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<p>Metal bellows expansion joints are critical components in fluid catalytic cracking (FCC) units and accommodate large thermal displacements under extreme conditions. This study systematically investigates the degradation and ultimate failure of a retired 904L stainless steel bellows through combined experimental characterization and coupled thermal–structural finite element analysis. Chemical analysis revealed a carbon content of 0.029&#xa0;wt.%, which slightly exceeds the specified limit and thereby increases susceptibility to sensitization at a service temperature of approximately 600&#xa0;°C. Mechanical testing indicated significant work hardening, with the yield strength increasing to 575&#xa0;MPa. Thermal–structural finite element simulations identified severe stress concentration at the bellows wave crest. The calculated maximum von Mises stress was 205.5&#xa0;MPa, exceeding the allowable stress of the material at the elevated temperature. These findings reveal a synergistic degradation mechanism driven by structural and environmental factors. Localized pitting corrosion first develops at the wave crest due to the gravity-driven accumulation of sulfate- and phosphate-containing condensates during shutdown periods. Subsequent operation under high thermal stress provides a strong driving force for these pits to transition into microcracks, indicating a high susceptibility to stress-assisted corrosion (SAC) and posing a severe risk to the long-term structural integrity of the bellows.</p>

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Synergistic Degradation Mechanism of 904L Stainless Steel Bellows in FCC Units: Multi-scale Characterization and Coupled Thermal–Structural Evaluation

  • 期刊论文

  • 发表日期：

    2026年7月10日

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  • Wang Sui,
  • Liu Wencai,
  • Xu Xiuqing,
  • Su Hang

摘要

Metal bellows expansion joints are critical components in fluid catalytic cracking (FCC) units and accommodate large thermal displacements under extreme conditions. This study systematically investigates the degradation and ultimate failure of a retired 904L stainless steel bellows through combined experimental characterization and coupled thermal–structural finite element analysis. Chemical analysis revealed a carbon content of 0.029 wt.%, which slightly exceeds the specified limit and thereby increases susceptibility to sensitization at a service temperature of approximately 600 °C. Mechanical testing indicated significant work hardening, with the yield strength increasing to 575 MPa. Thermal–structural finite element simulations identified severe stress concentration at the bellows wave crest. The calculated maximum von Mises stress was 205.5 MPa, exceeding the allowable stress of the material at the elevated temperature. These findings reveal a synergistic degradation mechanism driven by structural and environmental factors. Localized pitting corrosion first develops at the wave crest due to the gravity-driven accumulation of sulfate- and phosphate-containing condensates during shutdown periods. Subsequent operation under high thermal stress provides a strong driving force for these pits to transition into microcracks, indicating a high susceptibility to stress-assisted corrosion (SAC) and posing a severe risk to the long-term structural integrity of the bellows.