Analysis of heat treatment stress and its effects

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Analysis of heat treatment stress and its effects Heat treatment residual stress refers to the final stress remaining after heat treatment of the workpiece, 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 will cause deformation of the workpiece. When it exceeds the strength limit of the material, it will cause the workpiece to crack. This is its harmful side and should be reduced and eliminated. However, by controlling the stress to make it reasonably distributed under certain conditions, the mechanical properties and service life of the parts can be improved, turning harmful effects into beneficial effects. Analyzing the distribution and change patterns of stress during the heat treatment process of steel and making it reasonably distributed has far-reaching practical significance for improving product quality. For example, the impact of reasonable distribution of surface residual compressive stress on the service life of parts has attracted widespread attention. 01 Heat treatment stress of steel During the heating and cooling process of the workpiece, due to the inconsistent cooling speed and time between the surface layer and the core, a temperature difference is formed, which will lead to uneven volume expansion and contraction and generate stress, that is, thermal stress. Under the action of thermal stress, since the initial temperature of the surface layer is lower than that of the core, the shrinkage is also greater than that of the core, causing the core to be stretched. When cooling is completed, the final cooling volume shrinkage of the core cannot proceed freely, causing the surface to be compressed and the core to be stretched. That is, under the action of thermal stress, the surface layer of the workpiece is eventually compressed and the core is stretched. 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, the uneven plastic deformation generated under the action of thermal stress during the cooling process is greater, and the final residual stress is greater. Practice has proved that during the heat treatment of any workpiece, as long as there is a phase change, thermal stress and tissue stress will occur. It’s just that thermal stress has already been generated before the tissue transformation, while tissue stress is generated during the tissue transformation process. During the entire cooling process, the combined effect of thermal stress and tissue stress is the actual stress in the workpiece. Fe-Fe3C phase diagram 02 The influence of heat treatment stress on quenching cracks There are factors that can cause stress concentration in different parts of the quenched parts (including metallurgical defects), which promote the generation of quenching cracks, but they will only appear in the tensile stress field, especially under the maximum tensile stress. If there is no cracking 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. It is also a factor that can have an important or 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 part in the high temperature section and make it exceed the critical quenching cooling rate of the steel to obtain the martensite structure. As far as residual stress is concerned, this can increase the value of thermal stress that offsets the effect of tissue stress, so it can reduce the tensile stress on the surface of the workpiece to suppress longitudinal cracks. The effect will increase with the acceleration of high-temperature cooling. Moreover, if the workpiece can be hardened, the larger the cross-sectional size of the workpiece, although the actual cooling rate is slower, the risk of cracking will be greater. Quenching Cracks 03 Summary 1. The stress generated during heat treatment is inevitable and often harmful. But we can control the heat treatment process to make the stress distribution as reasonable as possible, so that the harmfulness can be reduced to the minimum, or even become harmful. 2. When thermal stress dominates, the stress distribution is that the core is under tension and the surface is under compression. When tissue stress is dominant, the stress distribution is that the core is under compression and the surface is under tension. 3. Longitudinal cracks are easy to form in highly hardenable steel parts, arc cracks are often formed in non-hardenable workpieces, and transverse and longitudinal splits are easy to form in large non-hardenable workpieces. 4. Carburizing reduces the starting temperature (Ms) of martensite in the surface layer, which can cause the martensite transformation sequence to be reversed during quenching. Martensite transformation occurs first in the center and then spreads to the surface. The residual compressive stress of the surface layer can be obtained to improve the fatigue strength. 5. Isothermal quenching after carburizing can ensure that the martensite transformation in the core is fully carried out before the surface structure transformation proceeds, so that the workpiece can obtain a greater surface residual compressive stress than direct quenching, which can further improve the fatigue strength of carburized parts. 6. The composite surface strengthening process can make the residual compressive stress distribution on the surface layer more reasonable and significantly improve the fatigue strength of the workpiece.