The tentative phase diagram of an Fe-Cr-N system in the composition range of 12 to 60% Cr, N up to about 2% at the partial pressure of nitrogen in air has been constructed by thermal analyses of Fe-Cr-N alloys which contained less nitrogen in the previously reported experimental formula [Cr] =32.9 log [N] +4.1.5, and X-ray analyses as well as microscopic examinations. Besides molten alloys, Fe-Cr-N alloys were prepared by the nitrogen-absorption at 1250°C and at one atmospheric preesure of pure nitrogen, and their structures have been examined. The denitrogenization of the nitrogen-absorbed alloys during the heating in the vacuum has also been clarified. The results are summarized as follows.
(1) It was presumed from the study of the heterogeneous equilibrium of the Fe-Cr-N system that a peritectic reaction L+δ γ exists in the Fe-rich melt of the Fe-N system as seen in that of the Fe-C system, and it was also assumed that a eutectic reaction L δ+Cr2N exists in the range up to 11.6%N at Cr2N in the Cr-N system.
(2) The peritectic reaction at the Fe-N alloy-side lowers the temperatures as the Cr content in the melt increases, and the eutectic reaction at the Cr-Cr2N alloy-side also decreases the temperatures as the Fe content in the melt increases. The above two mono-variant reactions meet at the point s in Fig. 1, which is found to exist at 16%Cr and 0. E9%N and at 1328.., and a peritecto-eutectic reaction L+δ γ+Cr2N is formed, the composition range of the non-variant reaction being in the hatched area formed by the following four points: q (δ), u (γ), s (L) and Cr2N.
(3) It is presumed from x-ray diffraction analyses of alloys quenched immediately after their solidification that Cr2N does not dissolve Fe, Cr or N; the nitride primarily separated from melts may be shown as the simple form of Cr2N.
(4) When Fe-Cr alloy sheets are heated at 1250°C at one atmospheric pressure of nitrogen, the higher the chromium content of the alloys, the more the nitrogen-absorption and the shorter the time for the saturation of the absorbed nitrogen.
(5) When Fe-Cr alloys in the composition range of 12 to 40%Cr are nitrogen-absorbed at 1250°C, austenite is formed irrespective of the concentration of chromium in the alloys, and the phase containing chromium up to 23% decomposes into martensite on quenching and decomposes into α + Cr2N on slow cooling to room temperature, while the austenite containing above 23% Cr is retained as austenite without being decomposed by water-quenching from high temperatures.
(6) Nitrogen-absorbed Fe-Cr-N alloys are denitrogenized by heating in the vacuum, the degree being less in higher chromium alloys.