Slow strain rate tensile test properties of iron-based superalloy SUH660 in hydrogen gas

To investigate dependence of strain rate of tensile test for iron-based superalloy SUH660 (A286), tensile tests were conducted for the specimens in 70 MPa hydrogen gas and air at 150°C. Nominal stress-nominal strain curve of each strain rate in 70 MPa hydrogen gas showed same behavior to maximum load via yield point in comparison with that in air, however, each elongation at breaking point in 70 MPa hydrogen was a little shorter than that in air. The values of tensile strength didn't depend on the strain rate in 70 MPa hydrogen as well as those in air. In addition, the difference in tensile strength wasn't observed between that in 70 MPa hydrogen gas and that in air for the strain rate. However, it's proved that relative reduction of area in 70 MPa hydrogen to that in air was significantly affected by strain rate of tensile test. Those values were 80%, 51%, and 32% in the case of strain rate 5.0×10^-5, 7.5×10^-6, and 1.25×10^-6 s^-1, respectively. The morphology of fracture surface also changed from dimple to quasi-cleavage (QC), with a decrease in strain rate. Simulation of hydrogen gas diffusion form surface to inside during experiment showed that the hydrogen diffusion layer of specimen with QC fracture surface (RRA 51%, strain rate 7.5×10^-6 s^-1) was only 0.25 mm in depth. That implies that hydrogen content at crack tips is much higher than that of simulation due to hydrogen concentration by a couple of defects. That tendency seems to become stronger with a decrease in strain rate.