采用硬度计、轮廓仪、光学显微镜、扫描电子显微镜等对服役后的U71Mn钢轨气压焊接头的硬度、裂纹形貌及显微组织进行测试与观察，分析接头不同区域的损伤行为。结果表明：焊接接头两侧距接头中心30～50 mm处存在低凹区域，其位置与软化区位置吻合；轮轨接触使接头硬度提高，但是软化区的硬度始终低于母材；根据珠光体形态，接头从中心向两侧热影响区可依次分为层片状珠光体区域、部分珠光体球化区I、珠光体球化区、部分珠光体球化区Ⅱ；软化区位于珠光体球化区；珠光体球化区与部分珠光体球化区的裂纹深度和裂纹尖端扩展角都较大，这是由于粒状珠光体对裂纹扩展的阻力较小导致的。

The hardness, crack morphology and microstructure of U71Mn steel rail gas pressure welded joint after service were tested and observed by hardness tester, profilometer, optical microscope and scanning electron microscope. The damage behavior in different regions of the joint was analyzed. The results show that the depressions were located at 30-50 mm away from the joint center on both sides, and their positions were consistent with the softening zones in the joint. The hardness of the joint increased because of wheel-rail contact, but the hardness of the softening zone was always lower than that of the base metal. According to morphology of pearlite, the joint was divided into four regions from the joint center to heat-affected zone on both sides: lamellar pearlite area, partial pearlite spheroidization area I, pearlite spheroidization area and partial pearlite spheroidization area Ⅱ. The softening zone was located in pearlite spheroidization area. The depths of cracks and the expansion angles of crack tips in pearlite spheroidization and partial pearlite spheroidization areas were relatively large, because of the relative small resistance of granular pearlite to crack growth.