Development of new technology for gear heat treatment

Austenite Bainite Ductile iron has good comprehensive mechanical properties of high strength, high elongation and high impact value, as well as high bending fatigue strength and good wear resistance. In recent years, Austenite Bainite Ductile iron has been actively used as structural material, and gear is a typical application of Austenite Bainite Ductile iron. Using Austempered bainite ductile iron instead of 20CrMnTi steel to manufacture the transmission gear of small four-wheel tractor can reduce the gear cost and shorten the production cycle. The residual austenite will have a strengthening effect in the work, that is, the austenite in the surface layer of the gear tooth will have a work hardening effect, so that the surface has excellent wear resistance. Because the surface is continuously strengthened in the work process, it has nothing to do with the wear amount of the gear surface, which can not be achieved by general carburizing, nitriding and other surface treatments. In terms of gear structure and production process, Austempered bainitic ductile iron is more suitable for making large gears, because generally, bars are used to directly process gears with small diameter, which has low material consumption and low cost. If larger gears are made of alloy steel or carbon steel, they need to go through blanking, forging, normalizing and other processes, which has high material consumption, long cycle and high cost.

Heat treatment technology is the key to the production of Austempered bainite ductile iron gears. At present, the traditional nitrate isothermal quenching is mostly used at home and abroad to obtain obei structure. Although this heat treatment process is relatively mature, it has large investment in production equipment, high energy consumption, complex production process, great corrosion to workpieces, great pollution to the environment, and the health of workers is affected. Moreover, there are two difficult problems in the use of salt bath furnace, namely, difficult start-up temperature rise and large heat loss at the mouth of the furnace, resulting in the salt furnace becoming a heating equipment with high energy consumption. In addition, after the nitrate quenching treatment, the nitrate is easy to bond to the gear teeth, so it must be cleaned and dried thoroughly after cleaning, thus increasing the drying equipment and processes.

There is a new isothermal quenching process using high-temperature oil instead of salt bath in foreign countries. This process overcomes some shortcomings of salt bath, but there are still some disadvantages, such as long production cycle, high cost, long-term high oil temperature, easy aging, low flash point of oil, and local exceeding flash point is easy to cause combustion and unnecessary loss.

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