Fundamentals of Metal Materials: Metal Segregation

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The phenomenon of non-uniform chemical composition that occurs in metals during the solidification process due to certain factors is called 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 composition of the solid and liquid phases continues to change, the parts of a single crystal that solidify later will have a different composition from those that solidify earlier. In other words, the areas closer to the center are richer in components with higher melting points, while the areas 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 as the temperature decreases, the diffusion rate drops sharply. Therefore, under normal cooling conditions in actual production, the diffusion process often lags behind the solidification and cooling processes. Due to insufficient diffusion, there is a non-uniform distribution of components within the crystal range in the solidified metal; this is known as intragranular segregation. This segregation often leads to the formation of dendritic structures in metals; hence it is also known as dendritic segregation. For the same reason, in solid solution metals, the composition of the crystals that solidify later also differs from that of the crystals that solidify first. Furthermore, in any metal, the last-to-solidify portion between the various dendritic crystals usually consists 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 non-uniformity between crystals, that is, intergranular segregation. Carbide segregation is a type of intergranular segregation that frequently occurs in alloy tool steels and high-speed steels in the as-cast state. During forging or hot rolling, these large carbides are broken up and deformed along the direction of processing, resulting in discontinuous band-like carbides distributed throughout the steel matrix. This leads to differences in the longitudinal and transverse mechanical properties of the steel; in particular, its transverse plasticity is significantly reduced. During the processing and use of steel, due to this segregation, other defects may occur. Therefore, in alloy tool steels and high-speed steels, every effort is made to avoid or reduce such segregation. ② Regional segregation occurs during the casting of ingots (or castings) due to intense directional 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 (cast), but rather begins in the outer layer that is in contact with the mold walls. As a result, the primary crystals enriched with the high-melting-point component precipitate right against the mold wall, 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, the composition of the solution in the edge region becomes identical to that of the solution in the core. Consequently, this inevitably leads to the outer region being enriched with high-melting-point components, while the core becomes enriched with low-melting-point components; it also accumulates non-metallic impurities and gases precipitated 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 components manufactured from it in use. Sulfur segregation can disrupt the continuity of the metal; it causes hot shortness during steel forging and leads to laminations when rolling steel plates, thereby severely affecting the cold bending properties of the plates. In components subjected to alternating loads, sulfur segregation often serves as one of the main causes of fatigue fracture during service. 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 inhomogeneity in chemical composition is called specific gravity segregation. The greater the density difference between the crystals and the remaining solution, the greater the gravity segregation. This density difference depends on the density difference between the metal components, as well as the compositional difference between the crystal and the solution. The slower the cooling process, the more slowly the number of primary crystals increases as the temperature drops; consequently, the temperature range within which the crystals can freely float and settle in the solution becomes wider, leading to more pronounced density segregation. 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.