The constitutional diagrams of 18%Cr-Fe-C-N quaternary system were examined up to 0.5% of both carbon and nitrogen in the temperature range of 1300°to 700℃, and the sectional diagrams were described at fixed contents of 0.1 and 0.2% nitrogen, and 0.1, 0.2 and 0.3% carbon. In the present range of composition, ferrite and austenite existed in equilibrium in the range of 1300°to 1100℃, and the austenite-promoting effects of carbon and nitrogen were very nearly equivalent and additive. The minimum concentration of carbon plus nitrogen required to obtain a completely austenitic structure was about 0.4%. Below 1100℃, Cr_C_6, Cr_7C_3 and Cr_2N precipitated and the phase regions recognized were γ+Cr_C_6, γ+Cr_7C_3, γ+Cr_2N, γ+Cr_C_6+Cr_2N, α+γ+Cr_C_6, α+γ+Cr_2N, α+γ+Cr_C_6+Cr_2N, α+Cr_C_6+Cr_2N, α+Cr_C_6 and α+Cr_2N. It was presumed that the four-phase region of α+γ+Cr_C_6+Cr_2N arose from the eutectoid reaction γ→α+Cr_C_6+Cr_2N. The compositions of Cr_C_6 and Cr_2N were generally represented by (Fe_7Cr_)C_6 and (Fe_Cr_)C_N_, respectively
