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

研究了晶间氧化对淬火齿轮弯曲疲劳强度的影响。采用气体渗碳和真空渗碳两种方法对试验齿轮进行了淬火，并对齿轮的晶间氧化状态和晶粒尺寸进行了检测。通过对淬硬齿轮的弯曲疲劳试验，阐明了晶间氧化对淬硬齿轮弯曲疲劳强度的影响。在此基础上，对齿轮进行了喷丸、抛丸等机械表面处理，并通过弯曲疲劳试验研究了表面硬化后的机械表面处理对齿轮弯曲疲劳强度的影响。结果表明，在气体渗碳的情况下会发生晶间氧化，而在真空渗碳的情况下不会发生晶间氧化。真空渗碳齿轮的晶粒尺寸比气体渗碳齿轮的晶粒尺寸大。气体渗碳齿轮的弯曲疲劳强度一般比真空渗碳齿轮小，但低温处理的气体渗碳齿轮的弯曲疲劳强度随晶间氧化的深度而变大。气体渗碳齿轮和真空渗碳齿轮在淬火后进行喷丸强化和抛丸处理均能提高齿轮的弯曲疲劳强度。真空渗碳齿轮的弯曲疲劳强度随表面处理切削量的增加而降低，而气体渗碳齿轮的弯曲疲劳强度随表面处理切削量的增加而增加。阐明了晶间氧化对淬硬齿轮弯曲疲劳强度的影响，为淬硬齿轮的设计提供了有益的数据。

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.