某20Cr2Ni4A钢轴经两次渗碳处理，静置4 d（天）后，发生开裂。采用宏观分析、化学成分分析、断口微观分析、金相检验、气孔及夹杂物分析等方法对钢轴的开裂原因进行了分析。结果表明：不合理的热处理工艺使钢轴产生了过大的内应力，在表层的夹杂物和气孔等缺陷处产生了应力集中。裂纹在缺陷处萌生并扩展，最终导致钢轴发生静置开裂。

A 20Cr2Ni4A steel shaft was carburized twice and cracked after standing for four d (days). The cracking causes of the steel shaft were analyzed by means of macro analysis, chemical composition analysis, fracture micro analysis, metallographic examination, blowhole and inclusion analysis. The results show that the unreasonable heat treatment process caused the excessive internal stress of the steel shaft, and the stress concentration was formed at the defects such as inclusion and porosity. The cracks initiated and propagated at the defects, which eventually led to the static cracking of the steel shaft.