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<p>For a U75V flash-butt weld joint that failed due to early fracture on a heavy-haul railway line, a comprehensive approach combining ICP-AES chemical analysis, microhardness testing, SEM/EDS characterization, and EBSD phase identification was employed to systematically elucidate the failure mechanism whereby martensite induced by segregation zones led to early brittle fracture of the joint. The results indicate that the central segregation zone formed during continuous casting, after being transversely reoriented during the FBW upsetting stage, becomes enriched at the fusion line. The synergistic enrichment of carbon (0.94 wt%) and manganese (1.38 wt%) causes the local Ms point to decrease from 191 to 98&#xa0;°C. During natural cooling after welding, a local martensitic transformation occurs in the segregation zone, with a peak hardness exceeding 641 HV and a BCC martensite volume fraction of approximately 78%–85%. The hardness mismatch of about 300 HV between the martensite and the surrounding pearlitic matrix generates significant stress concentration at the interface, ultimately inducing quasi-cleavage crack initiation and early brittle fracture. This failure mechanism stems from inherent metallurgical defects in the raw material and lies beyond the scope of conventional non-destructive testing capabilities. Quality control for U75V steel FBW joints should be advanced through a coordinated approach addressing both the management of segregation during continuous casting and post-weld tempering heat treatment.</p>

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Failure analysis of premature fracture in flash-butt welded U75V rail joints

  • 期刊论文

  • 发表日期：

    2026年7月23日

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  • Chenyang Wang,
  • Chao Xu,
  • Xin Xu,
  • Jie Hu,
  • Xinwang Cao

摘要

For a U75V flash-butt weld joint that failed due to early fracture on a heavy-haul railway line, a comprehensive approach combining ICP-AES chemical analysis, microhardness testing, SEM/EDS characterization, and EBSD phase identification was employed to systematically elucidate the failure mechanism whereby martensite induced by segregation zones led to early brittle fracture of the joint. The results indicate that the central segregation zone formed during continuous casting, after being transversely reoriented during the FBW upsetting stage, becomes enriched at the fusion line. The synergistic enrichment of carbon (0.94 wt%) and manganese (1.38 wt%) causes the local Ms point to decrease from 191 to 98 °C. During natural cooling after welding, a local martensitic transformation occurs in the segregation zone, with a peak hardness exceeding 641 HV and a BCC martensite volume fraction of approximately 78%–85%. The hardness mismatch of about 300 HV between the martensite and the surrounding pearlitic matrix generates significant stress concentration at the interface, ultimately inducing quasi-cleavage crack initiation and early brittle fracture. This failure mechanism stems from inherent metallurgical defects in the raw material and lies beyond the scope of conventional non-destructive testing capabilities. Quality control for U75V steel FBW joints should be advanced through a coordinated approach addressing both the management of segregation during continuous casting and post-weld tempering heat treatment.