Effects of precipitation phases on the hydrogen embrittlement sensitivity of Inconel 718

We investigated the effects of precipitation phases on the hydrogen embrittlement (HE) sensitivity of Inconel 718 by means of tensile tests. Hydrogen was charged into the test specimens via a cathodic charging process prior to the tensile tests. Various heat treatments were applied to conventionally aged specimens to fabricate specimens with different precipitation conditions for the γ″ phase and the δ phase. For each precipitation condition, we fabricated two specimens, one of which was charged with hydrogen before the tensile test. All specimens were tensioned under identical tensile conditions. The percent loss of the reduction of area (RA) caused by pre-charged hydrogen was used to assess HE sensitivity. Both the δ phase and the γ″ phase were found to play significant roles in altering HE sensitivity of Inconel 718. When these phases were totally dissolved, the HE sensitivity of the alloy was very low. The percent loss of RA decreased along with a decrease in the fractional volume of γ″. The δ-free aged alloy had greatly enhanced HE resistance, the same level as that of conventionally annealed alloy, and its strength was equal to that of the conventionally aged alloy. Fracture origins noted on the specimens were located on the surface layers and displayed brittle cleavage when pre-charged hydrogen was utilized. Local transgranular cleavages initiated from the δ/matrix were also observed in conventionally aged specimens, where there was a presence of pre-charged hydrogen. Therefore, the δ phase was considered to promote HE by initializing micro-cracks from δ/matrix interfaces. Since the δ-free aged alloy has both good strength and good ductility, we propose that it is advantageous for fabricating some hydrogen-containing parts.