The microstructures and the characters of nitrides in various alloys of the Fe-Cr-N system in the range of 0.5 to 40% chromium with up to about 1% nitrogen were studied by optical- and electron-microscope and by X-ray diffraction and chemical analysis of the extracted residue. The results are summarized as follows : (1) The nitrides detected were Fe_4N, CrN and Cr_2N. (2) In the content of about 0.1% N, the changes in microstructure with the lowering of temperature were as follows : 0.5% Cr : γ→α+γ→α+γ+CrN→α+CrN+Fe_4N ; about 0.5～6% Cr : γ→α+γ→α+γ+CrN→α+CrN ; abaut 6～7% Cr : γ→α+γ→α+γ+Cr_2N→α+Cr_2N+CrN ; about 7～13% Cr : γ→α+γ→α+γ+Cr_2N→α+Cr_2N ; about 13～26% Cr : α+γ→α+γ+Cr_2N→α+Cr_2N ; above about 26% Cr : α→α+Cr_2N. (3) In the case of alloys containing chromium above about 13%, the minimum content of nitrogen to obtain a completely austenitic structure increased with the chromium content ; for example, the minimum nitrogen contents in 13, 18 and 16% Cr alloys were about 0.1, 0.4 and 0.8%, respectively. The microstructures of these alloys revealed such a change as γ→γ+Cr_2N→α+γ+Cr_2N→α→Cr_2N. (4) The very fine acicular phase was observed within ferrite in the Fe-Cr-N alloys, which contained chromium above about 15% and were quenched in water from above about 1100℃. It was confirmed that this phase was Cr_2N precipitated during cooling clue to a remarkable decrease of the solubility of nitrogen in ferrite, and that the suppression of this precipitation was very difficult
