Experimental analysis on heat treatment of one speed gear

1. Microstructure analysis of pre heat treatment

After ordinary normalizing, the metallographic structure of the first speed gear blank is flake pearlite and ferrite, which is grade 3, the banded structure is grade 2, and the hardness is hb190-240. After isothermal annealing, the metallographic structure of one speed gear blank is flake pearlite and ferrite, grade 1, band structure is grade 1, and the hardness is hb160-180. Because isothermal annealing is rapidly cooled to 560 ℃ after full heat preservation at 960 ℃ and then fully heat preservation, according to the transformation diagram of Austenite Isothermal curve. It can be seen that isothermal annealing treatment makes it too late or rarely precipitate ferrite when it is cooled at high temperature, and fully heat preservation at the nose tip to obtain uniform and fine flake pearlite and a small amount of ferrite structure. Ordinary normalizing is naturally cooled in air, but according to the transformation diagram of Austenite Isothermal curve, ferrite is precipitated first, and then pearlite is precipitated. With the decrease of temperature, ferrite and pearlite are continuously precipitated. In the actual cooling process, although we spread the workpiece for cooling, the cooling speed of each part is still inconsistent, resulting in uneven structure, granular bainite structure and uneven hardness. It can be seen that the metallographic structure after isothermal annealing is more uniform and fine than that after normalizing, the banded structure is significantly improved and the hardness is uniform.

2. Deformation analysis of first speed gear

According to the change of common normal line and the broken line diagram of m value, the deformation of gear blank is obviously improved after isothermal annealing. The change of common normal line and m value of gear blank after isothermal annealing are less than that of gear blank after ordinary normalizing. The microstructure of isothermal annealing is smaller than that of ordinary normalizing. Therefore, in the process of carburizing, the microstructure of gear austenitized by isothermal annealing is more uniform than that obtained by ordinary normalizing, and the microstructure stress is smaller. After carburizing and quenching, the deformation is smaller than that of ordinary normalizing. This has been verified on many automobile gears of our company.

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