The purpose of pre heat treatment is to obtain uniform and fine structure and completely eliminate stress. The pre heat treatment of cylindrical gear blank generally adopts normalizing treatment, but the cooling speed range for obtaining the best microstructure and hardness of 20CrMnTi forgings of general size during normalizing is too narrow (33-38 ℃ / min), which is difficult to implement in production. Due to the limitation of field, the cooling in the actual normalizing process is heap cooling in many cases, resulting in uneven cooling of cylindrical gear blank of the whole furnace, resulting in uneven structure, This directly leads to the inconsistent deformation of cylindrical gear during heat treatment. Serious improper forging or normalizing will also promote the generation of Widmanstatten structure, which will increase the deformation of cylindrical gear heat treatment.
In addition, the uneven hardness caused by improper normalizing will not only affect the cutting quality, but also increase the cold working stress, which will also aggravate the deformation of cylindrical gear heat treatment. It is difficult to improve the banded structure after normalizing, and the grain size of cylindrical gear blank after normalizing is uneven, which are the factors to increase the deformation of cylindrical gear after carburizing and quenching. Therefore, it is imperative to adopt isothermal annealing for the pre heat treatment of cylindrical gear blank.
Isothermal annealing process refers to a process method that rapidly cools the fully austenitized parts to a certain temperature near the tip of the isothermal curve of the material in a short time, and fully maintains heat within this temperature to ensure the completion of its microstructure transformation, so as to obtain uniform flake ferrite + pearlite structure and uniform hardness. Because isothermal annealing is to heat and austenitize the cylindrical gear blank, and then quickly cool it to the isothermal temperature in the pearlite transformation area until the transformation is completed, and then free air cooling. It can prevent the out of tolerance of banded structure and the generation of granular bainite. At the same time, the structure and hardness are uniform, which provides better structural preparation for subsequent carburizing heat treatment, and finally stabilize and reduce the carburizing and quenching deformation of cylindrical gear.
Since 2007, the pre heat treatment of cylindrical gear blank of ZHY gear has adopted isothermal annealing. Compared with ordinary normalizing, its microstructure uniformity and hardness uniformity have been greatly improved, which lays a good foundation for stabilizing and improving the deformation of subsequent carburizing and quenching.