Experimental and numerical analyses of the mechanism for decreasing hydrogen-embrittlement sensitivity of aged Inconel 718 by laser surface annealing

Laser surface annealing (LSA) treatments were conducted to decrease the hydrogen embrittlement (HE) sensitivity of Inconel 718. Hydrogen was charged into the alloy by a cathodic-charging process. The LSA treatment significantly reduced the strength loss of the hydrogen-precharged notched specimen. Tensile tests and elasto–plastic simulations were performed to study how LSA affects the HE sensitivity of the as-aged Inconel 718. Various heat treatments were applied to the notched tensile specimens to obtain different precipitation states of γ″. All tensile tests were conducted in air at room temperature under identical conditions. One specimen of each kind was free of hydrogen; another was charged with hydrogen prior to the tensile test. A correlation between the HE sensitivity and the precipitation state of the alloy was confirmed by assessing the loss in the notch tensile strength due to hydrogen pre-charging. The γ″ particles, especially those in the surface layers, were proved to have deleterious effects on the HE sensitivity of Inconel 718. Fractures originated from the surface vicinity in the as-aged specimen when there was precharged hydrogen. However, hydrogen-induced fractures in the LSA-treated specimen originated in the deep region of the laser-annealed surface layer. Stress concentration conditions at the notch roots were studied via elasto–plastic finite element method simulations. The area of most highly concentrated stress of the LSA-treated specimen was in deep layer of the laser-annealed notch region where the material was partly annealed. The formation of the annealed surface layer that has better HE resistance than the as-aged alloy, and the migration of the fracture origin to deeper layers were considered to be the mechanism for decreasing the HE sensitivity by the LSA. Further, the partly annealed raw alloys with strength comparable to the LSA-treated specimens were found to have much higher HE sensitivities than the corresponding LSA-treated specimens. Both the experimental results and the simulation results suggested that the LSA caused the fracture origins to migrate from surface to deep layers. It is possible to reduce the HE sensitivity of Inconel 718 without sacrificing its strength by applying LSA to produce locally annealed surface zones.