Research On The Mechanism Of Heat Treatment Distortion And Its Control Methods For Case-hardened Gear Of Rolling Mill

Aiming to the control of heat treatment distortion of gear, the research work in this dissertation is carried out integrated theoretical, experimental and industrial method. The object of study is large helical gears made of 20CrMnMo and its addendum circle is more than 1 meter in diameter. It is listed as following:1) to analysize the general topography of heat treatment distortion of carburized gears in order to get the distortion rule and prepare for the discovery of the mechanism. 2) to analysize the influence factors in heat treatment distortion, considering the layout, pre-heat treatment, carburizing and hardening process and so on.3) to design the digital measurement methods of heat treatment distortion.Some innovative results are obtained in this research, and they are listed as following:1) the gear heat treatment distortion rules are deducted, including the variation of the common normal length, helix angle, end face, and the minor and major diameter; 2) the test method to study the effect of cooling process on the heat treatment distortion after carbonitriding is designed, especially including changing pre-cooling temperature after carbonitriding in old pit gas carburizing furnace; 3) the new cooling process after carbonitriding is designed. It takes furnace cooling and air cooling process for carbonitriding gear in controlled atmosphere pit gas carburizing furnace; 4) the digital measurement method of heat distortion using FARO Platinum Arm is designed to measure the gear heat treatment distortion.The research results show as following:1) the variation of normal length of the lager gear turns lower when pre-cooling to 450℃ after carbonitriding; 2) carbonitriding structure can be amended and the gear heat treatment reduces clearly in the controllable atmosphere furnace, due to its online automatic control system and controllable fast cooling system.3) it is an digital measurement method using FARO Platinum Arm to obtain the normal length of the lager gear, helix anale, the hole diameter and the diameter of the addendum circle.