Abstract
An alloy, of Fe-31.3Mn-8.92AI-5.96Cr-0.86C composition, was heated from 800 to 1000° C in atmospheres of oxygen, nitrogen and dry air respectively. A needle-like structure was observed between the alloy matrix and the external oxidation layer in the nitrogen-containing atmosphere at temperatures higher than 800° C. The needle-like phase was identified as AIN by both X-ray and STEM diffraction methods. Nitriding first occurred in the austenitic grains adjacent to the free surface, with subsequent AIN growth towards the alloy matrix. The ferrite phase, formed due to the precipitation of chromium carbide, prevented the growth of AIN. For the alloy oxidized in air, AIN formed and the growth front of AIN was ahead of the oxides. The aluminium content of the alloy matrix in the nitrided region was depleted by the formation of AIN. Due to the rapid nitriding of AI, the formation of a protective oxide layer was retarded and the oxidation resistance became less promising. © 1988 Chapman and Hall Ltd.