For a long time, the most common steel grade used in truck gears in my country is 20CrMnTi. This is a medium-sized 18XTr steel grade (ie 20CrMnTi steel) for automobile gears imported from the former Soviet Union in the 1950s. The steel has fine grains, has a small tendency to grow grains during carburization, and has good carburizing and quenching performance. It can be quenched directly after carburizing. The literature pointed out that before 1980, my country's carburized alloy structural steel (including 20CrbinTi steel) only guaranteed the chemical composition of the steel and the mechanical properties measured with samples when the steel was delivered. However, the chemical composition and mechanical properties often appeared in the production of automobiles. Qualified steel, due to the excessive fluctuation range of hardenability, which affects the quality of the product. For example, if the hardenability of 20CrMnTi carburized steel is too low, the core hardness of the gear after carburizing and quenching will be lower than the value specified in the technical conditions, and the fatigue life of the gear will be reduced by half during the fatigue test; if the hardenability is too high If the gear is high, the shrinkage of the inner hole of the gear after carburizing and quenching is too large, which will affect the gear assembly.
Because the hardenability of steel has an extremely significant impact on the hardness and distortion of the tooth core, the Ministry of Metallurgy issued my country's technical conditions for guaranteed hardenability structural steel (GB5216-85) in 1985, which was included in this technical condition The chemical composition and hardenability data of 10 carburizing steels including 20CxMnTiH and 20MnVBH steels are presented. The standard stipulates that the hardenability index of 20CrMnTi steel used to manufacture gears is 30-42HRC at a point 9 bar away from the water-cooled end. After that, the problem of too low hardness and too large distortion in the tooth core of the gear produced by using 20CrMnTi steel was basically solved. However, it is obviously unreasonable to use the same steel grade 20CrMnTi regardless of the size of the gear module and the thickness of the steel section. Due to the improvement of the level of steel smelting technology in my country and the improvement of the supply of alloy structural steel, there are conditions to further narrow the hardenability band of gear steel and develop it according to the requirements of different products (such as transmission gears and rear axle gears, etc.) The new steel grade meets its requirements.
Through negotiation with steel mills, in 1997, Changchun FAW signed agreements with gear steel manufacturers to supply the hardenability of 20CrMnTi steel in different stages. The hardenability groups of 20CrMnTiH steel used for the primary and secondary gears of the small transmission and the larger section size of the rear axle main and driven bevel gears are I and II, and the corresponding hardenability groups are J9: 30—36HRC and J9=36-42HRC.
Around 1960, due to the tight supply of nickel and chromium steel in China, the production of nickel and chromium-containing steel in my country was affected. At that time, my country's automobile industry was based on technology imported from the former Soviet Union, and the Soviet Union used a large number of nickel and chromium-containing steels. Therefore, at that time, my country's automobile industry vigorously developed the development and research work of boron steel, using 20MnVB and 20Mn2TiB steel instead of 20CrMnTi carburized steel to make gears. This is because adding trace boron (0.0001 percent -0.0035 percent ) to structural steel can significantly improve the hardenability of steel. Therefore, adding trace boron to steel can replace a certain amount of precious alloying elements such as manganese, nickel, chromium, and molybdenum. Therefore, boron steel is widely used. Changchun FAW has used 20MnTiB and 20Mn2TiB steels in the production of "Jiefang" brand automobile gears.
The "Dongfeng" brand 5 produced by Dongfeng Motor Company is made of 20CrMnTi and 20MnVB steels respectively for the transmission and rear axle gears of trucks. Similarly, an agreement was signed with steel mills to narrow the steel hardenability band and supply it in stages. The steel for the main and driven bevel gears of the transmission and rear axle are 20CrMnTiH(3) and 20MnVBH(2), 20MnVBH(3), and the corresponding hardenability is J9=32-39HRC and J9=37-44HRC, J9=34. 42HRC.
my country's Qijiang Gear Factory has introduced the German company's heavy-duty automobile transmission gear production technology, and successfully trial-produced the company's Cr-Mn-B series boron-containing gear steel in accordance with German Ⅲ company standards. The hardenability of the gear material is J10=31-39HRC
Engrenagem de dentes retos
Engrenagem de dentes retos
Of course, boron{{0}}containing steels such as 20CrMnTi steel, 20MnTiB steel, and 20MVB steel also have shortcomings. It is generally believed that 20CrMnTi and other carburized steels are intrinsically fine-grained steels, and the grains will not be coarsened after carburization and can be quenched directly. But in fact, due to the influence of the quality of steel smelting, grain coarsening often occurs under normal conditions. In the actual grain size test of many batches of materials, it is found that a considerable part of the actual grain size is only 2-3 (under the condition of holding at 930 degree for 3h). The literature believes that due to the high Ti content of 20CrMnTi, there are many TiN inclusions in the steel, especially the large TiN inclusions are the source of fatigue when the gear is fatigued, and its existence will reduce the contact fatigue performance of the gear. This kind of inclusion has a cubic structure and is prone to cleavage and cracking when subjected to force, leading to early failure of the gear. Another problem is that the steel has limited hardenability and cannot meet the requirements of large-diameter and large-modulus gears. The effective hardened layer depth and core hardness of carburizing cannot meet the requirements of heavy-duty gears. In addition, 20CrMnTi steel is prone to internal oxidation and non-martensitic structure during the heat treatment process, which reduces the fatigue life of the gear. However, in my country's gear carburizing steel, there is no steel as mature and reliable as 20CrMnTi steel in the carburizing process. Therefore, it is still the most commonly used carburizing steel in China. Boron steels such as 20MnVB, 20MnTiB and 20Mn2TiB also have some shortcomings. For example, due to poor deoxidation and nitrogen removal during smelting, boron cannot increase the hardenability. Therefore, the performance of boron steel is unstable. After carburizing and quenching The gear distortion increases and affects the quality of the product. At the same time, because the mixed crystals and grains are easy to be coarse, the deformation is difficult to control and the toughness is poor, and the roots of the boron steel gears are prone to produce the troostite structure and the black mesh and black belts of the carbonitriding gears. Therefore, many factories have discontinued the use of this steel. However, it must not be concluded that boron steel is not suitable for gear carburizing steel. Carburized steel containing boron is still used abroad. For example, the famous IV gear factory in Germany has always used the reserved steel grade ZF7 formulated by its own factory, which is a low-carbon chromium-manganese steel containing boron. The main chemical composition (mass fraction, percent ) of the steel is 0.15-0.20C, 0.15-0.40S, 1.0-1.3Cr, 1.0-1.3Mn, 0.001-0.003B. American automobile transmission gears and rear axle main and driven gears are also made of boron-containing carburized steel, such as 50B15, 43BVl4 and 94B17. Therefore, as long as the smelting technology of steel plants keeps up, the above-mentioned problems of boron steel can be solved.
Os blanks de forjamento de engrenagens de aço 20CrMnTiH, 20MnVBH e 20MnTiBH são processados em um forno contínuo de normalização isotérmica para garantir uma distribuição uniforme de lascas de perlita e ferrita. Desta forma, a distorção do tratamento térmico da engrenagem pode ser bastante reduzida, a precisão da engrenagem pode ser melhorada e a vida útil da engrenagem pode ser prolongada.
A dureza de normalização isotérmica de peças de forjamento de engrenagens é 156-207HB. 6
Cromo-manganês-aço molibdênio e cromo-aço molibdênio
Os aços 22CrMnMo, 20CrMnMoH e 20CrMoH são usados em engrenagens automotivas- de tamanho médio devido à sua alta temperabilidade. Este tipo de aço pode ser temperado diretamente após a cementação. Como o cromo-manganês-aço molibdênio e cromo-aço molibdênio contêm cromo e molibdênio e outros elementos que formam carbonetos, o teor de carbono na superfície dos dentes da engrenagem aumentará durante o processo de cementação, e é provável que uma grande quantidade de carbonetos apareça na estrutura da camada cementada, fazendo com que o desempenho da camada de cementação se deteriore. Portanto, quando a engrenagem é cementada com cromo-manganês-aço molibdênio e cromo-aço molibdênio, uma atmosfera de cementação fraca deve ser usada para evitar a formação de carbonetos excessivos. Os blanks de forjamento de engrenagens 22CrMnMo e 20CrMnMoH são normalizados e submetidos a tratamento de têmpera de alta temperatura a 650-670 graus. A estrutura metalográfica é perlita em flocos mais uma pequena quantidade de ferrita, e a dureza é 171-229HB. O blank de forjamento de engrenagem 20CrMnH é melhor processado em um forno de normalização isotérmico contínuo, aquecido a 935-945 graus, pré-resfriado a 640-650 graus e depois isotermicamente, para obter uma estrutura uniforme de ferrita mais perlita com uma dureza de 156-207HB. A literatura aponta que o processo de fundição do aço 20CrMoH é estável, a faixa de temperabilidade é estreita e de fácil controle. Comparado com a engrenagem de aço 20CrMnTi, tem menos distorção do tratamento térmico; a camada cementada tem boa temperabilidade e estável; a estrutura metalográfica, cementação e têmpera A dureza da superfície e do núcleo pode atender melhor aos requisitos técnicos; o desempenho à fadiga é bom, e é mais adequado para engrenagens de módulo pequeno e médio em automóveis. Considerando as condições de serviço das engrenagens de forma abrangente, não só garante a vida em fadiga das engrenagens, mas também reduz a distorção do tratamento térmico das engrenagens. Quando usado para fabricar engrenagens de transmissão, deve ser J9=30-36HRC, e quando usado para fabricar engrenagens do eixo traseiro, deve ser J9=37- 42HRC.
