On the Improvement of Various Properties of 20% Cr-Mn-Fe Alloys by Nitrogen-Absorption Treatment

On the function of nitrogen as an alloying element in heat-resisting materials-X

Abstract

The formation of the nitrogen-bearing austenite in 20% Cr-Mn-Fe alloys containing manganese up to 10% by the authors' nitrogen-absorption method and the thermal behavior of the formed austenite were studied. The creep property of the nitrogen-absorbed alloys at elevated temperature was also investigated. The results obtained are as follows:
(1) The depth of the single austenite zone formed by the nitrogen-absorption in 20% Cr-Mn-Fe alloys is widened with increase of the manganese content of alloys, and the nitrogen content of the nitrogen-absorbed alloys is also increased as the manganese content is raised. In the alloys containing 8% or more of manganese, the single austenite zone contains about 0.8% nitrogen.
(2) When the manganese content of the alloys is 2% or more, the nitrogen-bearing austenite is retained by quenching in water. In the alloys containing 4% or more of manganese, no martensite is formed even by subzero-treatment using the liquid oxygen.
(3) In the alloys containing manganese lower than 2%, the austenite decomposes readily by aging at 700°C, while, in the alloys containing 4% or more of manganese, the lamellar phase of nitride is developed from the surface zone to the inner one during aging at 700°C. The hardness of the single austenite zone of the alloys containing about 8% manganese is maintained as high as about Hv 400 even after aging forup to 800 hours.
(4) Both the bending creep property and the tensile creep property at elevated temperature of the nitrogen-absorbed 20% Cr-8% Mn-Fe alloy are excellent in the same degree as 316L type alloy shows, and the property is further improved by an intermediate and light rolling prior to solution-quenching of the nitrogen-absorbed alloy.
(5) The oxidation resistance of these alloys is lowered to a certain extent by nitrogenabsorption treatment.