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Improvement of Gear Vacuum Carburizing and Quenching Process

The rotating shaft of the motorcycle starter is an important part of power transmission. Its toothed end is prone to wear and tooth chipping during operation. The rotating shaft is made of 20CrMo steel, the modulus of the end gear is 0.6, the number of teeth is 11, and the diameter of the pitch circle is 6mm. The gear part requires a carburized layer depth of 0.2~0.3mm, and the surface hardness after quenching is 80~87HRA. After installation, the starting life of the starter should be more than 10,000 times.

Gear carburizing and quenching are carried out in SIMUWU vacuum carburizing furnace. The original carburizing process is shown in Figure 1. After installing and starting the machine more than 10 times, all 11 teeth of the gear collapsed. The metallographic examination shows that the depth of the carburized layer of the gear is uneven, the depth of the carburized layer at the tooth root is about 0.22mm, the depth of the carburized layer at the top of the tooth is about 0.35mm, and the depth of the carburized layer near the pitch circle is 0.28~0.32mm, which exceeds the technical requirements , indicating that the carburizing time is too long. There is no reticular carbide in the surface structure of the carburized layer, but the martensitic needles are very thick, indicating that the austenite grains have grown significantly after long-term high-temperature carburizing, resulting in obvious mechanical properties of the material after quenching, especially the toughness. reduce. Therefore, the new process is mainly to ensure the thickness of the carburized layer by precisely controlling the carburizing time, and adopts the secondary quenching process to refine the grain and improve the toughness (see Figure 2).

The key points of the process are:

(1) Oven In order to ensure that the state of the vacuum carburizing furnace is basically the same before carburizing each batch of workpieces, if the furnace is shut down for two days, the carburizing fixture must be put into the vacuum carburizing furnace for pre-carburizing until the test until it is carbonized. In order to judge whether the sample is carburized, a thin iron wire can be put into the carburizing furnace from the sample hole in advance. After carburizing for a certain period of time, take it out and water-cool it. If the iron wire can be broken, it is carburized.

(2) Preparation of carburizing agent Because the carburizing layer is thin, there is no diffusion stage, and the carbon potential in the furnace is controlled by controlling the ratio of kerosene and alcohol. The ratio we use is kerosene: alcohol = 3 1.

(3) The vacuum carburizing temperature is 900~920°C to reduce the carburizing speed and make the carburizing layer easy to control.

(4) Carburizing time After the vacuum carburizing furnace reaches temperature, it will be exhausted for about 10 minutes, and the exhaust stage will end when the flame color of the exhaust pipe changes from red to orange. At this time, the sample can be put in, and the carburizing time can be calculated, which is generally 20~30min (each vacuum carburizing furnace should be determined by the test). The workpiece is vacuum carburized and then cooled in a heat preservation barrel.

After testing, the thickness of the pitch circle and carburized layer at the root of the gear after vacuum carburizing and quenching by this process is 0.2~0.25mm, and the surface structure is fine martensite. After the installation test, it successfully passed the life test of 10,000 times, and the wear of the gear surface is extremely small.