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

For the purpose of fabricating ultrahigh nitrogen austenitic steels (> 1 mass pet 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 absorp- tion 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 tak- ing account of interactions among Cr, Mn, and N atoms, and the condition for fabrication of ultra- high 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 pet) alloy has 830 MPa in 0.2 pet proof stress, 2.2 GPa in true vtensile stress, and 75 pet 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).