Microstructure and Phase Composition of High-Nitrogen Steel Fabricated by Electron Beam Additive Manufacturing

A comparative study of microstructure and phase composition of high-nitrogen Cr-Mn steel billets obtained by the casting following by conventional solid-solution treatment and by electron beam additive melting method was carried out. Both studied steel billets possess two-phase structure (austenite/ferrite). However, the microstructure, grain structure and volume fraction of delta-ferrite are strongly dependent on the method of steel's production. Coarse austenitic and ferritic grains are peculiar for cast solution-treated steel, but additively manufactured billet possess columnar coarse austenitic grains with dendritic ferrite inside of them. The volume fraction of delta-ferrite in cast steel (V approximate to 20%) is significantly lower than that in steels obtained by the additive manufacturing method (V approximate to 50%). The microhardness of the additively produced steel (3.3 GPa) is close to the value peculiar for cast steel (3.4 GPa). Cast steel shows only slight variations of microhardness values across the specimens, whereas in the case of additively manufactured steel, a considerable scatter in microhardness values are observed along the height of the billet, which is associated with inhomogeneity in phase composition of the as-built dendritic microstructure.