Comparison of Contact Fatigue Strength of Carbon Case Hardening and Laser Hardening of Gears

Carbon case hardening of gears is a widely used surface treatment in the manufacturing industry. However, an evident shortcoming is that gear distortion after carbon case hardening can be very large and necessitate regrinding. Laser hardening is a new heat treatment which is superior to carbon case hardening in some aspects, including a lower degree of distortion. For low quality gears, where there is no need for regrinding, laser hardening of gears can replace carbon case hardening in some fields. However, the key to whether laser hardened gears can replace carbon case hardened gears is mainly dependent on their relative tooth contact fatigue strength and tooth bending fatigue strength. The present paper compares the contact fatigue behaviour of laser hardened gears with that of carbon case hardened gears. The contact fatigue test results from actual gear tests show that the contact fatigue strength limit of laser hardened gears is 92% of that of carbon case hardened ones. The reason is that laser hardened gears have too much retained austenite in the hardened case, which leads to a lower contact fatigue strength. However, the spread of contact fatigue life of laser hardened gears is lower than that of carbon case hardened gears. SE/509