On the Nitrogen Absorption of High Cr-Fe Alloys with Various Chromium Contents in Nitrogen

On the functionof nitrogen as an alloying element in heat-resisting materials-VII

Abstract

Studies were made on the influence of the chromium content of high Cr-Fe alloys in the range from 15 to 25% of chromium on the nitrogen absorption at 1250°C in pure nitrogen. The results obtained were as follows:
(1) When the chromium content was increased in high Cr-Fe alloys which had not been -added with both carbon and nitrogen, the nitrogen abrosption was promoted remarkably. While, the single austenite zone formed by the nitrogen absorption was not widened with increase of the chromium content.
(2) High Cr-Fe alloys containing 0. 1% of carbon absorbed nitrogen more readily than those without carbon. The addition of 0. 1% of carbon formed a considerable amount of austenite in 15% Cr-Fe alloy before the nitrogen absorption, and the austenite accelerated The nitrogen absorption. The promoting effect of carbon on the nitrogen absorption was found to be more remarkable in this alloy than in alloys of higher chromium contents and with the same carbon content, in which almost all the structural constituents were consisted of ferrite.
(3) Nitrogen pre-alloyed in high Cr-Fe alloys was found to retard the nitrogen absorplion though the austenite forming tendency of nitrogen was very similar to that of carbon. The effect of 0.1% of nitrogen in preventing the nitrogen absorption of high Cr-Fe alloys was not so marked in 15% Cr-Fe alloy as in alloys with higher chromium contents because of the formation of an appreciable amount of austenite in 15% Cr-Fe alloy by the prealloying of 0. 1% of nitrogen.
(4) It was not always true that the higher the nitrogen content of high Cr-Fe alloys before the nitrogen absorption, the less the absorption rate. The nitrogen c3ntent of 25% Cr-Fe alloy containing 0.3% of nitrogen before the nitrogen absorption was found to have a high level of about 1. 0% in the single austenite zone by the nitrogen absorption at 1250°C for 4 hours. The strong absorption was presumably caused by the high chromium content and the fact that the pre-alloyed nitrogen was much enough to produce an appreci-able amount of austenite.
(5) The single austenite zone of 15% Cr-Fe alloys consisted wholly of martensite when quenched in water immediately after the nitrogen absorption, while a considerable amount of austenite was retained in 20% Cr-Fe alloys. The austenite thus retained, however, was fully hardened by the subzero-treatment using liquid oxygen. In 25% Cr-Fe alloys martensite was no longer formed even by the subzero-treatment.
(6) The retained austenite by the quenching in the single austenite zone which was formed by the nitrogen absorption was fairly stable against the heating at temperatures below about 600°C the nitrogen-containing austenite decomposed, however, rather readily by the tempering at above 600°C. in even 25% Cr-Fe alloy. The transformation from ferrite toaustenite on heating occurred at higher temperatures as the chromium content of alloys were raised, the change being found at about 1050°C in 25% Cr-Fe alloys.