Material selection of low speed and heavy load mechanical gears

  1. Comparison of machinability and forgeability between Ni Cr and Ni Cr Mo steels

The machinability of different materials is mainly related to the hardness of carbon steel. After normalizing, the hardness of Ni Cr Mo carbon steel will decrease slightly, and the machinability will be further improved. The Ni Cr steel is a mixture of ferrite, bainite, martensite and austenite, and its hardness is higher than that of Ni Cr Mo carbon steel, but its machinability is relatively poor. With the increasing of alloy elements in carbon steel, the machinability of Ni Cr Mo carbon steel is higher At the same time, the forging performance of carbon steel is also improving, which is the main reason why the performance of NiCrMo is higher than that of NiCrMo.

  1. Comparison of overload resistance and wear resistance between Ni Cr and Ni Cr Mo steels

Because the overload resistance of carburized steel depends on the strength and toughness of the steel core, it is found that the impact fracture stress and critical overload load are related to the fracture toughness of the material after a large number of tests. Molybdenum, nickel and other elements can improve the overload resistance of carburized steel. Reducing the content of manganese and chromium in the material can reduce the internal oxidation, and then reduce the content of decomposition products of surface quenched cold austenite The results show that the fatigue resistance of the gear surface is improved, the surface hardenability of the material is improved, and the hardness gradient distribution is rationalized. Therefore, the overload resistance of Ni Cr Mo steel is better than that of Ni Cr steel.

Because the carburized structure of the two steel systems is mixed martensite, the residual austenite in the structure will produce plastic deformation, improve the contact condition of the gear, ensure the accuracy of the gear, but reduce the wear resistance and fatigue strength of the gear. Generally, the content of retained austenite is 10-20%. The content of retained austenite in carburized layer of Ni Cr Mo steel system is better than that of Ni Cr steel system. Therefore, the carburizing performance, hardness and wear resistance of Ni Cr Mo steel system are worse than that of Ni Cr Mo steel system.

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