某公司生产的20MnCr5钢齿轮轴经渗碳淬火处理，精加工后发现齿轮轴表面局部可见明显斑疤。采用宏观观察、化学成分分析、扫描电镜分析和金相检验等方法对齿轮轴表面缺陷的产生原因进行了分析。结果表明：渗碳热处理前使用的防渗碳涂料中的金属铜渗入钢材的晶界，使钢材发生脆化开裂，继而防渗碳涂料中其他低熔点物质也渗入钢材的晶界，破坏钢材基体的连续性，因此精加工后齿轮轴表面出现斑疤状的缺陷。

The 20MnCr5 steel gear shafts was treated with carburizing and quenching. After fine machining, obvious spotted scars were found on the surface of the gear shafts. The causes of surface defects of the gear shafts were analyzed by means of macro observation, chemical composition analysis, scanning electron microscopy analysis, metallographic examination and so on. The results show that the copper in the anti carburizing coating used before carburizing heat treatment penetrated into the grain boundaries of steel and caused embrittlement cracking of steel material. Then other low melting point substances in the anti carburizing coating penetrated into the grain boundaries of steel and destroyed the continuity of the steel matrix. Therefore, the surface of gear shaft appeared spotted scar defects after fine machining.