Investigating Low Toughness Events in SA – 350 LF2 CL1 Flanges: A Case Study

Abstract The industry has seen a rise in low toughness events involving standard SA-350 LF2 Cl1 flanges in recent years, leading to conservative approaches by EPC Contractors and End Users whenever they face an issue in an impact test. This might result in the rejection of complete lots without thorough root cause analysis, contrasting with the lenient sampling criteria of SA-350, which allows certification with minimal lot sampling. A recent case during a major project involved three low toughness events out of ten flanges used for welding qualification. Impact tests on these flanges showed values below SA-350 thresholds, prompting an investigation. The inquires revealed that improper extraction of test specimens and a lengthy post-weld heat treatment (PWHT) were the main issues, rather than inherent poor toughness of the flanges. To verify this, six mock-ups were welded together, illustrating the effect of PWHT parameters on toughness when specimens were correctly extracted. An optimized PWHT cycle within ASME B31.3 requirements was then selected to minimize toughness deterioration while maintaining hardness below 248 Hv10. This experience reveals the importance for EPC Contractors to specify supplementary requirements for the fabrication and certification of SA-350 flanges beyond the Code requirements. It also calls for a measured response whenever an impact test failure occurs, to avoid misidentifying as a low toughness event.