What Is Alloy Structural Steel

This kind of steel has suitable hardenability, after suitable metal heat treatment, the microstructure is uniform sorbite, bainite or very fine pearlite, so it has high tensile strength and yield strength. Ratio (usually around 0.85), higher toughness and fatigue strength, and lower toughness-brittle transition temperature, can be used to manufacture machine parts with larger cross-sectional dimensions.

The role of alloying elements in structural steel

There are three aspects:

① Increase the hardenability of steel. Hardenability refers to the depth of the martensite layer from the surface layer when the steel is quenched, and is the main parameter for achieving good overall performance. Except for Co, almost all alloying elements such as Mn, Mo, Cr, Ni, Si and C, N, B, etc. can improve the hardenability of steel. Among them, Mn, Mo, Cr, B have the strongest effect, followed by Ni , Si, Cu. The strong carbide forming elements such as V, Ti, Nb, etc., can increase the hardenability of steel only when they are dissolved in austenite.

② Affect the tempering process of steel. Since alloying elements can hinder the diffusion of various atoms in steel during tempering, compared with carbon steel at the same temperature, they generally delay the decomposition of martensite and the accumulation and growth of carbides, thereby improving The tempering stability of steel, that is, the improvement of the resistance to tempering and softening of steel, the effects of V, W, Ti, Cr, Mo, and Si are more significant, while the effects of Al, Mn, and Ni are not obvious. Steel containing higher content of carbide forming elements such as V, W, Mo, etc., when tempered at 500-600℃, precipitates fine and dispersed special carbide spots such as V4C3, Mo2C, W2C, etc., instead of part of the coarser alloy Cementite, so that the strength of the steel no longer decreases but increases, that is, secondary hardening occurs (see tempering). Mo can prevent or weaken the temper brittleness of steel.

③ Affect the strengthening and toughening of steel. Ni strengthens ferrite by solid solution strengthening; carbide forming elements such as Mo, V, and Nb improve the yield strength of steel by both dispersion hardening and solid solution strengthening; carbon has the most significant strengthening effect. In addition, adding these alloying elements generally refines austenite grains and increases the strengthening effect of grain boundaries. The factors affecting the toughness of steel are more complex. Ni improves the toughness of steel; Mn is easy to coarsen austenite grains and is sensitive to temper brittleness; reducing the content of P and S and improving the purity of steel are important for improving the toughness of steel. Function (see strengthening of metal).

Alloy structural steel is generally divided into quenched and tempered structural steel and surface hardened structural steel.

① The carbon content of quenched and tempered structural steel is generally about 0.25% to 0.55%. For structural parts with a predetermined cross-sectional size, if quenched and tempered along the cross-section during quenching and tempering treatment (quenching and tempering), the mechanical properties Good. If the quenching is not through and free ferrite appears in the microstructure, the toughness will decrease. For steels with a tendency to temper brittleness, such as manganese steel, chromium steel, nickel-chromium steel, etc., it should be cooled quickly after tempering. The quenching critical diameter of this type of steel increases with the increase in grain size and alloying element content. For example, 40Cr and 35SiMn steels are about 30-40mm, while 40CrNiMo and 30CrNi2MoV steels are about 60-100mm. They are often used in manufacturing to withstand Structural parts such as shafts and connecting rods with larger loads.

② Surface-hardened structural steel is used to manufacture parts with hard and wear-resistant surface and flexible heart, such as gears and shafts. In order to make the core part of the parts high in toughness, the carbon content in the steel should be low, generally 0.12 to 0.25%, and there are also appropriate amounts of alloying elements to ensure proper hardenability. Nitrided steel also needs to add alloying elements that easily form nitrides (such as Al, Cr, Mo, etc.). Carburized or carbonitrided steel, after carburizing or carbonitriding at 850～950℃, quenched and used under low temperature tempering (about 200℃). Nitrided steel is subjected to nitriding treatment (480～580℃) and used directly without quenching and tempering treatment.