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The normalized test piece was air-cooled after heating for 1 h at 940e, and its hardness was 224 HBS, and the metallographic structure was granular bainite, ferrite and pearlite (B F P), as shown. Normalizing, as a traditional tooth blank preparation heat treatment process in China, although it has the advantages of short production cycle, simple process and low cost, it also has obvious deficiencies. First of all, due to the use of air cooling in normalizing, the cooling rate of the parts is unstable and there are many influencing factors. For different climates, the cooling speed varies greatly. In addition, when the normalizing air is cold, the parts are usually stacked together, so that the parts and the internal parts are stacked. The cooling rate of the parts is also different. Due to the different cooling rates, even the same furnace normalized parts have inconsistent metallographic structure and performance. This heterogeneity and instability of the structure and properties is one of the main factors that cause the deformation of the gear carburizing heat treatment.
Secondly, it can be seen that for the steel with higher content of alloy elements such as 21NiCrMo5, the shape of the C curve changes significantly, the upper part is the pearlite transformation zone, the lower part is the bainite transformation zone, and the pearlite transformation zone The supercooled austenite is very stable and its transition curve shifts to the right. Thus, under normalizing air-cooling conditions, a large amount of granular bainite will appear in the structure, resulting in an increase in hardness and a decrease in cutting performance. Therefore, it is not appropriate to use normalizing as a preliminary heat treatment for the 21NiCrMo5 steel blank.
The isothermally annealed specimens were heated and incubated at 940e for 1 h, and then equilibrated at 580e, 600e, 620e, 650e and 670e for 2 h, respectively, and their hardness and metallographic structure. F ferrite, B granular bainite, P pearlite from and know, using 650e isothermal annealing, hardness of 170HBS, metallographic structure of F P, uniform structure, moderate hardness, good cutting performance. Moreover, this temperature is located at the tip of the nose of the C-curve pearlite transformation zone. The supercooled austenite is the most unstable, and the time required to complete the transformation is the shortest. Therefore, the optimal isothermal annealing process for the 21NiCrMo5 steel tooth blank should be 940e@1h. , 650e@1h. The isothermal temperature is lower or higher than this temperature, the stability of supercooled austenite is improved, and the isothermal time should be extended appropriately, otherwise abnormal tissue may occur.
The normalized and high-temperature tempered specimens were air-cooled after being heated for 1 hour by 940e, and then kept at 600e, 640e, 650e, and 660e for 2 hours, and their hardness and metallographic structure. Isothermal annealing requires a special production line. If the factory does not have such equipment conditions for the time being, 940e@1h air cooling, 660e@2h normalizing and high temperature tempering can be used as the preliminary heat treatment for the 21NiCrMo5 steel tooth blank.
Conclusion (1) 21NiCrMo5 steel is heated by 940e for 1h, and then 650e is isothermal for 1h, which can obtain ideal microstructure and hardness. It can also be heated by 940e for 1h air cooling, and then tempered at 660e for 2h. (2) Forging waste heat isothermal annealing not only saves energy, but also obtains ideal structure and performance, and is the best heat treatment process for the tooth blank [2].