Almost all gears used for power transmission of automobiles have been case-hardened by carburizing. Recently, strict demand for reducing running noise and vibration from the power transmission gears requires, in most cases, an additional finishing operation such as grinding and/or honing after carburizing. Nitriding is conducted at a temperature of about 820 K which is lower than the transformation temperature, and thus quenching is not required, resulting in smaller heat treatment deterioration. However, nitrided gears hardly used in practice as for power transmission gears.

In the present investigation, experiments were conducted, using test gears case-hardened by two different methods, carburizing and plasma-nitriding. Test results showed that the fatigue strength of carburized gears was higher than that of nitrided gears in most cases when the test gears were made from the same steel. However, the impact fatigue strengths of nitrided gears made from a high tension steel with additional alloy elements Mo and V were higher than those of carburized gears made from the carbon and alloy steels which have been, used as for gear material. The other high tension steel containing neither Mo nor V could not bring about a sufficiently high fatigue strength in comparison with the conventional carburized gears. It should be noted that the impact fatigue strength of carburized gears made from the high tension steel was higher than the ones made of conventional carburizing steel.

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