对某汽车发动机曲轴用40CrNiMoA钢进行低倍酸蚀检查时，发现其心部存在疑似裂纹的缺陷。采用低倍酸蚀对比、化学成分分析、金相检验、磁粉检测等方法分析了缺陷产生的原因。结果表明：材料心部组织不致密及心部存在的异常贝氏体组织导致其不耐腐蚀，在酸蚀过程中产生了腐蚀坑，这些腐蚀坑串连在一起形成了“假裂纹”缺陷。对材料连铸及轧钢过程的工艺参数进行优化后，其致密度提高，心部贝氏体异常组织消失，心部酸蚀30 min后无明显腐蚀坑存在。

The defects of suspected cracks in the heart of 40CrNiMoA steel used for crankshaft of an automobile engine were found during the low magnification acid corrosion test. The causes of defects were analyzed by means of acid etching contrast, chemical composition analysis, metallographic examination and magnetic particle inspection. The results show that the core microstructure of the material was not dense, and the abnormal bainite structure in the core led to its non-corrosion resistance. Corrosion pits were generated in the acid corrosion process, and these corrosion pits were linked together to form a "pseudo-crack" defect. After optimizing the process parameters of the continuous casting and rolling process of the material, the density was increased, the abnormal bainite microstructure in the heart disappeared, and there was no obvious corrosion pit in the heart after 30 min of acid etching.