Development Of Preheating Treatment For Gear Steel Of Automobiles

The influence of common and isothermal normalizations on microstructures and properties of forged blanks for 22CrMo and 20MnCr steel were studied by different technological experiments of preheating, hardness measurements and metallographic analyses. Results show that both the size and number of pre-eutectoid ferrite decrease and the number of pearlite increases with the increase of cooling speed of common normalization, leading to the improvement of hardness of the carburizing alloy steel. Further increase of the cooling speed will result in the formation of bainite, which will cause sharp increase of hardness. Therefore, the range of cooling speed of common normalization to get best microstructure and hardness is narrow, and is difficult to control. While, for isothermal normalization, appropriate microstructure and hardness can be obtained in a wide range of isothermal temperatures, and is easy to control in practical production. Isothermal normalizations using the remaining heat of forging were also studied. It is found that the equilibrium ferrite and pearlite formed in original large austenite grains in the process of isothermal normalizations using the remaining heat of forging can transfer to fine and even austenite grains after carburizing. This kind of new technology can not only save energy and improve production, but also obtain stable and reliable microstructure and hardness required, getting good cutting performance and stable distortion regularity of work-pieces after carburizing and quenching. The experience proved that adopting this new isothermal normalization in steel 22CrMo and 20MnCr. we can obtain stably and reliably microstructure of Ferrite and Pearlite . The hardness ranges from 160-180HB and save much more money . The cost of normalization can reduce from 0.31 yuan per kilogram to 0.18 yuan per kilogram , so it will save 2064800 yuan together one year since you adopt this methord. To get good results for isothermal normalization of forged blanks, three parameters i.e. cooling rate before isothermal treatment, isothermal temperature and...