Effect of different microstructural features on the hydrogen embrittlement susceptibility of alloy 718

Due to its high strength and acceptable corrosion resistance, Alloy 718 is a viable material for the use in subsea applications. The alloy's susceptibility to hydrogen embrittlement is however limiting its viability. In this work the effect of microstructure on the hydrogen embrittlement susceptibility of Alloy 718 was examined by the use of slow strain rate testing on three different heat treatments. Cathodic pre-charging and polarization during testing were used to introduce hydrogen into the samples. A severe reduction in the ductility of the alloy due to the precipitation phases γ″ and γ′ were found. In addition, one of the heats had a continuous film of a Nb and C rich phase on the grain boundary giving an intergranular fracture mechanism. This intergranular fracture mode was further enhanced by the presence of hydrogen. In the solution annealed condition, the presence of hydrogen reduced the dimple size on the fracture surface.