treatment process. As far as its development process is concerned, there are only two types, namely thermal stress and tissue stress. When the direction of action is opposite, the two cancel out, and when the direction of action is the same, the two are superimposed. Regardless of whether they cancel each other or superimpose each other, the two stresses should have a dominant factor. When the thermal stress is dominant, the result is that the core of the workpiece is pulled and the surface is compressed. The result of the effect when the tissue stress is dominant is the tension of the compressed surface of the workpiece core.
2. The effect of heat treatment stress on quenching cracks
The factors that can cause stress concentration (including metallurgical defects) in different parts of the quenched parts have a promoting effect on the generation of quenching cracks, but only in the tensile stress field (especially under the maximum tensile stress). If there is no crack promoting effect in the compressive stress field.
Quenching cooling rate is an important factor that can affect the quality of quenching and determine the residual stress, and it is also a factor that can have an important and even decisive influence on quenching cracks. 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 to obtain the martensite structure.
In terms of residual stress, this can increase the thermal stress value that offsets the effect of the tissue stress, so it can reduce the tensile stress on the surface of the workpiece and achieve the purpose of suppressing longitudinal cracks. The effect will increase as the high temperature cooling rate increases. Moreover, in the case of hardening, the larger the cross-sectional 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 type 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, and finally the tensile stress mainly formed by the structural stress acts on the workpiece. Caused by the characteristics of the surface. And it is very different from the traditional concept that the slower the cooling, the smaller the stress. For this type of steel, only longitudinal cracks can be formed in high hardenability steels that are quenched under normal conditions.
The principle to avoid quench cracking is to try to minimize the unequal time of martensite transformation inside and outside the section. Merely implementing slow cooling in the martensite transformation zone is not enough to prevent the formation of longitudinal cracks. Under normal circumstances, only arc cracks can occur in non-hardenable parts. Although the overall rapid cooling is the necessary forming condition, the real reason for its formation is not the rapid cooling (including the martensite transformation zone) itself. But the local position of the quenched part (determined by the geometric structure), the cooling rate in the high temperature critical temperature zone is significantly slowed down, so there is no hardening. The transverse fractures and longitudinal splits produced in 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. At the center of the quenched section of the quenched part, cracks are first formed and caused by Caused by expansion from the inside out.
In order to avoid such cracks, the water-oil double liquid quenching process is often used. In this process, the purpose of implementing rapid cooling in the high temperature section is only to ensure that the outer layer of metal has a martensitic structure; from the point of view of internal stress, rapid cooling is harmful and useless at this time. Secondly, the purpose of slow cooling in the later stage of cooling is not to reduce the expansion rate of martensite transformation and the value of organizational stress, 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 The purpose of suppressing cracking.