Effect of Ni Content and Tempering Temperature on Hydrogen Embrittlement of SA372 Steels for Pressure Vessel

This study investigates the influence of grain boundary characteristics on hydrogen embrittlement in SA372 steels by varying Ni content and tempering temperature. Microstructural characteristics and mechanical properties were analyzed to understand hydrogen trapping behavior, followed by an in-situ slow strain-rate test (SSRT) to assess hydrogen embrittlement resistance and hydrogen-induced fracture behavior. A higher tempering temperature reduces the density of reversible hydrogen trap sites, such as dislocations, thereby decreasing the steel’s resistance to hydrogen embrittlement. However, key factors influencing hydrogen trapping, including lattice sites, dislocations, grain size, and tensile strength, remained unchanged regardless of Ni content. Consequently, the hydrogen content measured through thermal desorption analysis (TDA) was also consistent. The fraction of Σ3 grain boundaries was the only factor that increased with higher Ni content, indicating a potential role in hydrogen embrittlement. The SSRT results demonstrated that at elevated tempering temperatures, higher Ni content reduced hydrogen embrittlement resistance, leading to intergranular fractures on the fracture surface. This reduction in resistance was attributed to hydrogen accumulation along prior austenite grain boundaries, facilitated by dislocation pile-ups and its diffusion along Σ3 grain boundaries. At lower tempering temperatures, however, the effect of Ni content on hydrogen embrittlement resistance was less significant, as intergranular fractures were primarily associated with high dislocation density rather than Σ3 grain boundaries.