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Vacuum Heat Treatment Process for Mining Gears

1 Selection of mining machinery gears

Coal mine machinery bears a high dynamic load, and is often eroded by underground coal (powder) dust and harmful gases, and the working conditions are very harsh. With the increase of coal production, the power of coal mine machinery is increasing day by day, and the requirements for the carrying capacity of gear transmission devices are also increased accordingly. The peripheral speed of downhole gear transmission is low (2~12 m/s), and the design calculation contact stress σH is generally greater than 500 MPa, which belongs to low speed, heavy load, high reliability and long service life gear transmission, and hard tooth surface is mostly used Gears, which put forward higher requirements for gear material selection and vacuum heat treatment.

The design of mining machinery gears follows the criteria of the contact fatigue strength between the working tooth surface and the layer depth and the bending fatigue strength of the tooth root. The high toughness of the gear tooth center and the high hardness of the tooth surface are reasonably combined, and the material selection and treatment method are determined according to the load (contact and bending stress) of the gear teeth.

It can be seen from Fig. 1 that the material strength is divided into 9 grades according to the contact fatigue limit σHim and the bending fatigue limit σFlim, and then divided into 2 groups according to different vacuum heat treatment methods: the hard tooth surface vacuum carburizing and quenching group is from H16, 6 to H10 , 3 is divided into 5 categories, and the quenched and tempered group is divided into 4 categories from Q10, 2.5 ~ Q5, 1.6 (H means hard tooth surface, Q means quenched and tempered treatment). The first number in the box is one percent of the contact fatigue limit, and the second number is one percent of the bending fatigue limit during pulsating cycles.

2 Vacuum heat treatment requirements and typical process for mine hardened gears

Mine-used hard-toothed gears mostly use vacuum heat treatment processes of vacuum carburizing, quenching and nitriding (mainly used for planetary transmission inner gears).

3 material selection and vacuum heat treatment process

According to the calculation results, the gear is classified according to the method shown in Figure 1, and the material can be 20CrMnMo or 20Cr2Ni4A. From the perspective of improving reliability, 20Cr2Ni4A is finally selected, and its vacuum heat treatment requires an ice-cooling process. The flow chart is shown in Figure 2.

20Cr2Ni4A alloy steel is a high-quality alloy steel with high strength, high toughness and good hardenability. Due to the high content of alloy elements, the forging deformation resistance is large, and the workmanship is poor. After normalizing, the forging can obtain martensitic structure and high hardness. After normalizing, high-temperature vacuum tempering is required. After the rough car is formed, heat it to 650°C, keep it warm for a period of time and cool it slowly to about 350°C, so as to eliminate the machining stress and reduce the deformation. Gears often need to be cut after vacuum carburizing. At this time, the hardness is relatively high, so vacuum tempering after vacuum carburizing is required to soften it. Vacuum quenching is the most important part. After vacuum quenching, the hardness reaches HRC 61 ~ 63, and the metallographic structure is martensite. After vacuum quenching, the gear is cooled to room temperature, and then put into a -70°C refrigerator for 2 to 3 hours, then taken out and returned to room temperature in the air, and the residual austenite is controlled below 15% during the ice-cooling treatment to prevent cracks during gear grinding. And stabilize the gear size, improve the hardness.

4 Improved process and effect of non-icing treatment

When the above-mentioned reducer gear was produced in the factory, it was negotiated with the factory to adopt a vacuum heat treatment process without ice cooling for 20Cr2Ni4A and 18Cr2Ni4WA steel. The vacuum heat treatment process flow is shown in Figure 3.

5 Conclusion

1) According to the strength classification method shown in Figure 1, it is one of the more scientific and reasonable methods to select materials for mining hard-toothed gears, which can avoid blindness in material selection.

2) High-reliability transmission gears in industries such as mining and construction machinery are generally made of low-carbon NiCr alloy steel, and the vacuum heat treatment process requires high requirements. The process improvement without ice treatment introduced in this article has been proved by production practice that the performance of the gear after heat treatment meets the design requirements, and it is safe and reliable. It reduces the production cost while ensuring the mechanical properties, and has certain economic value.

Vacuum furnace selection: In order to manufacture excellent evaporator products, the equipment must have the necessary performance indicators. The vacuum heat treatment furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment process of tools and molds. The good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum heat treatment process. SIMUWU company focuses on the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.