Common carburizing (carbonitriding) steel
20, 20Cr, 20CrMnTi and other low carbon (low alloy) steels with carbon content of 0.15%～0.25%, after carburizing (carbonitriding) heat treatment, the surface (0.5～2.0 mm) carbon content reaches 0.8%～ 1.05%, while the heart still maintains the original carbon content. After quenching and low-temperature tempering, the surface structure is composed of high-carbon martensite and carbides, with high hardness (HRC55～65) and wear resistance; the core structure is low-carbon martensite or low-carbon martensite and ferrite Equal composition, low hardness (HRC≤43), maintaining high plastic toughness. It is widely used for parts that require surface wear resistance and tough heart.
Low carbon steels such as 15, 20 have poor hardenability and low core strength after carburizing (carbonitriding) quenching. It is only suitable for small workpieces such as shaft sleeves, chains, small water valves, etc., which have abrasion-resistant surface, small load-cutting, slight impact, and the core does not require high strength.
The surface of the part is required to be wear-resistant, and the core is required to have good strength and toughness. Low-alloy carburizing (carbonitriding) steel with good hardenability such as 20Cr, 20CrMnTi steel is often used. If working under friction conditions for a long time, piston pins and pin shafts that bear a certain alternating load and impact load are often carburized (carbonitrided) quenched by 20Cr steel; for gears with heavy alternating loads and greater impact (cross-section ≤30～35 mm), then use 20CrMnTi steel carburizing (carbonitriding) quenching. 20CrMnTi steel carburizing (carbonitriding) quenching grains are fine, hardenability is good, and heat treatment deformation is small, which can ensure that the core is composed of low-carbon martensite, and the core strength is high (HRC38～43) , At the same time, it has higher plastic toughness (αk≥100 J/cm2); it can be used with heavy-duty tractors for carburizing (carbonitriding) gears with heavier loads and large cross-sections (workpiece wall thickness ≥35-40 mm) , As with automobiles, 30CrMnTi steel is used to ensure high core strength, and the strength of the transition zone between the core and the carburized (carbonitriding) layer is also high.
Low-carbon steel carburizing (carbonitriding) quenching and medium-carbon steel quenching and tempering (normalizing) high-frequency surface quenching, although both improve the surface hardness, wear resistance and fatigue strength of parts, they should be distinguished when used . Generally speaking, low-carbon steel quenching is mainly used for large load with σb=700～1000 MPa, large impact, medium and low speed gears, spline shafts and other parts; while high-frequency surface quenching of medium carbon steel is used for relative load Lighter (400～700 MPa), less impact gears, shafts and other parts. Because the wear resistance of the medium-carbon martensite high-frequency quenching layer and the strength and toughness of the quenched and tempered core are lower, the carburized (carbonitriding) quenched carburized (carbonitriding) layer and low-carbon horses The core of the body is low. In addition, due to the influence of high-frequency hardening process, for heavy-duty gears and bevel gears with larger modulus (m=5～6), the tooth surface high-frequency hardening layer is distributed along the tooth profile and cannot be completed; especially for large bevel gears. The hardness difference of the arc tooth surface is large, and early damage such as broken teeth often occurs during use, which affects normal operation. In this regard, it should be considered to use 20CrMnTi steel carburizing (carbonitriding) quenching instead of 40Cr steel quenching and tempering and high-frequency quenching. Although the manufacturing cost is higher, the advantages of several applications still outweigh the disadvantages.
The material selection of carburizing (carbonitriding) steel is also based on hardenability, and the core requires high strength and toughness. Generally, low-carbon alloy steel is used. Otherwise, low-carbon steel is used. Carburizing (carbonitriding) is the material to give full play to. Therefore, it is necessary to choose materials that meet the performance requirements of parts as much as possible to improve economic benefits.