1. Material selection of mechanical gears
Other countries in the world mainly use Cr Ni and Cr Ni Mo carbon steel. Compared with Mn Cr steel which is commonly used in mechanical gears in our country, it has greater advantages in strength and toughness. In China, 18 Cr Mn Ti, 20 Cr Mn Ti and 20 MN VB are commonly used in small power machinery. In recent years, 18 CR 2 Ni 4 W and 20 CR 2 Ni are mostly used. The main reason is that they not only have relatively high toughness and hardenability, but also have the advantages of high wear resistance and hardness after quenching. They have relatively good impact toughness at low temperature and relatively good machinability after normalizing, In addition, the service life of 18 CR 2Ni 4 W and 20 CR 2Ni 4 steel gears is longer than that of 8 Cr Mn Ti, 20 Cr Mn Ti and 20 MN.
The above two kinds of steel gear have chromium, nickel, manganese and other elements, among which chromium can improve the hardenability of steel, nickel can reduce the probability of excessive carbon concentration on the surface of steel and the sharp change of equilibrium concentration, which can also greatly improve the strength and toughness of carbon steel. Compared with chromium, manganese can not only improve the strength and toughness of carbon steel, It can also improve the hot working ability of carbon steel.
The main function of molybdenum element is to dissolve austenite to form carbon compound, so as to achieve the goal of solid solution strengthening and precipitation strengthening, and make the carbon steel still have high toughness, at the same time, the strength can be further improved.
2. Technical requirements for mechanical gears
There are two main technical requirements for mechanical gears, one is the general working conditions of pulsating cyclic stress, bending stress and surface friction stress, the other is the special working conditions including short-term overload and irregular impact load. Therefore, the gear is required to have high strength and good overload capacity and impact toughness. Secondly, the limitation of working space requires that the size of gear should not increase too much with the increase of power of mechanical tools, otherwise, the surface contact fatigue stress and bending stress of gear will increase. Therefore, in addition, the contact fatigue and bending fatigue performance of material must be ensured.
3. Material selection of mechanical gears
3.1 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, but 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, However, the cutting performance is relatively poor, and with the increasing alloy elements of carbon steel, 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.
3.2 comparison of overload resistance and wear resistance between Ni Cr and Ni Cr Mo steels. Since 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 through a large number of tests, and molybdenum and nickel can improve the overload resistance of carburized steel and appropriately reduce the content of manganese and manganese in the material The results show that the content of chromium can reduce the internal oxidation, reduce the formation of decomposition products of surface quenched cold austenite, improve the fatigue resistance of gear surface, improve the surface hardenability of material, and rationalize the hardness gradient distribution. 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.