Assessing hydrogen embrittlement of alloy 718: Hollow and conventional tensile tests

• Comparison of the hollow specimen technique with conventional tests in a high pressure hydrogen autoclave and pre-charged specimen. • Slow strain rate tensile tests on alloy 718 in wrought and additive manufactured state at hydrogen gas pressures up to 200 bar • The fracture surfaces are examined fractographically and the damage mechanisms in the different test methods are discussed. • Influencing factors on hollow specimens such as surface roughness and multiaxial stresses due to internal pressure are discussed. Testing of metals for hydrogen embrittlement is of great interest for a safe hydrogen infrastructure. However, the costs and efforts involved in in-situ tests in hydrogen pressure autoclaves are high. Simpler alternatives are ex-situ testing with hydrogen pre-charging or in-situ testing with the hollow specimen technique, in which the specimen is pressurized with hydrogen gas from the inside through a longitudinal hole. However, the comparability of the techniques has not been conclusively clarified. Slow strain rate tensile tests are carried out with the three techniques on the nickel-based alloy 718 in wrought and additive manufactured states. Experiments are performed at hydrogen gas pressures up to 200 bar. Testing in nitrogen gas is used as a reference. Hydrogen pre-charging was conducted at 350 °C and 100 bar pressure. The in-situ techniques show increasing hydrogen embrittlement with increasing hydrogen pressure. The most severe embrittlement could be achieved with hydrogen pre-charging. Conventional and hollow specimens agree on yield strength and tensile strength but differ in elongation to failure and reduction of area. Influencing variables such as surface qualities and multi-axial stresses in hollow specimens resulting from the internal pressure are discussed. The damage mechanisms of all techniques are analyzed by fracture surface examinations using SEM and EBSD.