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<p>The current work focuses on the premature failure analysis of gate valve bonnet bolts made of ASTM A193 GR B7 grade steel, used in gas pipeline systems. Visual inspection revealed severe corrosion at threads adjacent to the fractured surface, suggesting a corrosion-induced failure mechanism. To investigate the root cause, optical microscopy, scanning electron microscopy, hardness testing, and fractography analysis were employed. These methods revealed significant non-uniformity in both chemical composition and hardness at the thread ends of the bolts. The chemical analysis revealed a deviation from ASTM A193 GR B7 specification, with the measured carbon content falling below the specified minimum, indicating a heat chemistry non-compliance in the bolt material. Additionally, microstructural examination confirmed surface decarburization, i.e., loss of carbon from the surface layer due to improper heat treatment during manufacturing. The decarburized regions also exhibited reduced hardness approximately 272 HV at the decarburized surface and ~ 550 HV at the core, exceeding the specification limit of 332 HV, indicating inadequate tempering. Decarburization on the surface makes the material more prone to corrosion. Fractography revealed a mixed mode of failure. At the thread roots, corrosion and stress concentration at the decarburized surface initiated cracking. In the bolt core, a high density of fine voids and intergranular cracking were identified consistent with hydrogen-assisted cracking of the over-hardened martensitic region. The failure is attributed to the combination of surface decarburization, core over-hardening from inadequate tempering, chloride-assisted corrosion, and hydrogen-assisted cracking, all driven by non-compliant manufacturing practices compounded by aggressive service conditions. To mitigate such failures in future, it is recommended to refine and control the heat treatment process,&#xa0;to enhance surface finishing procedures, and follow quality control measures to ensure the long-term performance and safety of components in pressurized gas systems.</p>

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Investigation on Surface Decarburization Induced Corrosion Failure of ASTM A193 Gr B7 Gate Valve Bonnet Bolts in Gas Pipeline Service: Evidence of Hydrogen-Assisted Cracking

  • 期刊论文

  • 发表日期：

    2026年5月6日

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  • Sushma Jyothirmai Kurapati,
  • J. N. Mohapatra,
  • Mrunmaya Pasupalak

摘要

The current work focuses on the premature failure analysis of gate valve bonnet bolts made of ASTM A193 GR B7 grade steel, used in gas pipeline systems. Visual inspection revealed severe corrosion at threads adjacent to the fractured surface, suggesting a corrosion-induced failure mechanism. To investigate the root cause, optical microscopy, scanning electron microscopy, hardness testing, and fractography analysis were employed. These methods revealed significant non-uniformity in both chemical composition and hardness at the thread ends of the bolts. The chemical analysis revealed a deviation from ASTM A193 GR B7 specification, with the measured carbon content falling below the specified minimum, indicating a heat chemistry non-compliance in the bolt material. Additionally, microstructural examination confirmed surface decarburization, i.e., loss of carbon from the surface layer due to improper heat treatment during manufacturing. The decarburized regions also exhibited reduced hardness approximately 272 HV at the decarburized surface and ~ 550 HV at the core, exceeding the specification limit of 332 HV, indicating inadequate tempering. Decarburization on the surface makes the material more prone to corrosion. Fractography revealed a mixed mode of failure. At the thread roots, corrosion and stress concentration at the decarburized surface initiated cracking. In the bolt core, a high density of fine voids and intergranular cracking were identified consistent with hydrogen-assisted cracking of the over-hardened martensitic region. The failure is attributed to the combination of surface decarburization, core over-hardening from inadequate tempering, chloride-assisted corrosion, and hydrogen-assisted cracking, all driven by non-compliant manufacturing practices compounded by aggressive service conditions. To mitigate such failures in future, it is recommended to refine and control the heat treatment process, to enhance surface finishing procedures, and follow quality control measures to ensure the long-term performance and safety of components in pressurized gas systems.