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The uneven distribution of chemical composition in metals during the condensation process, caused by certain factors, is known as segregation. There are several types of segregation that occur in metals. ① Intra-grain segregation and inter-grain segregation: During the solidification process of solid solution metals, as the compositions of the solid and liquid phases continue to change, the areas that solidify later within the same crystal will have different compositions from those that solidify earlier; in other words, the areas closer to the center are richer in components with higher melting points, while those closer to the edges are richer in components with lower melting points. This difference in composition can be homogenized through the diffusion of components, thereby bringing the metal composition to an equilibrium state. However, diffusion in crystals is a slow process, and the diffusion rate decreases sharply as the temperature drops. Therefore, under the normal cooling conditions in actual production, the diffusion process often lags behind the solidification and cooling processes. Due to insufficient diffusion, compositional inhomogeneity within the crystals exists in the solidified metal, that is, intragranular segregation. This segregation often leads to the formation of dendritic structures in the metal, which is why it is also known as dendrite segregation. For the same reason, in solid solution metals, the crystals that solidify later also have a different composition from those that solidify first. Furthermore, in any metal, the last-to-solidify parts between the various dendritic crystals usually consist of low-melting-point components and inevitable impurities, which differ from the composition of the crystal itself. Both of the above situations are instances of compositional inhomogeneity between crystals, that is, intergranular segregation. Carbide segregation is a type of intergranular segregation that often occurs in cast alloy tool steels and high-speed steels. During forging or hot rolling, these large carbides are broken apart and deformed in the direction of processing, resulting in discontinuous strip-like carbides distributed within the steel matrix. This leads to differences in the mechanical properties of the steel in the longitudinal and transverse directions, with the plasticity in the transverse direction decreasing significantly. During the processing and use of steel, the presence of such segregation can lead to the formation of other defects. Therefore, in alloy tool steels and high-speed steels, every effort should be made to avoid or mitigate such segregation. ② Regional segregation occurs during the casting of ingots (or castings) due to intense directed heat loss through the walls of the mold, which creates a large temperature difference within the alloy that is in the process of solidifying. Solidification does not occur simultaneously across the entire cross-section of the ingot (casting); it begins first in the outer layer in contact with the mold wall. As a result, the primary crystals enriched with the high-melting-point component precipitate in close proximity to the mold walls, while the solution in the area in contact with these crystals becomes enriched with the low-melting-point component. Under favorable conditions, before solidification in the core area, the composition of the solution in the edge region becomes consistent with that of the solution in the core. As a result, the outer regions inevitably become enriched in components with high melting points, while the core is enriched in components with low melting points, as well as in non-metallic impurities and gases that precipitate during solidification. This type of segregation is called regional segregation. Regional segregation in steel ingots, particularly sulfur and phosphorus segregation, significantly reduces the quality of the steel and causes various difficulties during subsequent processing; it can even lead to severe damage to the material and failure of the manufactured components in use. If sulfur segregation disrupts the continuity of the metal, it can cause thermal brittleness during steel forging and lead to interlayers during steel plate rolling, severely affecting the cold bending properties of the steel plates. In components subjected to alternating loads, sulfur segregation is often one of the main causes of fatigue fracture during use. Figure 1 shows the sulfur print of sulfur segregation. Phosphorus segregation endows steel with cold brittleness and promotes its temper brittleness. Figure 1 Sulfur segregation ③ Specific gravity segregation: During the solidification of metal, if the density of the crystals that form differs from that of the remaining solution, these crystals tend to sink or float within the solution. The resulting unevenness in chemical composition is known as specific gravity segregation. The greater the density difference between the crystal and the remaining solution, the greater the specific gravity segregation. This density difference depends on the density difference of the metal components, as well as the compositional difference between the crystal and the solution. The slower the cooling, and the slower the increase in the number of primary crystals as the temperature drops, the wider the temperature range within which the crystals can float freely in the solution; consequently, the density segregation becomes more pronounced. Due to the spatial separation of various organizational components, it is impossible to eliminate or reduce specific gravity segregation through heat treatment; instead, special measures such as melting or pouring (for example, rapid pouring) must be employed to prevent it.