(1) General gears with light load, low speed or medium speed, small impact force and low precision are made of medium carbon steel, such as Q235, Q275, 40, 45, 50, 50Mn and other steels. They are usually made of soft gear by normalizing or tempering heat treatment, with normalizing hardness of hbs160-200 and general tempering hardness of hbs200-280. Because of the moderate hardness, the fine cutting tooth profile can be carried out after heat treatment, the process is simple and the cost is low. If the hardness of tooth surface is not high, it is easy to run in, but the bearing capacity is not high. This kind of gear is mainly used in standard series reductiongear, metallurgical machinery, medium load machinery and some secondary gears in machine tools.
(2) For gears with medium load, medium speed, certain impact load and relatively stable movement, medium carbon steel or alloy quenched and tempered steel shall be selected, such as 45, 50Mn, 40Cr, 42SiMn and other steels, and low hardenability steel such as 55tid and 60tid can also be used. The final heat treatment adopts high frequency or medium frequency quenching and low temperature tempering to make the hard tooth surface gear. The hardness of the tooth surface can reach hrc50-55. The center of the gear is kept in normalizing or tempering state and has good toughness. Due to the small deformation of induction heating surface hardened gear, if the accuracy requirement is not high (such as below grade 7), it is unnecessary to grind the gear again. Most gears in machine tools are of this type. For case hardened gears, it is necessary to control the depth of hardened layer and the reasonable distribution of hardened layer along the tooth profile.
(3) For the gears with heavy load, high speed or medium speed and subjected to large impact load, low carbon alloy carburizing steel or carbonitriding steel shall be selected, such as 20Cr, 20CrMnTi, 20crni3, 18Cr2Ni4WA, 40Cr, 30crmnti and other steels. The heat treatment is carburizing, quenching and low temperature tempering. The surface of the gear has high hardness of hrc58-63. Because of the high hardenability, the gear core has high strength and toughness. The surface wear resistance, fatigue strength, bending strength and impact resistance of the tooth root of this kind of gear are higher than those of the surface hardened gear. When the accuracy is required to be higher, grinding is generally arranged at last. It is suitable for gearbox and rear axle gear of automobile and tractor with bad working conditions. Compared with carburizing, carbonitriding has the advantages of small heat treatment deformation, short production cycle and high mechanical properties. It is also used in medium carbon steel or medium carbon alloy steel, so many gears can be replaced by carbonitriding. The load and working conditions of variable speed gears on internal combustion engine tanks and airplanes are heavier and worse than those on automobiles, which require higher performance of materials. Alloy carburizing steel with high alloy elements should be selected to obtain higher strength and wear resistance.
(4) For precision transmission gear or hard tooth surface gear (such as internal gear) with difficult gear grinding, the main requirements are high precision and small heat treatment deformation. Nitriding steel, such as 35CrMo, 38CrMoAlA and other steels, should be used. The hardness of the gear surface after nitriding is as high as hv850-1200 (equivalent to hrc65-70), the thermal stability is good (it can still maintain high hardness at 500-550 ℃), and it has certain corrosion resistance. Its disadvantages are thin hardening layer and impact resistance, so it is not suitable for heavy load gears with frequent load changes, but for precision transmission gears with stable load and good lubrication or internal gears with difficult gear grinding. In recent years, due to the development of soft nitriding and ion nitriding technology, the process cycle has been shortened, the selection of steel grades has been widened, and the selection of nitriding gear has been gradually extensive.