Influence of Cooling Rate on the Ferrite Prediction Diagram of Duplex Stainless Steel Castings
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Six different composition steels were designed based on CD3MWCuN, a common industrial alloy formulated to have a microstructure consisting of 50 pct ferrite and 50 pct austenite. The steels were designed to have ± 10 pct Cr, Ni, and Mo over the nominal composition of CD3MWCuN and were obtained by remelting a keel bar of the base alloy and adjusting the composition. The + compositions variations were achieved simply by adding additional Cr, Ni, or Mo, while the − variations were obtained by adding additional amounts of the remaining major elements to the nominal composition of the keel bar. In this way it was possible to produce a series of alloys where the Cr/Ni equivalent ratio, Creq/Nieq, as calculated using the equations first employed by Schoffler varied from ≈ 1.35 to 1.65. A series of homogenization temperatures ranging from 1000 °C to 1250 °C were used to produce a uniform starting microstructure and the alloys were cooled at rates ranging from 500 °C to 1000 °C/hour from these homogenization temperatures. Air-cooled and water-quenched samples were examined as well. Based on observations of the microstructure a generalized ferrite prediction diagram was developed that takes into account homogenization temperature, cooling rate, and chromium-to-nickel equivalence ratio (Creq/Nieq) to predict the amount of ferrite that can be expected in the alloy. This diagram represents the first major evaluation of ferrite content as a function of homogenization temperature and cooling rate specifically for cast duplex alloys.
This work was funded jointly by the Steel Founder Society of America and their members and the Iowa State University Center for Industrial Research and Service. Portions of this work were carried out using facilities present at the Ames Laboratory. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. DE-AC02-07CH11358.
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