Synthesis of High-Nitrogen Steel by SHS Reduction with Aluminum

Abstract

High-nitrogen austenitic steel is characterized by high strength, plasticity, and corrosion resistance. However, its production by traditional methods (under high nitrogen pressure) calls for energy-intensive and complex equipment. In terms of energy conservation, a simpler alternative is the reduction of metal oxides by means of aluminum under nitrogen pressure. The present work is devoted to thermodynamic modeling of such reactions. High-nitrogen nickel-free stainless steels (of Cr–N and Cr–Mn–N type) with around 1% nitrogen are produced by the proposed method. The structure of the steel samples is investigated by X‑ray diffraction, metallography, and transmission electron microscopy; their mechanical properties are also determined. Thermodynamic analysis shows that the reduction processes are incomplete. In synthesis, the most important parameter is the ratio of the aluminum and oxygen concentrations in the batch. Correct selection of this ratio entails a compromise between the completeness of reduction of the oxides, the content of aluminum and oxygen in the steel (the degree of reduction), and its contamination with aluminum nitride. Cast ingots of Cr–N steel are characterized by the structure of nitrogen-containing pearlite (ferrite–nitride mixture), while ingots of Cr–Mn–N steel are characterized by ferrite–austenite structure, with traces of discontinuous austenite decomposition, accompanied by the precipitation of the nitride Cr2N. Quenching leads to complete austenitization of both types of steel. The agreement of the austenite lattice parameter obtained from the diffraction patterns for quenched Cr–Mn–N steel and the value expected from the concentration dependence for Cr–Mn–N steel indicates that all the alloying elements (including nitrogen) dissolve in austenite as a result of holding at the quenching temperature and are fixed in the solid solution by quenching. Tests of the mechanical properties shows that quenched Cr–Mn–N steel is characterized by both high strength and high plasticity. It may be concluded that reduction of oxides by means of aluminum under nitrogen pressure permits the production of high-nitrogen steel whose mechanical properties match those of corresponding steels produced by electroslag remelting under nitrogen pressure.