Effect of structural factors on the mechanical properties of high nitrogen austenitic steels

Solute nitrogen in steels is substantially a strong austenite former and lowers Ms temperature markedly. This means that austenitic stainless steels can be obtained in Fe-Cr-N ternary alloy without adding expensive alloying elements like nickel. In this investigation, 12 to approximately 23%Cr ferritic stainless steels were selected in order to examine the influence of nitrogen and chromium content on austenite stability and on mechanical properties of Fe-Cr-N ternary alloys produced by nitrogen absorption treatment in solid state. The results obtained are as follows: (1) Nitrogen content of specimens can be controlled within the chemical composition range which gives austenite single phase at 1473 K in the ternary alloys. The structures at room temperature are different depending on the chromium content; martensite or (martensite+austenite) below 20%Cr, metastable austenite between 20%Cr and 23%Cr, stable austenite above 23%Cr in case the steels are rapidly cooled from 1473 K, because austenite partially undergoes an eutectoid transformation (decomposition from austenite to (ferrite+Cr₂N)) during air cooling. (2) Metastable austenitic steels fracture in brittle mode due to the formation of lenticular martensite during deformation. (3) Stable austenitic steels with 23%Cr are strengthened to 0.7 GPa in 0.2% proof stress by absorbing about 1% of nitrogen with excellent ductilities (40% in elongation). Such an excellent ductility is probably due to high work-hardening rate.