45钢外套螺母表面裂纹分析

45钢外套螺母装配时发现表面存在笔直的纵向裂纹。经后续排查，同批交付的1 069件螺母中有116件存在相似的表面缺陷。通过对外套螺母进行裂纹和断口的宏微观观察、能谱成分分析以及金相组织检查，并结合生产工艺检查的结果，最终确定了外套螺母的开裂原因。结果表明:外套螺母开裂的主要原因是原材料表面存在折叠缺陷，热处理淬火时在淬火应力的作用下，裂纹由折叠缺陷起始并向厚度方向扩展；建议增加无损检测工序并严格工艺纪律管理，对此类故障进行避免和防范。

Analysis on Surface Crack of 45 Steel Cap Nut

A 45 steel cap nut was found to have surface crack during assembly. After subsequent screening, 116 of the 1 069 cap nuts delivered in the same batch had the same surface defects as the failure one. In order to find out the cracking reason, macro and micro observation on crack and fracture, energy spectrum analysis, and metallurgical structure examination were carried out. Based on these experimental results and production process inspection result, the final cracking reason of cap nut was determined. The results show that fold defects existed in raw material, which is the main reason for cracking. The primary fold defects became cracking sources when quenching, and extended in the thickness direction under the stress of quenching. It is proposed that nondestructive inspection should be used and process discipline management should be strict in order to avoid and prevent such failure.