Enhancing heavy load wear resistance of AISI 4140 steel through the formation of a severely deformed compound-free nitrided surface layer

The tribological application of gas nitriding on alloy gears used under high-speed and heavy-duty conditions has been limited owing to the intrinsic characteristics of the traditional gas-nitrided layers. Complex phenomena of wear failure are involved under such harsh conditions, therefore a tougher nitrided layer with excellent comprehensive performance is necessary to prevent them. To this aim a severely deformed compound-free nitrided layer with an outermost ultra-fine-crystalline layer and a subsurface deformed nitrogen diffusion zone was prepared on an AISI 4140 sample through the combined treatment of controlled gas nitriding and ultrasonic shot peening. The effect of the severely deformed compound-free nitrided layer on the hardness, toughness and wear properties was investigated. The enhanced wear resistance under heavy load after the combined treatment can be attributed to the good combination of surface hardness and toughness, as well as the significantly enhanced residual compressive stress. The two-step surface treatment provides an effective approach to enhancing the wear resistance of heavy-duty gears.