Hydrogen induced stress cracking of alloy 718: Effects of chemical composition and microstructure
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This master thesis have examined the effect of variations in composition and microstructure on the HISC susceptibility of Inconel-718. 6 different Inconel-718 variants with different compositions and heat treatments were examined. Tensile samples were pre-charged with hydrogen at 120*C in an Orthophosphoric-acid/Glycerol electrolyte for 5 days, and subsequently subjected to stepwise constant-load tensile tests. The tensile tests were performed in synthetic saltwater with a negative polarization of -1050mA Ag/AgCl, corresponding to a galvanic coupling with aluminium or zinc in seawater. Hydrogen free samples were tested in air for comparison. An optical microscope was used to examine the sample surfaces during testing. Fracture surfaces were examined in SEM, and the samples were also examined for secondary cracks. All the Inconel-718 samples pre-charged with hydrogen showed a significant reduction in ductility and contained Inter Granular (IG) areas on the fracture surfaces. A decrease in fracture strength of approx. 5-10% of Yield Strength (YS) was seen in all the variants examined, but no significant trend between the Inconel-718 variants was detected here. Smaller grain sizes were correlated with an increasing ductility loss measured by loss of Reduction of Area (RA) and loss of Elongation prior to fracture. This was attributed to increased Grain Boundary (GB) diffusion of hydrogen during pre-charging, resulting from more GB areas. A Hydrogen melt extraction analysis done by SINTEF confirmed the elevated hydrogen levels expected in these fine-grained Inconel-718 variants.