Comparison of Internal Hydrogen Embrittlement of Superalloys 718 and 625

Abstract

Mechanical tests were used to compare the internal hydrogen embrittlement susceptibility of alloy 718 to that of alloy 625. Slow, constant extension rate tests (CERT), on edge-notched strip specimens were performed at 25'C for alloy 718 (both aged and annealed) and alloy 625 (annealed). The results for internal hydrogen were compared to those for (i) 101 kPa external gaseous hydrogen environments, and (ii) dynamic cathodic charging at current densities up to 1000 A/m2 in a 0.7 M HzS04 plus 1 g/l of NaAs02 solution. Hydrogen precharging was carried out at 250°C in a molten salt bath for times sufficient to produce uniform hydrogen concentrations up to 50 wt ppm H (0.3 atomic %). Subcritical crack growth (SCG) rates at constant load also were measured on aged 718 and annealed 625. Fractographic features for various testing environments and internal hydrogen contents were compared for the two alloys. Both were found to be susceptible to internal hydrogen embrittlement as evidenced by decreases in notch tensile strength, reduction of area and threshold stress intensity for subcritical crack growth. Microscopic evidence suggests that hydrogen enhanced the localization of plasticity and led to planar slip and finally failure.