Novel Microstructural Necessities for High Durable Automotive Speed Gears

In general, for automotive transmission gears, carburizing / carbonitriding process is being followed globally for its superior bending and rolling contact fatigue properties. These properties are widely influenced by various factors like material chemistry, hardness, hardening depth, microstructure and residual stress, etc. Out of these factors, microstructural requirements are vital for transmission gear’s durability which being controlled by various process methods. This research work explains, experimental research on microstructure characteristics improvement for highest durability of the gear. Higher retained austenite in the microstructure is detrimental for bending fatigue life in longer cycle regime due to drop in strength. But, it is beneficial for pitting resistance due to ability of deforming plastically. To balance both bending fatigue & pitting resistance, retained austenite level is aimed to control in surface & sub-surface of transmission gear by modifying carburizing process. Comparative study on conventional and modified carburizing process carried-out for hardness profile, phase analysis, retained austenite profile and residual stress. Durability testing on transmission gear revealed that survival life with modified carburizing cycle is more than two times of conventional process.