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Macroscopic residual stress. Macroscopic residual stress, also known as the first type of residual stress, is the average stress that is distributed over a macroscopic area and spans multiple grains. Its size, direction, and nature, etc., can be measured using conventional physical or mechanical methods. Based on the causes of their formation, macroscopic residual stresses can be classified into the following three types. 1. Residual stresses caused by non-uniform plastic deformation: Materials often undergo non-uniform plastic deformation due to processing; that is, the degree of plastic deformation varies across different parts of the material. This inevitably leads to relative compression or tensile deformation between these parts, thereby generating residual stresses. Processing techniques such as rolling, drawing, extrusion, cutting, and shot peening all cause non-uniform plastic deformation. 2. Residual stresses induced by heat influence The residual stresses caused by heat influence are complex. During heating or cooling, a temperature gradient develops within the material. This uneven heating or cooling leads to non-uniform thermal expansion and contraction, thereby generating thermal stresses. When organizational changes cause non-uniform volume changes within the material, phase transformation stresses are generated. When plastic deformation occurs due to thermal effects, the mechanical characteristic values of the material itself, such as its yield strength and elastic modulus, are also affected, which in turn influences the stress changes. 3. Residual stress caused by chemical actions This type of residual stress arises from chemical or physical changes that are transmitted from the surface to the interior. Take porcelain, for example: a glaze material is applied to its surface and then heated to form the glaze. Since the glaze has a relatively high coefficient of thermal expansion, cooling causes tensile stresses to develop in it, resulting in cracking. The cracks are regular; most of them are interconnected with one another. This kind of cracking can eliminate the tensile stress acting perpendicular to them. The cracks formed by the cracking of the soil are star-shaped cracks with intersection angles of exactly 120°. When steel is nitrided, a compound layer with a relatively large specific volume forms on its surface, resulting in significant residual compressive stress on the surface. A similar situation also occurs during carburizing. This is mainly due to changes in density caused by chemical changes. Microscopic residual stresses are stresses that exist within the microscopic range. Based on their area of influence, they can be further divided into two categories: Type II and Type III residual stresses. The second type of residual stress acts between grains or sub-grains (in the range of approximately 0.01–1 mm); it is the average stress within this range. The third type of residual stress acts within the grains (in the range of approximately 10⁻⁶ to 10⁻² mm). Based on the causes of residual stress, microscopic residual stresses can be classified into the following types. 1. Microscopic residual stresses arising from the anisotropy of grains. These residual stresses result from the anisotropy of the thermal expansion coefficient, elastic modulus, etc., of crystals, as well as the differences in orientation between grains. Taking the anisotropy of crystal elastic moduli as an example, in single crystals of lead, the elastic modulus varies by a factor of 1 to 3 depending on the crystal orientation; for single crystals of zinc, this variation is from 1 to 4 times. The elastic modulus of the vast majority of metals is anisotropic; generally, it is maximum in certain crystal directions and minimum in others. In polycrystals, due to the different orientations of individual grains, even when the applied external force is uniform, the deformations of the grains may vary. In such cases, if plastic deformation occurs, it will also be non-uniform among the grains; consequently, residual stresses are inevitably generated. 2. Residual stresses caused by plastic deformation inside and outside the grains. These 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 a grain, dislocations accumulate at the grain boundaries; they may also cross the boundaries and undergo slip over a wider area, resulting in phenomena such as kink bands. Since dislocations pass through grains without disappearing, various internal defects also form unevenly within the structure at this time. These become the main causes of microscopic residual stresses after the removal of external forces. 3. Microscopic residual stresses arising from the formation of different phases due to inclusions, precipitate phases, or phase transformations. In the metallographic structure, when inclusions, precipitate phases, or phase transformations lead to the formation of different phases, considerable microscopic residual stresses may be generated due to volume changes and thermal stress effects.