Controlled dissolution of colossal quantities of nitrogen in stainless steel

Abstract

The solubility of nitrogen in austenitic stainless steel was investigated thermogravimetrically by equilibrating thin foils of AISI 304 and AISI 316 in ammonia/hydrogen gas mixtures. Controlled dissolution of colossal amounts of nitrogen under metastable equilibrium conditions was realized, with nitrogen contents as high as corresponding to an occupancy of y N=0.61 of the interstitial sublattice, i.e., about 38 at. pct N. Associated with the dissolution of these unprecedented nitrogen contents in an austenitic matrix a reversible volume expansion of the austenite lattice occurred for y N > 0.17. A simplistic model based on a statistical distribution fo the nitride forming elements over the octahedrons constituting the solid state agrees favorably with the experimental data.