Distortion control of big disc conical gear during carburizing and hardening processes

A outer diameter 495 mm disc conical gear made of SAE8822H steel was treated as follows.The original process was pre-isothermal normalizing→machining→gas carburizing in all-case furnace with directly heating from ambient temperature to 930 ℃,using overlapped charging method,then air cooling,second heating to 850 ℃ in rotary hearth furnace,manually transmitting the workpiece to press quenching with oil cooling,tempering in all-case furnace.The result showed that the distortion of the gears only 60% was qualified and followed some surface decarburization.The improved process is pre-isothermal normalizing,machining,gas carburizing in double array pusher continuous carburizing production line with step heating(from ambient temperature to 480 ℃,and then to 900 ℃ and 930 ℃,using separated charging method),then pre-cooling in the furnace to 860 ℃,by manipulator transmitting the workpiece to press quenching with oil,tempering in all-case furnace,the final results show that the distortion of the gears are all controlled in the qualify range,and the surface decarburizing can be avoided basically.