Fabrication of ultrahigh nitrogen austenitic steels by nitrogen gas absorption into solid solution

For the purpose of fabricating ultrahigh nitrogen austenitic steels (>1 mass pct N), the phenomenon of nitrogen absorption into solid solution was thermodynamically analyzed and applied to Fe-Cr-Mn system ternary alloy. During the annealing of the steel in a nitrogen gas atmosphere of 0.1 MPa at 1473 K (nitrogen absorption treatment), the nitrogen content of the steel was increased with the absorption of nitrogen gas from the material surface and then saturated when the system reached a state of equilibrium. Effect of the steel composition on an equilibrium nitrogen content was formulated taking account of interactions among Cr, Mn, and N atoms, and the condition for fabrication of ultrahigh nitrogen austenitic steels was clarified. The nitrogen addition to ultrahigh content markedly increased proof stress and tensile stress of the austenitic steels without losing moderate ductility. For example, Fe-24Cr-10Mn-1.43N (mass pct) alloy has 830 MPa in 0.2 pct proof stress, 2.2 GPa in true tensile stress, and 75 pct in total elongation. As a result of tensile tests for various nitrogen-bearing austenitic steels, it was found that the proof stress is increased in proportion to (atomic fraction of nitrogen)^2/3.