Question: We are a local gear manufacturing company. We are currently experiencing a great deal of distortion on carburizing and quenching spiral-bevel pinion gears. What are the causes of the distortion? The material is forged AISI 4320.
Answer: Distortion can be attributed to a number of different aspects of the manufacturing procedure.
This procedure can contribute significantly to the distortion, particularly when hot forging. Hot forging is conducted at 2200-2300°F. The procedure is usually followed by normalizing at around 1650°F with air cool. If the carburizing temperature is conducted at 1700°F, then any residual forge stresses will come out in the form of distortion of the teeth during the carburizing procedure. This is due to the induced forging stress not being fully relieved at normalizing. Therefore, one should consider normalizing at temperatures of 30-50°F above whatever the final carburizing temperature would be. If 1700°F is the final carburize temperature, then normalize at 1730-1750°F
Because the gear does not have a uniform and constant cross-sectional area, the action of the spiral gear cutting will induce distortion. The occurrence of the distortion is due to cutting across the grain flow of the steel. Therefore, if possible, design the gear to cut the distortion into the gear teeth so that when the high-temperature distortion occurs (and it will), the teeth will move into the required position. This is difficult (but not impossible) to do. Furthermore, if the spiral bevel gear is large enough, it could be press quenched so that the teeth will not move in relation to the back, flat face. This would require the use of, for example, a Gleason press-quench unit. After quenching, it is necessary to wash the gear thoroughly to remove all oil contamination and then temper immediately.