Austenite grain growth in alumina-forming austenitic steel
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Hailian Zhang School of Economics and Management, Yanshan University, Qinhuangdao, 066004, China
Qiang Huo School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China (Received 5 February 2016; accepted 8 April 2016)
Microstructures and austenite grain growth behavior of the alumina-forming austenitic (AFA) steel subjected to normalizing and annealing at various temperatures were investigated. A modified kinetic model of austenite grain growth was constructed based on consideration of the heating history. Abnormal growth of austenite grain occurs when the temperature is increased to 1473 K, and some special large particles of the precipitates located at grain boundaries form when the sample is normalized at the temperature of 1523 K. Both NbC and NiAl precipitates are identified using routine x-ray diffraction. The fitted data based on the kinetic model used and the consideration of the heating history is in agreement with the changes in the austenite grain growth in the AFA steel even when there is abnormal grain growth. The grain growth exponents are shown to be 2.85 and 2.42 for normalizing and annealing, respectively. I. INTRODUCTION
A new family of alumina-forming austenitic (AFA) stainless steels is currently being developed at the Oak Ridge National Laboratory for structural application in aggressive oxidizing environments at 873–1173 K.1,2 The AFA steels exhibit a useful combination of high creep strength and oxidation resistance at elevated temperatures. The excellent oxidation resistance of the AFA steels is due to the formation of an alumina scale replacing the traditional chromia scale to protect the conventional austenitic steels.3,4 The adjustment of alloying elements was accomplished based on the chemical composition of the high-temperature ultrafine precipitation strengthened steel family of austenitic steels, and with the additions of Al, B, Nb, C, and other elements, to form nanoscale precipitates with dispersive distribution, and a stable alumina layer on the surface of the AFA steels. Generally, the microstructure of the AFA steels is a single-phase, face centered cubic austenite, with a number of nanoscale NbC and NiAl based precipitates with dispersive distribution.1,5,6 The level of Al in the AFA steels is higher than that in conventional austenitic steels and this produces alumina on the surface of the steel. A transition from protective alumina scale to internal oxidation of the Al may also occur. In addition, the Al element will also combine with the Ni element to form a NiAl precipitate in
which the small particle size increases creep strength at high temperature.5,7,8 During the past ten years, most research has focused on the adjustment of the addition of the alloying elements, especially Al and Ni,9 the improvement of creep strength and oxidation resistance,10,11 and the effect of alloying elements on the related properties.9,12 Until now, however, the heat treatment before the related properties test was still carried out using the traditional process
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