Estimated Nitrogen Solubility and Phase Composition in Fe-13% Cr Steel after Its Additional Alloying with (Mn, Mo, V, Nb)

The solubility of nitrogen is estimated in Fe-13% Cr model steels of various compositions with additional alloying (by a small amount from several hundredths of a percent to 1.5%) with elements (Mn, Mo, V, Nb), which increase the solubility of nitrogen in iron-based solid solutions. 60 versions of compositions have been studied. Based on our earlier experimental data obtained for nitrogen-bearing steels with 16% Cr5% Ni-Nb, we estimate the compositional stability coefficient of nitrogen K-s, which is used in the calculations of the nitrogen solubility, for martensitic steel. The calculated value of K-s is lower than that of austenitic steels. For steels of all composition versions with the calculated nitrogen content and various contents of additional alloying elements (Mn, Mo, V, Nb), their phase compositions are estimated using a modified Schaeffler-Delong diagram and a nonequilibrium Potak-Sagalevich diagram for steels heat treated to form a solid solution. All Fe-13% Cr-Mn, Mo, V, Nb compositions with a low carbon content (0.03-0.05%) at the maximum nitrogen concentration in a metal are shown to be in a martensite-ferrite region. The temperature of the onset of the martensitic transformation M-s is calculated using the empirical Finkler-Schirra formula, and a correlation between M-s and the ratio Ni-eqiv/Cr-eqiv (chromium and nickel equivalents calculated by the formulas used for the modified Schaeffler-Delong diagram) is revealed.