Abstract
The recently developed high powered engines require the increase of fatigue strength of related components, especially, the transmission gears, to meet the stringent high torque. To improve fatigue properties of transmission gears, the development of high strength ally steels, optimization of hot forging and carburizing process have been considered. In this research, the controlled cooling process which accelerated the cooling rate after hot forging to refine the grain size of prior austenite, has been developed and evaluated. The optimized condition of controlled cooling was proposed above cooling rate of 200 'C/min. in the austenite and austenite/ferrite 2 phase zone. The bending and contact fatigue properties of treated specimens were evaluated to confirm the effect of accelerated cooling rate and grain refinement after carburizing. The relationship between crack propagation and grain size was also revealed by SEM.