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Coolant selection The coolant should be cool, permeable and lubricious. We use 10 oils in winter and 20 oils as coolant in summer. Although these oils are slower than water or water-based solutions, they are superior to water and water-based solutions in terms of permeability and lubricity, and are non-toxic, rust-proof, and transparent in color, which is easy to observe and difficult to age. Vmax——the heat source speed in the middle of the tooth; vmin——the heat source speed of the gear end face. The speed change of the heat source on the tooth width when the grinding wheel exceeds the difference. The selection of the ambient temperature in the production practice, we find that the winter grinding tendency is serious, this is mainly steel. In order to ensure the normal grinding, the gear can be preheated to about 10 °C to grind the teeth to eliminate the occurrence of grinding cracks. After grinding the teeth, the gears are quickly placed near the radiator to prevent cold brittleness. The generation of cracks. In 1994, our branch built a grinding chamber, which effectively overcome this problem.
The effect of carburizing surface carbon concentration on grinding cracks has been learned through a large number of scientific research and production practices. The surface carbon concentration of the carburized layer is preferably between 0.8 and 1.1. If the surface carbon concentration is too low, it is not wear-resistant and the fatigue strength is also low. With the increase of carbon concentration, the solid solution strengthening due to supersaturated solid solution of carbon in martensite and the dispersion strengthening of fine carbides precipitated during low temperature tempering, together with the increase of crystal distortion, the yield strength of the material It will increase accordingly. If the surface carbon concentration is too high, the amount of martensite, the amount of carbides and the amount of retained austenite are too high. At the same time, the shape of the carbide is chicken-claw, and the fracture strength of the carburized layer is significantly reduced. serious. We control the surface carbon concentration from 0.85 to 0.95. After quenching and tempering, most of them are cryptic martensite with a small amount of retained austenite, which is beneficial to control the generation of grinding cracks.
The effect of quenching temperature on the grinding cracks was carburized at 930 °C, quenching at 860 °C, 840 °C and 820 °C, respectively. The test shows that as the quenching temperature decreases, the gear tooth cracking tendency also decreases. This is because as the quenching temperature is lowered, the carbon content in the martensite is gradually lowered, and the martensite breaking strength is increased. When the quenching temperature is 820 °C, the interval between the quenching of the workpiece and the quenching oil temperature must be strictly controlled, otherwise the ferrite precipitates more. In the production practice, we found that for the gear material of 20CrMnTi, the gear with low quenching hardness (about HRC54 4) has less chance of cracking when grinding.
Effect of tempering and heat preservation on grinding cracks Tempering and heat preservation is to refine the grains and eliminate the internal stress, and finally obtain the needle-shaped martensite, a small amount of retained austenite and granular carbide. As the tempering holding time is extended, the prevention of grinding cracks is more advantageous. For economic reasons, we control the tempering temperature to about 180 ° C for 4 h.
The effect of the surface quality of the workpiece on the grinding crack after carburizing and quenching. Before the grinding, the oil stain and the scale cleaning on the surface of the tooth are cleaned. If necessary, the surface of the tooth is polished with a coarse cloth, which can reduce the heat of grinding. , thereby reducing the occurrence of grinding cracks.
Conclusion (1) Grinding process parameters have an extremely important influence on the generation of grinding cracks. For 20CrMnTi carburized steel, a better white corundum grinding wheel, particle size 46, hardness K, ceramic bond A can be selected. It has been proved that the proper cutting speed and cutting amount can effectively prevent the occurrence of grinding cracks. In the winter, grinding the teeth after preheating is also advantageous for preventing the occurrence of grinding cracks. Conditional construction of a constant temperature grinding workshop can be used to effectively control the generation of grinding cracks.
(2) Heat treatment also has a great influence on the grinding crack. For 20CrMnTi steel, the carbon concentration of the carburized layer is controlled between 0.85 and 0.95, and the quenching temperature is controlled at about 820 °C, which is beneficial to control the generation of grinding cracks. Suitable low temperature tempering is also an indispensable process to prevent grinding cracks.