Metallographic Study on Nitrogen Absorption of 20% Cr-Fe Alloys (Effect of Carbon Content): On the function of nitrogen as an alloying element in heat-resisting materials-IV

High Cr-Fe alloys absorb nitrogen readily when they are heated in the gas at high temperatures. Although such absorbed nitrogen has very interesting effects on the alloys, few studies on this subject have been reported. This paper dealt with the influence of carbon content of 20% Cr-Fe alloys containing 0.04% C, 0.14% C or 0.22% C on the austenite formation due to the nitrogen absorption of these alloys, and clarified the behavior of the formed austenite due to various heat treatments. The results obtained were as follows: (1) The absorption of nitrogen during the heating for 4-6 hours in the extremely pure nitrogen, which was predeoxidized and predehydrated by metallic sodium chips, at 1250°C changed the surface zone of 20% Cr-Fe alloys into single austenite phase at the temperatures, and the depth of the austenite zone was enhanced as carbon content of the alloy increased from 0.04% to 0.22%. In inner region of the alloys, a duplex structure of austenite and ferrite was produced by the less content of nitrogen. The higher the carbon content of the alloy was, the more the amount of austenite became. (2) 20% Cr-Fe alloys heated for 6 hours in the nitrogen atmosphere at 1250°C was found to contain nitrogen from about 0.3 to 0.45% in the single austenite zone at the surface, and the nitrogen content decreased at the inner zone from the surface. At the higher carbon content of the alloys, however, the nitrogen content of these zones did not greatly concern the depth from the surface. As regards the carbon content, surface zone of the single austenite was likely to have fairly higher carbon level than that in the inner zone consisting of a mixture of both austenite and ferrite, which was presumed to be caused by the diffusion of carbon from the inner zone to the outer surface during the nitrogen absorption. (3) The austenite formed by the nitrogen absorption decomposed at about 850°C during the furnace cooling from 1050°C. Two stages of transformation were observed to occur during the air cooling of the nitrogen-absorbed alloys from 1050°C, of which one took place at about 650°C and the other at about 150°C. As regards the former, the lower the carbon content of the alloy was, the larger the change became. While, the latter stage became larger as the carbon content of the alloy increased. It was also found that the temperature range in which the change from ferrite to austenite occurred on heating was about 900°C-1050°C. (4) The austenite retained by quenching