Microstructure-composition relationships and M s temperatures in Fe-Cr-Mn-N alloys
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where M~ is in ~ and Cr, Mn, and N are the weight percentages of these elements. At higher N contents, the Ms generally falls more rapidly with increasing nitrogen content. Nitrogen solubility at 1050 ~ exceeds about 0.3 pct in all alloys and increases with increasing Cr and Mn content. In commercial purity steels, unstable anstenitic microstructures are expected to be obtained in compositions around 10 to 14 pct Cr, 8 to 12 pct Mn, and 0.1 to 0.3 pct N when the total level of these elements is selected to ensure the Ms is below room temperature.
I.
INTRODUCTION
I N recent years there has been considerable interest in austenitic stainless steels strengthened with nitrogen. As well as the strengthening effect, additions of nitrogen to an austenitic steel increase the stability of austenite. The nitrogen bearing austenitic steels which are now commercially available are therefore highly stable. This paper investigates steels in which the major alloying elements (Cr and Ni) have been reduced such that the austenite is unstable with respect to deformation despite the presence of significant quantities of nitrogen. The work reported here was undertaken as part of a program to develop steels for the arduous conditions found in hard-rock mines. The steels need to be corrosion resistant, wear resistant, tough, formable, weldable, and relatively cheap. A conceptual steel which could meet these requirements is one comprising more than 8 pct Cr to confer corrosion resistance to typical mine waters, an austenitic microstructure to confer toughness and formability, instability of the microstructure under deformation combined with a high interstitial content to promote high work hardening and wear resistance, absence of carbon to safeguard weldability, and low levels of expensive alloying elements such as Cr and Ni to minimize cost. The Fe-Cr-Mn-N system was identified as promising, and unstable austenitic steels from this system with the required properties have been developed at Fulmer Research Laboratories. 1 This paper describes the first stage of the develpment, which was to determine the microstructure-composition
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
relationships and martensite start temperatures (M0 in Fe-Cr-Mn-N alloys. Further developments in composition optimization and property determination followed, and these are reported in a companion paper. 2 In order to obtain an austenitic microstructure, it is necessary that austenite forms at the annealing temperature and that the austenite is retained to room temperature; i.e., Ms must be below room temperature. Nickel, manganese, and nitrogen favor the formation of austenite at annealing temperatures but chromium does not. The first objective of this work was to ascertain whether fully austenitic structures could be obtained during annealing of low chromium (8 pct to 12 pct) alloy
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