Study on Effect of Intergranular Oxidation on the Bending Fatigue Strength of Case-Hardened Gears

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The effect of intergranular oxidation on the bending fatigue strength of case-hardened gears was investigated. The test gears were case-hardened by gas carburizing and vacuum carburizing methods, and the state of the intergranular oxidation and the grain size of these gears were examined. The effects of the intergranular oxidation on the bending fatigue strength of casehardened gears were clarified by carrying out the bending fatigue tests for these gears.Furthermore, the various mechanical surface treatments (shot peening, shot blasting,etc.) were performed on these gears, and the effect of the mechanical surface treatments after case-hardening on the bending fatigue strength was investigated by carrying out bending fatigue tests.It was found that the intergranular oxidation occurs in the case of the gas carburizing, while does not occur in the case of the vacuum carburizing. The grain size of the vacuum carburized gear tends to be larger than that of the gas carburized gear. The bending fatigue strength of the gas carburized gear is generally smaller compared with the case of the vacuum carburized gear, but that of the sub-zero treated gas carburized gear becomes larger depending on the depth of the intergranular oxidation. The bending fatigue strength of both the gas carburized and the vacuum carburized gears increases by the shot peening and the shot blasting after case-hardening. The bending fatigue strength of the vacuum carburized gear decreases by grinding and skiving the tooth root after case-hardening, whicle that of the gas carburized gear increases depending on the cutting amount of the mechanical surface treatment.Thus the effect of the intergranular oxidation on the bending fatigue strength of casehardened gears was clarified, and the useful data for the design of the case-hardened gears were obtained from this investigation.