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<p>Stress-oriented hydrogen-induced cracking (SOHIC) of a 20# steel welded blowdown pipeline was investigated after approximately two years of service in an acrylonitrile/hydrogen cyanide (HCN)-containing medium. The crack initiated at the inner wall stagnation zone, propagated circumferentially along the weld toe, and exhibited multi-source initiation. From initiation to fast fracture, corrosion products evolved from cellular agglomerates to isolated whiskers, with Fe<sub>7</sub>(CN)<sub>18</sub> content decreasing from 44.6 to 3.2% and Fe<sub>3</sub>O<sub>4</sub> increasing from 0 to 64.7% as determined by micro-area x-ray diffraction (Micro-XRD). The crack path was confined to a high-hardness band (182–194 HV10) where bainite and Widmanstätten structure prevailed. Electron backscatter diffraction (EBSD) revealed that the geometrically necessary dislocation (GND) density at secondary cracks reached approximately 4.6 times that of the base metal, along with a strong <InlineEquation ID="IEq23"> <EquationSource Format="TEX">\(\left\langle {{\text{11}}0} \right\rangle\)</EquationSource> <EquationSource Format="MATHML"><math> <mfenced close="〉" open="〈"> <mrow> <mtext>11</mtext> <mn>0</mn> </mrow> </mfenced> </math></EquationSource> </InlineEquation>//X texture (73.4%) and a sharp rise of Σ3 boundaries (from 3.61 to 13.7%) at the expense of Σ5/Σ9 boundaries. These quantifications confirm the hydrogen-enhanced local plasticity (HELP) mechanism. The failure mode is identified as SOHIC dominated by welding residual stress, with HCN hydrolysis as the hydrogen source. Recommendations include low-point drainage, post-weld heat treatment, and the use of materials more resistant to hydrogen embrittlement.</p>

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Failure Analysis of Stress-Oriented Hydrogen-Induced Cracking of 20# Steel Welded Joints in Acrylonitrile-Containing Medium Environment

  • 期刊论文

  • 发表日期：

    2026年7月18日

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  • Ting Yu,
  • Dong Wang,
  • Zhiyu Zhao,
  • Yanbin Cheng,
  • Zhen Gao,
  • Hongbin Xie,
  • Yudong Zhang

摘要

Stress-oriented hydrogen-induced cracking (SOHIC) of a 20# steel welded blowdown pipeline was investigated after approximately two years of service in an acrylonitrile/hydrogen cyanide (HCN)-containing medium. The crack initiated at the inner wall stagnation zone, propagated circumferentially along the weld toe, and exhibited multi-source initiation. From initiation to fast fracture, corrosion products evolved from cellular agglomerates to isolated whiskers, with Fe7(CN)18 content decreasing from 44.6 to 3.2% and Fe3O4 increasing from 0 to 64.7% as determined by micro-area x-ray diffraction (Micro-XRD). The crack path was confined to a high-hardness band (182–194 HV10) where bainite and Widmanstätten structure prevailed. Electron backscatter diffraction (EBSD) revealed that the geometrically necessary dislocation (GND) density at secondary cracks reached approximately 4.6 times that of the base metal, along with a strong \(\left\langle {{\text{11}}0} \right\rangle\) 11 0 //X texture (73.4%) and a sharp rise of Σ3 boundaries (from 3.61 to 13.7%) at the expense of Σ5/Σ9 boundaries. These quantifications confirm the hydrogen-enhanced local plasticity (HELP) mechanism. The failure mode is identified as SOHIC dominated by welding residual stress, with HCN hydrolysis as the hydrogen source. Recommendations include low-point drainage, post-weld heat treatment, and the use of materials more resistant to hydrogen embrittlement.