某45钢齿轮轴在热处理后的精车加工过程中发现齿轮轴表面有纵向分布的发纹。采用宏观观察、化学成分分析、金相检验、扫描电镜及能谱分析等方法对发纹产生的原因进行了分析。结果表明：该齿轮轴表面的发纹是由硫化物夹杂聚集导致的，热处理过程中聚集的硫化物夹杂引起齿轮轴应力集中，产生沿夹杂物扩展的淬火裂纹，并最终形成发纹。

In the finish turning process of a 45 steel gear shaft after heat treatment, there were longitudinal hair cracks on the surface of the gear shaft. The causes of the hair cracks were analyzed by means of macroscopic observation, chemical composition analysis, metallographic examination, scanning electron microscope and energy spectrum analysis. The results show that the hair cracks on the surface of the gear shaft was caused by the accumulation of sulfide inclusions. The sulfide inclusions accumulated in the heat treatment process caused stress concentration of the gear shaft, resulted in quenching cracks extending along the inclusions and finally formed the hair cracks.