Chromium
Adding chromium to steel can significantly improve the steel’s anti-oxidation effect and increase its corrosion resistance. Chromium and iron form a continuous solid solution and form a variety of compounds with carbon. Among the elements that form carbides, chromium’s affinity for carbon is greater than that of iron and manganese, but lower than that of tungsten and molybdenum. It can replace part of iron to form a composite cementite, and this complex carbide can improve the wear resistance of steel. Chromium and iron can form intermetallic compounds. Chromium can also significantly increase the hardenability of steel, but it also tends to increase the temper brittleness of steel. High-chromium steel is also very easy to become brittle if heat treated improperly.
Nickel
Pure nickel steel is rarely used in industry at present. This is because nickel and other alloying elements (especially chromium and molybdenum) have better performance when used in combination. Nickel has a certain corrosion resistance. When nickel is added to iron, especially when the nickel content is high (reaching 15%-20%), it has strong corrosion resistance to sulfuric acid, hydrochloric acid, air and seawater, but cannot resist nitric acid corrosion. It exists in the α phase and γ phase in steel, strengthening it, and improving the low-temperature toughness of steel by refining the grains of the α phase. Nickel can improve the hardenability of steel, but its effect on temper brittleness is not as significant as that of manganese and chromium. When the nickel content is high, it can significantly improve the linear expansion coefficient, thermal conductivity and resistivity of steel, so it is also an important element for refining certain special materials. Nickel-containing steel is prone to banded structure and white spot defects, and must be strictly paid attention to during the production process.
Tungsten
Tungsten has a high melting point and high density. It often combines with carbon to form special compounds, and can also be partially dissolved in iron to form a solid solution. Tungsten in steel mainly increases tempering stability, red hardness, heat strength and wear resistance, and can also improve the creep ability of steel at high temperatures. Tungsten added to steel slightly increases hot melting and strongly reduces thermal conductivity, but can significantly improve the coercive force and residual magnetic induction of steel. Tungsten can also improve the stability of steel against hydrogen. The cast structure of high-tungsten steel is severely segregated, and there is a tendency for surface decarburization when heated.
Molybdenum
Molybdenum exists in the solid solution phase and carbide phase in steel. In the carbide phase, when the molybdenum content is low, it forms cementite with iron and carbon; when the carbon content is high, it forms special carbides. The role of molybdenum in steel can be summarized as improving hardenability, heat strength, preventing temper brittleness, improving remanence and coercivity, corrosion resistance in certain media, and preventing pitting tendency. Molybdenum has a solid solution strengthening effect on ferrite, and also improves the stability of carbides, so it also has a favorable effect on the strength of steel. Steel with a molybdenum content of more than 3% has poor oxidation resistance at high temperatures. High-molybdenum steel is prone to demolybdenum and decarburization when heated, which should be strictly paid attention to in production. Molybdenum is not easy to burn and can be added at any stage of smelting.
