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Influence of Vacuum Tempering Treatment on Carburized Gears

Remanufacturing technology provides a new way to rationally deal with scrapped automobiles. The so-called remanufacturing is to remanufacture new products with qualified performance after cleaning, testing, repairing and assembling scrapped mechanical and electrical products.

Common failures of gears are in the form of contact fatigue, bending fatigue and wear. Commonly used repair methods include thermal spraying, surfacing, laser cladding and other technologies. The heat generated by these treatments will increase the temperature of the workpiece substrate. For example, when the surfacing process is used to repair the connecting shaft of the gearbox, the workpiece temperature will reach 600 ~ 700 ℃. The failure of the gear mainly occurs on the surface, which is usually repaired by thermal spraying and other processes. In this experiment, the heat preservation at 350 ℃ is selected to simulate this heating process.

20MnCr5 steel carburized automobile gear products were selected, and the heat generated by the thermal spraying technology in the remanufacturing and repairing was simulated at 350℃ for 20 min and then air-cooled on the gear structure and hardness. 1, 3 and 5 heat preservation treatments were performed to simulate the gears undergoing multiple repair treatments. The experimental results can provide data for the remanufacturing of automotive gears and whether they can undergo multiple remanufacturing.

1 Experimental process

The experimental material is taken from the finished 20MnCr5 steel gear, and its chemical composition is listed in the table below. The carburizing heat treatment process curve is shown in the figure. Carburizing is carried out at 920 ℃, and it is divided into two stages: strong infiltration and diffusion, and the medium is methanol and acetone. To reduce the formation of oxidized tissue, the oxygen potentials were controlled to be 1134 mV and 1124 mV, respectively. After carburizing, the gear needs to be kept at 860 °C for 30 min, and the oxygen potential is controlled at 1098 mV at this stage. After quenching, it was vacuum tempered at 180 °C for 2 h.

Cut out 4 fan-shaped small samples on the gear. The temperature was kept at 350 °C for 20 min and then air-cooled. The three samples were treated for 1, 3 and 5 times respectively, and the other sample did not receive any treatment as a comparison. The samples were mounted and ground respectively, the observation surface was a fan-shaped cross section, and the corrosive solution was 2% nitric acid alcohol solution. The metallographic structure was observed with an optical microscope, and the microstructure was observed with a scanning electron microscope (SEM) with an accelerating voltage of 20 kV. The hardness was measured with a microhardness tester with a pressure of 0.3 kg and a holding time of 15 s.

2 Experimental results and analysis

2.1 Finished gear structure and hardness

The surface layer of the carburized gear to the area where the hardness starts to be less than 550 HV is called the effective hardened layer of the gear, which is about 1.4mm. In the superficial layer, retained austenite and high-carbon martensite coexist, and a large amount of retained austenite makes the overall hardness lower. As the distance of the metal increases, the retained austenite content decreases, the martensite content increases gradually, and the hardness also begins to increase. However, since the carbon content in martensite decreases with increasing distance, the hardness of martensite itself also decreases gradually. Therefore, the material hardness has a maximum value between 0.2 and 0.5 mm from the surface. When the carbon content is lower than 0.6%, the hardness of martensite decreases significantly with the decrease of carbon content, and the decrease of carbon content leads to a rapid decrease in hardness. When the distance reaches about 2.2 mm, the carbon content basically does not change, and the hardness of the heart tissue is stable between 450 and 460 HV.

3 Analysis and discussion

Heating of gears will reduce the hardness, which firstly reduces the ability of gears to resist surface chipping, wear resistance and pitting fatigue, which is detrimental to service performance and life. The loss of fatigue strength is related to the decrease of hardness and the magnitude and distribution of residual stress. When the hardness of the surface layer is lower than 680 HV, the fatigue limit decreases significantly. After treatment at 350℃, the hardness of the surface layer is below 620 HV, which indicates that the fatigue limit will decrease if the gear is overheated during remanufacturing.

In fact, the surface technology of remanufacturing often has the characteristics of high heating temperature and short time, and the heat-affected area is often small, and the influence on the organization and performance of gears is limited. At the same time, the remanufactured additive materials tend to have better performance, and the performance of the surface layer after remanufacturing will be improved, while the performance of the internal heat-affected zone may be reduced. The heat effect of remanufacturing treatment has limited influence on gear performance and life, but if the gear undergoes multiple remanufacturing, there will be superimposed heat-affected effects, which will enlarge the heat-affected zone. If the damage to the gear performance and life is accumulated to a certain extent, the metal will make the gear lose its use value.

4 Conclusion

( 1 ) The surface structure of 20CrMn5 steel carburized gear is retained austenite + fine carbide + coniferous martensite, and the core structure is lath martensite. The hardness first increased from the surface layer to 850 HV and then decreased, and after 2.2 mm from the surface, the hardness stabilized at 450-460 HV. The effective hardened layer thickness is approximately 1.4 mm.

(2) After vacuum tempering at 350℃ for one time, the residual austenite in the carburized layer is completely decomposed, the martensite recovers, and fine carbides are precipitated. The hardness of the surface layer decreased significantly, the hardness of the core decreased by 50HV, and the thickness of the effective hardened layer decreased to 0.76 mm. These changes will reduce the wear resistance of the gear surface, the ability of the gear to resist surface cracking and pitting fatigue, and lead to the reduction of the gear fatigue strength, which is prone to fatigue damage on the surface and core.

(3) The results of vacuum tempering at 350℃ for 3 times and 5 times have no obvious difference in structure and the hardness after one heat preservation treatment, and the hardness decreases slightly. Therefore, the heating temperature of the gear should be strictly controlled during remanufacturing.

Selection of heat treatment equipment: In addition to good process design, the selection of vacuum heat treatment equipment is also an important factor in completing the process. The RVT vacuum tempering furnace produced by SIMUWU is an excellent choice for this type of process. Its process performance can fully meet the needs of such thermal processing, with good temperature control accuracy, temperature uniformity and tempering uniformity. High process repeatability, stable production, quality output can be guaranteed.