Steel for machinery manufacturing, structural steel used to manufacture mechanical parts that bear loads or transmit work and force, is also called machine structural steel.
Steel for machinery manufacturing can be divided into quenched and tempered steel, surface hardened steel (including carburized steel, nitrided steel, low-hardenability steel), free-cutting steel, spring steel and rolling bearing steel according to use.

Quenched and tempered steel:

Before use, it is generally quenched and then tempered to ensure that the strength and toughness meet the requirements. The carbon content of carbon quenched and tempered steel is 0.03~0.60%. Due to its low hardenability, it is only used to manufacture small cross-section sizes and shapes. Simple or low load mechanical parts. Alloy quenched and tempered steel is based on carbon quenched and tempered steel with one or several alloying elements added. The total amount of alloying elements added generally does not exceed 5%. Alloy quenched and tempered steel has good hardenability, can be hardened in oil, has small quenching deformation, and has better strength and toughness. Commonly used steel grades include 40Cr, 35CrMo, 40MnB, etc. for important parts with large cross-section sizes and high load bearings. For example, commonly used steel types with high alloying element content, such as aero-engine main shafts, crankshafts and connecting rods of high-speed diesel engines, main shafts of steam turbines and generators, such as 40CrNiMo, 18CrNiW, 25Cr2Ni4MoV, etc.

Carburized steel:

It is used to manufacture parts that require a hard and wear-resistant surface and a strong and impact-resistant core, such as chain pins, piston pins, gears, etc. The carbon content of carburized steel is low, 0.10~0.30%, to ensure the core toughness of the part. After carburizing treatment, a high-carbon and high-hardness wear-resistant layer can be formed on the surface. Alloy carburized steel can be used for more important parts. Commonly used steel grades include 20CrMnTi, 20CrMo, 20Cr, etc.

Nitrided steel:

Steel contains alloying elements with strong affinity for nitrogen, such as aluminum chromium, molybdenum, vanadium, etc., to facilitate the penetration of nitrogen. The nitrided layer is harder, more wear-resistant and corrosion-resistant than the carburized layer, but the nitrided layer is thinner. The deformation of parts after nitriding is small, and it is often used to manufacture precision parts with small allowable wear, such as grinder spindles, plunger pairs, precision gears, valve stems, etc. The commonly used steel grade is 38CrMoAl.

Low hardenability steel:

Special carbon steel with low content of residual elements such as manganese and silicon. When parts made of this type of steel are quenched, the central part is more difficult to harden than ordinary carbon structural steel. The hardened layer is basically evenly distributed along the surface contour of the part, while the central part maintains a softer and tougher matrix to replace the penetration. Carbon steel is used to make gears, bushings, etc., which can save time-consuming carburizing process and save energy consumption. In order to properly match the toughness of the central part and the hardness of the surface, the carbon content is generally 0.50~0.70%.

Free cutting steel:

One or several elements such as sulfur, lead, calcium, selenium, etc. are added to the steel to reduce the cutting force. The amount added is generally only a few thousandths or less. The added element itself is used, or the added element is combined with other elements in the steel to form a Inclusions that reduce friction and promote chip breaking during the cutting process, thereby achieving the purpose of extending tool life, reducing cutting force, and improving surface roughness. Since the addition of sulfur will reduce the mechanical properties of steel, it is generally only used to manufacture light-load parts that are mass-produced on automatic machines. Modern free-cutting steels are also widely used in the manufacture of automotive parts due to their improved properties.