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Macro residual stress, also known as first-type residual stress, is the average stress that is distributed over a macroscopic area and spans multiple grains. Its size, direction, and properties can be measured using conventional physical or mechanical methods. Based on the causes of residual stress, macroscopic residual stress can be divided into the following three types. 1. Residual stresses resulting from uneven plastic deformation: Materials often undergo uneven plastic deformation due to processing; that is, the degree of plastic deformation varies in different parts of the material. This inevitably leads to relative compression or tensional deformation between these different parts, thereby generating residual stresses. Processing techniques such as rolling, drawing, extrusion, cutting, and shot blasting can all cause uneven plastic deformation. 2. Residual stresses induced by heat affect: The residual stresses resulting from heat affect are complex. During heating or cooling, a temperature gradient exists within the material. This uneven heating or cooling leads to uneven expansion and contraction, thereby generating thermal stress. And when organizational changes cause uneven volume changes within the material, phase transformation stresses are generated. When plastic deformation occurs due to thermal effects, the mechanical properties of the material itself, such as yield strength and elastic modulus, are also affected, which in turn influences stress changes. 3. Residual stress resulting from chemical reactions: This type of residual stress is generated as a result of chemical or physical changes that occur as energy is transferred from the surface inward. For example, in the case of porcelain, glaze materials are applied to its surface and then heated to form a glaze. Due to the high coefficient of expansion of the glaze, tensile stresses are generated in it after cooling, resulting in cracks. The cracks are regular, with most of them being connected to one another; such cracking allows the tensile stress acting in the direction perpendicular to them to be eliminated. The cracks formed by the cracking of the soil are star-shaped cracks with an angle of 120° between them. During nitriding of steel, a compound layer with a high specific volume is formed on the surface, resulting in significant residual compressive stress. A similar situation occurs during carburizing as well. This is mainly due to changes in density caused by chemical reactions. Micro residual stress refers to the stress within the microscopic scale. Based on the range of its influence, it can be further divided into two categories, namely second-type and third-type residual stress. The second type of residual stress acts between grains or subgrains (within a range of about 0.01–1 mm) and represents the average stress in this range. The third type of residual stress acts within the grains (in a range of approximately 10⁻⁶ to 10⁻² mm). Based on the causes of residual stress, micro-residual stress can be classified into the following types. 1. Microscopic residual stresses arising from the anisotropy of grains. Such residual stresses include those resulting from the anisotropy of properties such as the thermal expansion coefficient and elastic modulus of the crystal, as well as those caused by the different orientations of the grains. Taking the anisotropy of crystal elastic modulus as an example, the elastic modulus of lead single crystals varies by a factor of 1 to 3 depending on the crystal orientation, while that of zinc single crystals varies by a factor of 1 to 4. The elastic modulus of the vast majority of metals is anisotropic; it is generally highest in the direction of the crystal grains and lowest in directions perpendicular to them. In a polycrystal, due to the different orientations of the individual grains, even if the applied external force is uniform, the deformation of each grain may vary. If plastic deformation occurs, the plastic deformation of each grain will also be uneven, which inevitably leads to residual stresses. 2. Residual stresses resulting from plastic deformation within and outside the grains. Such residual stresses include microscopic residual stresses generated by slip within the grains, slip across grain boundaries, and the formation of twins. For example, when there is slip deformation within the grains, dislocations accumulate at the grain boundaries; they may also cross these boundaries to slide over a larger area, resulting in phenomena such as folded bands. Since dislocations do not disappear as they pass through grains, various internal defects are also formed unevenly within the structure at this time. These become the main causes of micro-residual stresses that arise after external forces are removed. 3. Microscopic residual stresses resulting from the presence of different phases due to inclusions, precipitates, or phase transformations: In the metallographic structure, when inclusions, precipitates, or phase transformations give rise to different phases, considerable microscopic residual stresses can be generated as a result of volume changes and thermal stresses.