Effect of morphology and volume fraction of δ-ferrite on hydrogen embrittlement of stainless steel produced by electron beam additive manufacturing

Abstract

The authors studied the influence of volume fraction and morphology of δ-ferrite on hydrogen embrittlement of austenitic stainless steel 08Kh19N9T obtained by electron beam additive manufacturing. It is experimentally shown that in additively-manufactured samples, long lamellae of δ-ferrite form a dense “net” of interphase boundaries (austenite/δ-ferrite, the volume fraction of the δ-phase is 20 %) and contribute to the hydrogen accumulation. Also, being the “easy” ways for the diffusion of hydrogen atoms, the dendritic lamellae of ferrite provide hydrogen transport deep into the samples. Post-production solid-solution treatment (at T = 1100 °C, 1 h) leads to a significant decrease in the fraction of δ-ferrite in steel (up to 5 %) and partial dissolution of dendritic lamellae. A decrease in the volume fraction of ferrite and a change in its morphology hinder the diffusion of hydro­gen deep into the samples and its accumulation during electrolytic hydrogen-charging and subsequent deformation. It contributes to a decrease in the total concentration of hydrogen dissolved in the steel samples. Despite the lower concentration of dissolved hydrogen in the solid-solution trea­ted samples, the solid-solution strengthening by hydrogen atoms is higher (\(\Delta \sigma _{0.2}^{\rm{H}}\) = 73 MPa) than for the initial samples with a high content of δ-ferrite (\(\Delta \sigma _{0.2}^{\rm{H}}\) = 55 MPa). The solid-solution treated samples are characterized by a smaller thickness of the brittle surface hydrogen-charged layer and a lower hydrogen embrittlement index compared to the post-produced samples ( D H = 55 ± 12 µm, I H = 32 % for initial samples and D H = 29 ± 7 µm, I H = 24 % for samples after post-production solid-solution treatment).