Phase 1: Simulating X65M Pipe Heat-Affected Zone Microstructures in Single Edge Notch Bending Toughness Samples to Assess Hydrogen Embrittlement

Abstract Significant variations in the hydrogen embrittlement behavior were observed in the heat-affected zones (HAZ) of pipeline steels after fusion welding. Therefore, a thorough study is needed on the characteristics of subzone microstructures within HAZs of pipeline steels, especially after multi-pass welding, and their individual impacts on hydrogen embrittlement. This work employed a two-step methodology: (1) physical simulation of representative HAZ subzone microstructures into bulk specimens using a Gleeble® 3500 following thermal profiles obtained from finite element (FE) models, followed by (2) single edge notch bending (SE(B)) toughness tests under high pressure gaseous H2. This Phase I manuscript focuses on the physical microstructural simulation methodology. The SE(B) sample geometry requires a large cross-section dimension of 7 mm by 16 mm. The crack deviation from centerline of SE(B) specimens was observed generally within ±2.5 mm, indicating the volume of interest demands a uniform microstructure within 5 mm along length direction. The Gleeble® specimen geometry was accordingly designed through finite element modeling to control the thermal gradients throughout the volume of interest. Gas quench cooling was used to replicate fast cooling curves from welding. This methodology was validated by thermal gradients measurement during Gleeble® experiments and microstructural characterizations.