Controlled dissolution of colossal quantities of nitrogen in stainless steel

The solubility of nitrogen in austenitic stainless steel was investigated thermogravimetrically by equilibratin- thin foils of AISI 304 and AIST 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 of the nitride forming elements over the octahedrons constituting the solid state agrees favorably with the experimental data.