某18CrNiMo7-6钢齿轮轴在机械加工结束后，在库存期间发生了轴向开裂。利用宏观分析、化学成分分析、渗碳层深度测定、扫描电镜观察、能谱分析等方法，对齿轮轴的开裂原因进行了分析。结果表明：齿轮轴原材料冶炼过程不当，混入了小颗粒状氧化铝类夹渣；在随后的锻打拉拔过程中小颗粒形成条带状，破坏了材料的连续性，形成应力集中；在较大的残余应力作用下，齿轮轴发生了延迟性开裂。

The 18CrNiMo7-6 steel gear shaft cracked along the axial direction during storing after machining processing. The cracking reasons of the gear shaft were analyzed by methods of macroscopic analysis, chemical composition analysis, depth measurement of carburized layer, scanning electron microscopy observation, energy analysis and so on. The results show that:there were some particles of aluminium oxides mixed into the raw material of the gear shaft due to the improper smelting process; the particles were banded during forging and drawing process, which destroyed the continuity of the material and resulted in the stress concentration; under the action of large residual stress, delayed cracking occurred to the gear shaft.