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

摘要

Six different composition steels were designed based on CD3MWCuN, a common industrial alloy formulated to have a microstructure consisting of 50pct ferrite and 50pct austenite. The steels were designed to have +/- 10pct 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, Cr-eq/Ni-eq, as calculated using the equations first employed by Schoffler varied from approximate to 1.35 to 1.65. A series of homogenization temperatures ranging from 1000 degrees C to 1250 degrees C were used to produce a uniform starting microstructure and the alloys were cooled at rates ranging from 500 degrees C to 1000 degrees 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 (Cr-eq/Ni-eq) 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. (C) The Minerals, Metals & Materials Society and ASM International 2019