Influence of Cooling Rate on the Ferrite Prediction Diagram of Duplex Stainless Steel Castings

Abstract

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.