Abstract
The effects of heat treatment on the hydrogen embrittlement (HE) susceptibility of selective laser melted 18Ni-300 maraging steel were investigated. The results demonstrate that the as-built steel has a microstructure consisting of an Fe–Ni martensite matrix and a cellular structure. Substantial microstructural changes are observed following various heat treatment processes (solution, direct aging, and solution-aging), resulting in varying degrees of increased HE susceptibility. Specifically, after solution treatment, the complete elimination of cellular structure and the growth of grains concurrently increased hydrogen diffusion coefficient of the steel. However, its lower proportion of ∑3 boundaries and higher proportion of ∑11 boundaries played an advantageous role that lead to a marginal increase in the HE susceptibility. In contrast, direct aging and solution-aging treatments enhance the HE susceptibility significantly, which is primarily related to the decomposition of the cellular structure and reversible hydrogen trapping and precipitation strengthening effects of the newly formed Ni3(Ti, Mo) precipitates. Compared with the solution-aging treatment, the direct aging treatment results in the formation of a localized austenite aggregation zone near the molten pool boundaries, which promotes the initiation of hydrogen-assisted cracking and leads to a higher HE susceptibility.
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