How are residual stresses generated by surface heat treatment?

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Residual stress from surface heat treatment: After heat treatments such as quenching, residual stress is generated within the material. If the shape and volume of various parts within the material change unevenly, the generation of residual stress is inevitable. The magnitude and distribution of residual stresses resulting from heat treatment have a significant impact on the mechanical properties of materials, and they are the cause of various defects. Whether residual stresses from heat treatment are harmful to a part depends mainly on the distribution of these stresses; this requires designers and manufacturers to make use of residual stresses effectively in order to improve the mechanical properties of the parts. For example, after carburizing, surface quenching, and thin-shell quenching, residual compressive stress can be formed on the surface of gears, and this compressive stress helps to increase the fatigue life of the gears. It is generated by thermal stress. When the specimen is quenched rapidly, no phase transformation occurs within the specimen; the diagram below shows the process by which thermal residual stress is generated under such conditions. During the rapid cooling of the specimen, a temperature difference arises due to the different cooling conditions in the surface layer (R) and the core (K), which in turn generates thermal stress. Figure a below shows the cooling curves of the surface and core layers of the specimen, while figure b shows the corresponding heat stress variation curves. The stress levels in the surface layer and the core vary depending on the temperature difference. Ⅰ represents the surface stress of the specimen in its elastic state, while Ⅱ and Ⅲ represent the actual stresses at the surface and in the core, respectively. Figure c below shows the residual stress distribution curve along the radius of the specimen. The surface layer of the specimen is under compressive stress, while the core is under tensile stress; this is the general pattern of residual stress distribution of the \"thermal stress type\". In this case, the magnitude of the residual stress in the specimen depends on the temperature difference during cooling and the yield strength of the material. The image is caused by phase transformation stress. Phase transformation stress is the stress that arises in metal materials during heat treatment phase transformations, including stress resulting from non-uniform phase transformations (microstructural stress) and stress resulting from asynchronous phase transformations (additional stress). Both types of phase transformation stresses are caused by the differences in specific volume among different microstructures. For example, during quenching of a part’s surface, since the specific volume of the martensite structure in the surface layer is greater than that in the core, residual compressive stress is generated in the surface layer, while tensile stress occurs in the core. This residual stress distribution is caused by non-uniform phase transformation. During the overall quenching of carbon steel parts, the parts are first heated above the austenite transformation temperature, held at that temperature for a certain period of time, and then rapidly cooled to obtain a martensitic structure. During such a heat treatment process, differences in the cooling rates of the surface layer and the core cause a delay in phase transformation, ultimately resulting in a stress distribution with tensile stress in the surface layer and compressive stress in the core. In other words, this residual stress distribution is caused by heterogeneous phase transformation. Final residual stress: In addition to thermal stress and phase transformation stress, changes in the material’s chemical composition can also generate stress. For example, chemical heat treatment methods such as carburizing and nitriding alter the chemical composition of the part’s surface, either by increasing the carbon content or raising the nitrogen content. After chemical heat treatment, a high residual compressive stress is generated on the surface of the parts. Conversely, if decarburization occurs in the part during heating, the carbon content in the surface layer decreases, and the residual compressive stress in the surface layer will transform into tensile stress. The figure below shows the various stress distributions that occur in large-section steel components after quenching and water cooling. Figure a below shows the thermal stress distribution, with compressive stress at the surface layer and tensile stress in the core ； Figure b below shows the stress induced by asynchronous phase transformation, with tensile stress in the surface layer and compressive stress in the core. On the other hand, due to the large cross-section of the steel component, it is not possible to harden the entire cross-section, which results in stresses caused by uneven phase transformation as shown in figure c below. All these stresses combine to form the final residual stress as shown in figure d below. The simple superposition of stresses in Figures 1–7 can only be used for a qualitative explanation; the actual situation is much more complex. The formation times of thermal stress, asynchronous phase transformation stress, and heterogeneous phase transformation stress are not the same. Obviously, the stress that appears first will inevitably affect the formation of subsequent stresses; similarly, the stresses that arise later will also cause the previously formed stresses to be redistributed. In short, they all have a significant impact on one another.