Effect of Nitrogen on Hydrogen Embrittlement of Austenitic Stainless Steels Based on Type 316LN

The effect of nitrogen on hydrogen gas embrittlement (HGE) in 1 and 70 MPa hydrogen and internal reversible hydrogen embrittlement (IRHE) of austenitic stainless steels of 17Cr11Ni2Mo(0.4 in max.)N alloys, based on type 316LN, was investigated by slow strain rate technique tests at room temperature in comparison with the effect of Ni on HGE and IRHE of Ni-added type 316 stainless-steel-alloys. For the nitrogen-added alloys, HGE and IRHE decreased with increasing nitrogen content, where α′ martensitic transformation occurred. HGE was not observed but IRHE was observed above the nitrogen content, where austenite is completely stabilized by nitrogen. Hydrogen-induced fracture related to the strain-induced α′ martensite structure was observed in HGE specimens and that together with brittle transgranular fracture was observed in IRHE specimens. HGE of the nitrogen-added alloys is larger than that of the Ni-added alloys in the Nieq range, where α′ martensitic transformation occurred. No HGE was observed in both the nitrogen-added alloys and the Ni-added alloys, but IRHE was observed in not the Ni-added alloys but the nitrogen-added alloys above the Nieq, where no martensite is identified in both alloys. It is discussed that the α′ martensite and the austenite of the nitrogen-added alloys were more sensitive to HGE or IRHE than those of the Ni-added alloys.

Internal Reversible Hydrogen Embrittlement and Hydrogen Gas Embrittlement of Austenitic Stainless Steels Based on Type 316

The internal reversible hydrogen embrittlement (IRHE) of austenitic Fe(10–20)Ni17Cr2Mo alloys based on type 316 stainless steel was investigated by tests using the slow strain rate technique from 80 to 300 K in comparison with its effect on the hydrogen gas embrittlement (HGE) of the alloys in hydrogen at a pressure of 1 MPa. The IRHE and HGE of the alloys in 70 MPa hydrogen at room temperature was also investigated. At low temperatures, IRHE occurred below a Ni content of 15% (Ni equivalent (Nieq):29%), increased with decreasing temperature, reached a maximum at 200 K, and decreased with further decreasing temperature, similarly to the temperature dependence of HGE. At room temperature, IRHE and HGE were observed below a Ni content of 14% (Nieq:28%) and decreased with increasing Ni content (Nieq). The dependence of HGE on hydrogen pressure increased with decreasing Ni content (Nieq). Hydrogen-induced fracture closely related to the strain-induced α′ martensite structure and twin boundaries mainly occurred for both IRHE and HGE. Dimple ruptures caused by hydrogen segregation occurred in only IRHE at 150 K. The content of strain-induced α′ martensite increased with decreasing temperature and Ni content (Nieq). Thus, the susceptibility to IRHE and HGE depended on Ni content (Nieq). It was concluded that both IRHE and HGE were controlled by the amount of strain-induced α′ martensite above 200 K, whereas they were controlled by the hydrogen transport below 200 K.