42CrMo钢曲轴连杆颈磁痕显示的成因分析

利用磁粉探伤对42CrMo钢曲轴锻件的连杆颈部位进行检测,发现明显的类似发纹的磁痕显示。通过OM、SEM形貌观察和EDS成分分析发现,连杆颈表层部位基体的显微组织不均匀,出现明显的成分偏析带,宽度约20~50 μm,C、Mn、S、Al、Cr元素含量相对基体较高,并含有少量的MnS夹杂。42CrMo钢棒状原料的中心偏析在模锻时沿着金属流动方向发生转动,偏析流线“遗传”至终态连杆颈表层而造成显微组织和化学成分的不均匀,进而导致磁导率的差异而出现磁痕显示。锻后的调质处理和表面感应淬火在一定程度上可以改善但不能消除这种宏观成分偏析。

Cause analysis of magnetic particle indication on 42CrMo steel crankshaft neck

The hair-line magnetic particle indication was detected by magnetic particle testing at the neck part of 42CrMo steel crankshaft. The microstructure just below the magnetic particle indication was studied by OM,SEM and EDS. The results show that the microstructure is considerably uneven,and significant segregation bands are found with a width of about 20-50 μm and enriched with the C, Mn, S, Al and Cr elements. In addition, individual MnS inclusion is also detected. The segregation bands paralleling with the axis of columnar 42CrMo steel are rotated during die forging process, especially at the neck part of 42CrMo steel crankshaft with severe plastic distortion.The macro-segregation is inherited to the final crankshaft product. The heterogeneous compositional distribution leads to the difference of magnetic permeability and thus the appearance of magnetic particle indication. The forging process and the following quenching-tempering as well as surface induction quenching treatment can mitigate to some extent but cannot totally eliminate the segregation.