Structure and properties of corrosion and wear resistant Cr-Mn-N steels
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I.
INTRODUCTION
AUSTENITIC stainless steels strengthened with nitrogen are gaining increasing industrial acceptance. The nitrogen beating austenitic steels which are commercially available have an austenitic microstructure which is highly stable. Unstable austenitic steels have the potential for transforming to hard martensite under deformation, particularly if the interstitial (nitrogen) content is high, conferring very high work hardening and, potentially, good wear resistance. Unstable austenitic nitrogen bearing steels have been developed at Fulmer Research Laboratories 1 and the first stage of the development, the determination of structurecomposition relationships in the Fe-Cr-Mn-N system, was reported in a companion paper.2 The present paper describes the preparation of a number of alloys and the determination of their microstructural, mechanical, corrosion, and wear properties. The alloy development program was aimed at developing steels which had moderate corrosion resistance, good wear resistance, good ductility and toughness, weldability, and relatively low cost, with potential applications in hard-rock mining and other arduous environments. It was expected that these properties could be obtained in unstable austenitic, nitrogen-beating, Cr-Mn steels. Corrosion resistance to typical mine waters is conferred by 8 to 12 pct Cr and ductility and toughness are conferred by an austenitic microstructure, obtained by balancing the composition with Mn and N. Instability of the austenite under deformation, coupled with a relatively high nitrogen content, was expected to lead to high work hardening and good wear resistance. Weldability is safeguarded by the absence of carbon, and costs are reduced (relative to commercial austenitic steels) by reduction in chromium and the absence of nickel. The study of microstructure-composition relationships2 showed that austenitic steels could be obtained in the U. R. LENEL, Research Manager, and B. R. KNOTT, Research Associate, are with Fulmer Research Laboratories Limited, Stoke Poges, Slough, Berks SL2 4QD, England. Manuscript submitted July 7, 1986. METALLURGICALTRANSACTIONS A
Fe-Cr-Mn-N system, and this paper reports on the investigation of the relevant properties of these steels. The compositions selected for study were based on the results of the microstructure-composition study2 carded out on high purity nitfided alloys. The first objective of the work was to confirm the composition range in which unstable austenitic microstructures could be obtained in alloys containing normal levels of impurities, produced from the melt rather than by nitriding. The compositions studied therefore included various nominal nitrogen levels (0.1 to 0.4 pct) in alloys containing 10 to 12 pct Cr and 5 to 10 pct Mn. It was expected that these compositions would cover the microstructural range from stable austenite through unstable austenite to martensite. Some of the commercially available austenitic stainless steels, such as 304, have an unstable microstructure and transform partially to
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