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Enhanced Steel Performance During Vacuum Carburizing of Gears

With the gradual deepening of railway development, the volume of railway cargo transportation is increasing, and the requirements for locomotives are becoming higher and higher. High-speed and heavy-duty electric locomotives have become the main development direction of freight electric locomotives. The electric locomotive traction driving gear is an extremely important part in the locomotive transmission system. Vacuum carburizing process is often used to make the surface of the gear have high hardness, good wear resistance and fatigue resistance, while the core retains good plasticity and toughness. Currently, locomotive driving gears use 18CrNiMo7-6 steel, which mainly uses the traditional controlled atmosphere carburizing process. After heat treatment, the product has defects such as internal oxidation and non-martensite, which affects product performance and service life. In order to solve these problems, the vacuum carburizing process using acetylene as the carburizing atmosphere has attracted the attention of researchers. This vacuum heat treatment method is clean, environmentally friendly, and the processed parts have no problems such as internal oxidation and decarburization.

Vacuum carburizing can also enable the product to obtain a deeper carburized layer and higher carbon distribution (to achieve the required carbon concentration). The highly hardened area (≥HRC 58) of the vacuum carburized gear carburizing layer is twice that of the atmosphere carburized gear. , can significantly improve the bending fatigue strength. For gears with a carburized layer of 1.5 mm, the high hardening zone of the gear treated in the atmosphere furnace is 0.4 mm, while the high hardening zone of the low pressure carburized gear can reach 0.9 mm.

Vacuum carburizing process

The vacuum carburizing process is carried out in a vacuum oil quenching furnace. During the heating and heat preservation periods, 100 Pa nitrogen was introduced to maintain the partial pressure state. During carburizing, acetylene and nitrogen were alternately introduced to achieve the pulse carburizing process. Follow the process route of vacuum carburizing → vacuum quenching → vacuum tempering.

The vacuum carburizing temperature is higher than the controlled atmosphere carburizing temperature, and the total carburizing time will be shortened accordingly. The ratio of carburizing time and diffusion (i.e., carburizing ratio) determines the depth and carbon concentration of the carburized layer. The expansion ratio of vacuum carburizing is different from the expansion ratio of traditional controlled atmosphere. The expansion ratios of vacuum carburizing are 1:41 and 1:53 respectively, and the carburizing time is much shorter than the diffusion time.

If heated rapidly or the temperature is too high during carburizing, the carbon concentration on the surface will be too high, resulting in massive and coarse carbides or network carbides appearing on the surface of the workpiece. The total process time of traditional controlled atmosphere carburizing is 1530 min, and the carburizing temperature is only 920°C, while the total carburizing time of the vacuum carburizing process is only 840 min, and the carburizing temperature is 950°C. At higher temperatures, there is insufficient diffusion time for the carbides to dissolve. The carbon content in the Austrian body is relatively high and precipitates along the product boundaries to form network carbides. On the one hand, the coarseness of martensite depends on the size of the original austenite grains. The larger the austenite grains, the coarser the martensite sheets; on the other hand, it depends on the heating temperature. High carburizing temperature will form coarse martensite. martensite.

The formation of retained austenite in the layer structure is mainly affected by the high C content. The higher the C content in austenite, the more retained austenite will be obtained during quenching. Excessive retained austenite will reduce the surface hardness.

Comparison between vacuum carburizing heat treatment and traditional heat treatment

1) A comparison of the results of three vacuum carburizing heat treatment processes for locomotive driving gears made of 18CrNiMo7-6 found that the process performance of 950°C vacuum carburizing + 830°C oil quenching + 180°C × 2h tempering can meet the process requirements.

2) After vacuum carburizing heat treatment, the carburized layer depth of 18CrNiMo7-6 gear is 2.2 mm, martensite level 4, carbide level 1, retained austenite level 1, core structure level 2, surface hardness HRC 61.5 , no internal oxidation, non-martensite and other undesirable structures, fully complying with the technical requirements of the locomotive driving gear.

3) Comparing the vacuum carburizing process of the locomotive’s driving gear with the traditional controlled atmosphere carburizing process, the vacuum carburizing process can save 18% of time.