Automobile gears are usually made by alloy carburizing. The gear blank generally needs to go through multiple cold and hot processing processes such as forging blank – pre heat treatment – cutting processing – carburizing quenching and finishing, so as to obtain high surface hardness and good core toughness, so that the gear has excellent properties such as wear resistance, fatigue resistance and corrosion resistance. Normalizing is a common process for pre heat treatment of automotive transmission gear forging blanks, which is used to improve grain size, obtain appropriate cutting hardness (160 ~ 190hbw), and prepare microstructure for carburization to reduce distortion. However, in the conventional normalizing treatment, due to the limitation of equipment, the stacking cooling method will cause great differences in the cooling speed, microstructure, stress and hardness of different parts or different parts of the same part, lead to the deterioration of cutting performance and the increase of heat treatment deformation, reduce the gear accuracy grade and affect the service performance of the gear.
1) Conventional normalizing is easy to produce non-equilibrium structure, which not only affects the cutting performance, but also aggravates the carburizing and quenching deformation of automobile gear blank. Ordinary normalizing is not suitable as pre heat treatment.
2) It is easy to obtain uniform microstructure and appropriate hardness by controlling the cooling rate and temperature of gear blank after forging, and the deformation is significantly reduced after carburizing and quenching.
3) Using forging waste heat instead of normalizing heating can save energy and improve production efficiency.
Since the forging of gear blank is stopped at 1050 ~ 1000 ℃, at this time, the blank is still in austenitic state. Through controlled cooling, it is possible to obtain microstructure and hardness similar to normalizing, that is, using forging waste heat instead of normalizing heating, so as to save energy and improve production efficiency. However, the austenite grain of the blank after stopping forging is significantly coarser than that of the normal reheating. This coarser grain characteristic will occur microstructure inheritance during carburizing heating, which will deteriorate the properties of the parts; At the same time, the austenite grain coarsening and uneven carbon concentration of the steel after stopping forging lag the F + P transformation, and it is easy to obtain bainite structure during cooling, resulting in poor machining performance. This experiment can completely avoid the above situation by controlling heat preservation and cooling, and realize isothermal normalizing by using forging waste heat (hereinafter referred to as forging heat isothermal normalizing).