Electron-beam additive manufacturing of high-nitrogen steel: Microstructure and tensile properties

Abstract

High nitrogen steel billets were successfully produced using electron-beam additive manufacturing technique with a Fe-20.7Cr-22.2Mn-0.3Ni-0.6Si-0.15C-0.53 N (wt. %) steel rods as a raw material. Additively manufactured steel possesses a dual-phase structure with a 40% of ferrite phase. An increase in volume content of ferrite and dendritic morphology of the microstructure is associated with the significant decrease in manganese concentration in the melt from 22.2 to 10.8 wt% during additive manufacturing process and change in solidification mode. A preferential formation of austenitic dendrites with high nitrogen content (about 0.9 wt% of nitrogen) during early stages of solidification process leads to a depletion of the melt by N, Mn, Cr and further formation of interstitial-free ferrite in interdendritic regions. The inhomogeneity in phase composition assists lower strain hardening and ductility in the additively manufactured high-nitrogen steel in comparison with conventionally produced material at the preservation of high values of the yield strength. In tension of additively produced high nitrogen steel, plastic deformation develops in both phases but more intensive deformation, strain localization and nucleation of microcracks occur in ferrite. Fracture of additively manufactured steel develops mainly in transgranular regime with ductile fracture of ferritic regions and brittle fracture of austenitic dendrites due to high nitrogen content in γ-phase.