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<p>This study investigates the microstructure, mechanical properties, and corrosion behavior of the cap and root regions in multi-pass shielded metal arc welded (SMAW) joints of API X70 pipeline steel. Comprehensive characterization techniques, including scanning electron microscopy/energy dispersive X-ray spectrometry, nanoindentation, and potentiodynamic polarization, were used. The root region, subjected to faster cooling, developed a fine mixture of acicular ferrite (AF) and bainite, resulting in higher hardness (5.5 ± 0.3 GPa and 4.1 ± 0.6 GPa, respectively). In contrast, the cap region exhibited softer polygonal and grain boundary ferrite due to slower cooling. Electrochemical tests in 3.5% NaCl solution revealed greater corrosion susceptibility in both regions compared to the base metal, with the root showing the poorest performance due to microstructural heterogeneity and surface roughness. These findings highlight the critical influence of welding thermal cycles on the local performance of HSLA X70 welds in corrosive environments.</p>

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Microstructural evolution, mechanical properties, and corrosion resistance of welded X70 pipeline steel: effects of thermal welding cycles on cap and root regions

  • 期刊论文

  • 发表日期：

    2025年7月14日

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  • Bouzid Maamache,
  • Naima Ouali,
  • Brahim Belkessa,
  • Azeddine Chebili,
  • Khawla Amara,
  • Malek Hebib,
  • Billel Cheniti

摘要

This study investigates the microstructure, mechanical properties, and corrosion behavior of the cap and root regions in multi-pass shielded metal arc welded (SMAW) joints of API X70 pipeline steel. Comprehensive characterization techniques, including scanning electron microscopy/energy dispersive X-ray spectrometry, nanoindentation, and potentiodynamic polarization, were used. The root region, subjected to faster cooling, developed a fine mixture of acicular ferrite (AF) and bainite, resulting in higher hardness (5.5 ± 0.3 GPa and 4.1 ± 0.6 GPa, respectively). In contrast, the cap region exhibited softer polygonal and grain boundary ferrite due to slower cooling. Electrochemical tests in 3.5% NaCl solution revealed greater corrosion susceptibility in both regions compared to the base metal, with the root showing the poorest performance due to microstructural heterogeneity and surface roughness. These findings highlight the critical influence of welding thermal cycles on the local performance of HSLA X70 welds in corrosive environments.