Study on Mechanism of Bending Ftigue Breakage and Strengthening Method of Gear Teeth

对喷丸强化的烧结金属齿轮进行了弯曲疲劳试验，测定了喷丸强化对烧结金属齿轮弯曲疲劳强度的影响，研究了离子渗氮、软氮化和表面硬化处理对齿轮齿形的影响，测定了这些强化方法对齿轮的变形量。对不同渗层深度的离子渗氮齿轮进行了均匀循环载荷下的弯曲疲劳试验，确定了离子渗氮齿轮的弯曲疲劳强度。在Miner假设的基础上，对离子渗氮齿轮在两级加载条件下的弯曲疲劳特性进行了研究，得到的主要结果如下：1.表面硬化的烧结金属齿轮的表面硬度增加，通过应用喷丸，但这些齿轮的硬化层深度不改变取决于喷丸。表面硬度不随含碳量（C0.3-0.5%）变化，但心部硬度随含碳量的增加而增加.经喷丸处理后，表面硬化的烧结金属齿轮的弯曲疲劳强度提高了20-40%。离子渗氮和软氮化齿轮齿廓变化量几乎相等且较小，而表面硬化齿轮齿廓变化量较大. S45 C和SACM 645离子渗氮齿轮的弯曲疲劳极限分别在渗氮时间为34和17小时时达到最大值，比正火齿轮提高约40%和80%。软氮化使S45 C齿轮的弯曲疲劳极限提高了30%左右.离子渗氮齿轮的弯曲疲劳断裂是由化合物层中萌生的小裂纹扩展引起的。

Bending fatigue tests (pulsator tests) for shot-peened sintered metal gears were carried out, and the effect of shot-peening on the bending fatigue strength of sintered metal gears was determined.The tooth profile changes of gears due to ion-nitriding, tufftriding and case-hardening were examined, and the deformations of gears due to these strengthening methods were determined. The bending fatigue tests for ion-nitrided gears with different case depths under uniform cyclic loading conditions were carried out, and the bending fatigue strength of ion-nitrided gears was determined. Furthermore, the bending fatigue characteristics of the ion-nitrided gears under two-step loading conditions was discussed on the basis of Miner's hypothesis.The main results obtained from this investigation are summarized as follows.1. The surface hardnesses of case-hardened sintered metal gears increase by applying shot-peening, but the case depths of these gears do not change depending on shot-peening. The surface hardnesses do not change with the carbon content (in the range of C0.3-0.5%), but the core hardnesses increase with the carbon content.2. The bending fatigue strengths of case-hardened sintered metal gears become 20-40% larger by applying shot-peening.3. The tooth profile changes of gears due to ion-nitriding and tufftriding are almost equal and fairly small, but those due to case-hardening are fairly large.4. The bending fatigue limits of S45C and SACM645 ion-nitrided gears become maximums for ion- nitriding time 34 and 17 hours, respectively, and are about 40 and 80% larger than the normalized gears. The bending fatigue limits of S45C gears become about 30% larger by tufftriding.5. The bending fatigue breakage of ion-nitrided gears occurs by the propagation of some of small cracks which initiate at the compound layr.