Alloy Structural Steel


The Alloy structural steel has suitable hardenability, after suitable metal heat treatment, the microstructure is uniform sorbite, bainite or very fine pearlite, so it has a higher tensile strength and yield ratio. (Generally 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

Alloy structural steel is generally divided into quenched and tempered structural steel and surface hardened structural steel.
①The carbon content of quenched and tempered structural steel is generally about 0.25% to 0.55%. For structural parts with a given 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.
②Surface-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.). Carburizing or carbonitriding steel, after 850~950℃ carburizing or carbonitriding, quenched and used in low temperature tempering (about 200℃) state. Nitrided steel is subjected to nitriding treatment (480~580℃) and used directly without quenching and tempering treatment.