某从动齿轮在中修时发现开裂。采用宏观检验、化学成分分析、金相检验和显微硬度测定等方法对开裂的从动齿轮进行了分析。结果表明，因中频淬火的不连续，使淬硬层出现了回火软区，组织为回火索氏体，降低了材料的硬度和疲劳极限，最终导致从动齿轮开裂。提出了建议。

Driven gear cracking was found in repair. In order to find out the reason of driven gear cracking,the methods of macro-examination,chemical composition analysis,microstructure examination and micro-hardness testing were carried out to analyze the cracked driven gear. The results showed that tempered soft zone in harden zone case produced in process of discontinuous medium frequency induction hardening which has microstructure of tempered sorbite and low hardness and low fatigue limit were major reason to cause driven gear cracking. According to analytic result,we put forward the suggestion.