Material selection principle of gear forgings

2019-03-13 12:26:15 4

There are many types of gear forgings. The commonly used materials for gear forgings are: 40Cr, 42CrMo, 20CrMnMo, 20CrMnTi, etc. 42CrMo and 40Cr forged gears are mainly gears forgings in the lifting industry. 20CrMnMo and 20CrMnTi are used in transmission machinery forging gears. Small pieces have no mains (reference). Most of the gears need gears to improve the quality. The heat treatment hardness of the gears can achieve good performance at 38-42HRC. The toughness of 42CrMo is much better than 40Cr, because the former has better heat treatment hardenability, and its material The contact is great. Also, the strength at the same hardness is very close. The good tensile strength of 40Cr is 60~75kg/mm2, the yield strength is 35~55kg/mm2; the tensile strength of 42CrMo is 110kg/mm2, the yield strength is 95kg/mm2, obviously the performance of 42CrMo is It is much better than 40Cr.


40Cr material has good hardenability, can be hardened to 28~60mm in diameter during water quenching, and can be hardened to 15~40mm in diameter during oil quenching. After quenching and tempering, the material has good comprehensive mechanical properties, and has good low notch sensitivity and low temperature impact toughness. 40Cr gear forgings are often subjected to surface induction hardening or nitriding after quenching and tempering treatment. When the hardness is 174~229HBS, the cutting performance is better, the relative machinability is 60%, and the carbon content of 40Cr forgings is kept at about 0.40%, which ensures the steel has good strength and toughness. The addition of Cr element is mainly to improve the hardenability of steel. When quenched and heated, the Cr element is completely dissolved in the austenite to improve the hardenability of the steel. After quenching, Cr element solid solution strengthens the matrix structure and improves the tempering stability of the matrix structure. At high temperature tempering, part of Cr element diffuses from the matrix structure to the precipitated cementite Fe3C to form alloy cementite (Cr, Fe) 3C.

The main chemical composition (mass fraction) of 40Cr steel for gear forgings is: 0.37%~0.44%C, 0.17%~0.37%Si, 0.50%~0.80%Mn, 0.80%~1.10%cr, ≤0.035%P, ≤0.035 %S.

The phase transition points are: Ac1770 ° C, Ac3805 ° C, Ms 328 ° C.

The initial forging temperature of 40Cr gear forgings is 1100~1150°C, and the final forging temperature is °C. After forging, the size is above 60mm and needs to be slowly cooled.


Compared with 40Cr forged gears, 42CrMo forged gears outperform the former, especially those with higher hardenability than 40Cr steel gears. After quenching and tempering treatment, the gear forging of 42CrMo steel has high fatigue strength limit and resistance to multiple impacts, and good low temperature impact toughness.

The main chemical composition (mass fraction) of forged gear 42CrMo steel is: 0.38%~0.45%c, 0.17%~0.37%Si, 0.50%~0.80%Mn, 0.90%~1.20j%Cr, 0.15%~0.25%Mo, ≤ 0.30% Ni, ≤0.030% P, ≤0.030% S.

The phase transition points are: Ac1730 ° C, Ac 3 ° C, Ms 310 ° C.

The forged 42CrMo gear has an initial forging temperature of 1130~1180°C and a final forging temperature of 850°C. After forging, the diameter is above 50mm and needs to be slowly cooled.


The carbon content of the 42CrMo gear is kept at about 0.42%, which ensures that the steel has good strength and toughness. The addition of Cr and Mo elements mainly improves the hardenability of the steel. When quenching and heating, the Cr and Mo elements are completely dissolved in austenite to improve the hardenability of the steel. After quenching, Cr and Mo elements solid solution strengthens the matrix structure and improves the tempering stability of the matrix structure. At high temperature tempering, part of Cr and Mo elements diffuse from the matrix structure to the precipitated cementite Fe3C to form alloy cementite (Cr, Mo, Fe) 3C. In addition, the addition of Mo element can eliminate temper brittleness. The addition of a small amount of Ni improves the toughness of the steel.


There are also many factors that should be considered when choosing. Under normal circumstances, the working conditions of the gear forgings are different, and the damage forms of the gear teeth are different, which is the basis for determining the calculation criteria of the gear strength and selecting the materials and heat treatment.


The material of the gear forging must first meet the requirements of working conditions. The requirements of working conditions are the first considerations when selecting gear forging materials. Alloy steel is often used to make gear forgings that work at high speed, heavy load and under impact load. For gears used in airplanes, it is necessary to meet small mass and high transmission power. High reliability requirements, so alloy steel with high mechanical properties must be selected. If the gear size is required to be as small as possible, high-strength alloy steel with surface hardening should be used. Gear transmission in mining machinery, generally high power, low working speed, extremely high dust content in the surrounding environment, often choose cast steel or cast iron and other materials. Household and office machinery have low power, but require smooth transmission, low noise or no noise, and can work normally with less lubrication or no lubrication. Therefore, engineering plastics are often used as gear materials.


The gear forging material should consider the size of the gear, the blank forming method and the heat treatment process. Large-sized gears generally use cast blanks, and cast steel or cast iron can be used as the gear material. For gears with medium or medium size requirements, forged blanks are often used, and forged steel can be used. When the size is small and the requirements are not high, round steel can be used as the blank.