Hydrogen embrittlement behavior of iron-based superalloy A286

In this study, slow strain rate tensile (SSRT) tests were conducted at 150°C in 70 MPa gaseous hydrogen and in air, assuming compressor components for hydrogen refueling stations. Strain rate dependence revealed that the nominal stress–nominal strain curves were almost the same up to the maximum load through yield point in 70 MPa hydrogen in comparison with those in air at each strain rate. On the other hand, the relative reduction in area (RRA) decreased significantly with decreasing strain rate, and the fracture surface morphology also changed from dimple, quasi-cleavages, to intergranular fracture. Next, the SSRT test environment was switched from 70 MPa hydrogen to air in the middle of the SSRT test in order to determine at which strain the hydrogen embrittlement occurs. The results showed that the RRA values rapidly decreased when exposed to the hydrogen environment, even if only in the elastic deformation region. Subsequently, the RRA recovered near the yield point, and the RRA value gradually decreased in the plastic deformation region. The RRA also decreased when stress cycles were applied in the elastic deformation region. This rapid decrease in RRA in the elastic deformation region indicates that the lattice spacing widened and inter lattice diffusion of hydrogen and the amount of dissolved hydrogen increased. In the plastic deformation region, on the other hand, defects such as dislocations begin to move, increasing defect density and increasing hydrogen trapping, but hydrogen diffusion decreases and the amount of dissolved hydrogen decreases. As a result, the amount of trapped hydrogen at the crack tip is considered to have decreased once and then increased. The results of this study indicate that the dominant hydrogen embrittlement mechanism of metallic materials may differ depending on the test conditions, i.e., the environment in which the material is used.