Hydrogen embrittlement susceptibility of L245NS pipeline steel after eight years of in-service exposure to pure hydrogen: a slow strain rate tensile (SSRT) investigation

To address the gap in long-term service research on hydrogen transportation pipeline steel, the hydrogen embrittlement (HE) susceptibility of L245NS pipeline steel is evaluated for the first time after 8 years of continuous pure hydrogen transportation (operating pressure: 2.13-2.87 MPa) via slow strain rate tensile tests. Results show that 8-year service significantly degrades the material's anti-HE performance, with the HE susceptibility index rising to 24.1% and the hydrogen content in serviced steel (1.502×10^-6) nearly twice that of unserviced steel (0.711×10^-6). Thermal desorption spectroscopy identifies two desorption peaks at ~100 °C (reversible traps) and ~370 °C (irreversible traps), while electron backscatter diffraction confirms the increased grain boundary length in serviced steel. Atom probe tomography (APT) reveals the synergistic modulation of grain boundary (GB) carbon segregation and hydrogen trapping, preferential hydrogen binding to carbides, and stress-driven hydrogen enrichment in nanoscale zones adjacent to GBs. Combined with transmission electron microscopy, APT verifies core hydrogen traps and enrichment characteristics. HE evolution law and microscopic mechanism of L245NS steel under long-term pure hydrogen exposure are clarified, providing critical experimental data and theoretical support for pipeline safety assessment.