The common materials for gears are 20Cr, 20CrMnTi and 18Cr2Ni4WA.
It has high strength and hardenability, but poor toughness. During carburizing, there is a tendency of grain growth, and the direct cooling quenching has a great influence on the impact toughness. Therefore, after carburizing, the secondary quenching can improve the toughness of the part center; the machinability is good, but the annealing is poor; 20Cr is pearlite, and the weldability is good, generally without heat treatment after welding.
20CrMnTi is a kind of carburizing steel with good properties, high hardenability, high strength and toughness after carburizing and quenching, especially high low temperature impact toughness, good machinability, small machining deformation and good fatigue resistance.
18Cr2Ni4WA belongs to high strength medium alloy carburizing steel. 18Cr2Ni4WA Steel is often used for alloy carburizing steel with high strength, toughness and hardenability. It can also be used without carburizing and tempering. It is generally used for important parts with large section, high load and good toughness.
For the automobile, because of its complex use conditions, the use of quenched and tempered steel can not guarantee the requirements, the selection of carburized steel is more appropriate. 20CrMnTi steel is carburized, Quenched and tempered at low temperature. The surface of gear can obtain high hardness of 55 ~ 63hrc. Because of high hardenability, the tooth center has high strength and toughness. Therefore, 20CrMnTi steel is selected.