Modeling M 6 C precipitation in niobium-alloyed ferritic stainless steel
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I. INTRODUCTION
THE reduction of harmful emissions from automobiles requires the exhaust gas temperature to be high so that the materials involved must be heat resistant. The exhaust manifold, in particular, has to have good thermal fatigue resistance, a property that is improved by the presence of niobium in solid solution.[1,2] Unfortunately, at service temperatures that can be in excess of 900 ⬚C, M6C carbide precipitates and coarsens rapidly.[2] The “M” in this stands for metal atoms, which, in the present case, consist of about equal amounts of iron and niobium to give Fe3Nb3C. The purpose of the present work was to develop a kinetic model for the precipitation process as a function of the steel composition and heat treatment. It was found at the outset that there is a lack of thermodynamic data for the kind of alloys relevant to the manufacture of automobile exhausts (i.e., Fe-19CrNb mass pct). Therefore, new experimental data had to be obtained to conduct the kinetic analysis. II. EXPERIMENTAL PROCEDURE Table I shows the chemical composition of niobiumalloyed ferritic stainless steel, with a nominal composition 19Cr-0.8Nb mass pct. The alloy was vacuum melted as a 10-kg ingot heated at 1250 ⬚C for 30 minutes in an argon atmosphere, hot rolled to a 12-mm-thick plate, and air cooled from a finishing temperature of about 900 ⬚C. The plate was annealed at 1000 ⬚C for 10 minutes and then quenched into water. Several samples were machined for a variety of aging conditions. They were aged at 950 ⬚C and 1000 ⬚C for up to 500 hours. After aging, electrolytically extracted residues were analyzed using X-ray diffraction to identify which phases exist in each aging condition. The microstructures were characterized mainly using carbon-extraction replicas examined using transmission electron microscopy (TEM), particularly to identify the NOBUHIRO FUJITA, Senior Researcher, and MASAO KIKUCHI, Chief Researcher, are with the Steel Products Lab.-1, Nippon Steel Corporation, Futtsu 293-8511, Japan. Contact e-mail: [email protected] H.K.D.H. BHADESHIA, Professor, is with the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, United Kingdom. Manuscript submitted September 14, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
phase by electron diffraction and energy dispersive X-ray spectrum (EDS) and to measure the particle size of each particle. To make the replica specimens, a carbon film was applied on the sample surface using vacuum evaporation. Then, the carbon replicas could be collected on 3-mmdiameter copper grids. For this kind of ferritic stainless steels, etching in a solution of 10 g/l tetramethylammonium chloride and 10 vol pct acetylacetone in methanol at the potential between 0 and 200 mV proves satisfactory. Because several types of precipitates are expected in the aged steels, replica specimens are a reliable method to detect M6C and to measure each particle size. The foil specimens are another method to characterize microstructure and superior to the replica method
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