Precipitation in Nb-Stabilized Ferritic Stainless Steel Investigated with in-situ and ex-situ Transmission Electron Micr
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I.
INTRODUCTION
IN the last decade, the steel market for automotive exhaust systems was characterized by an increased use of ferritic stainless steel at the expense of the austenitic grades. Although basic austenitic stainless steel grades offer better mechanical properties at high temperatures than similar ferritic grades, the high and fluctuating Ni prices are a major drawback of the former grades. This economical disadvantage has favored the development of ferritic grades with similar high-temperature properties as the austenitic grades, for example, Nb stabilized ferritic stainless steel. Nb increases the high-temperature strength of the steel by solid solution hardening.[1–3] The formation of Nb containing precipitates is reported to affect the strength and the creep resistance.[2,4] As the mechanical properties change with the distribution, size, and nature of the precipitates, it is desirable to know the stability, formation kinetics, and morphology of the different
A. MALFLIET, PhD, B. BLANPAIN and P. WOLLANTS, Professors, are with the Department of Metallurgy and Materials Engineering, Katholicke Universiteit Leuven, 3001 Heverlee, Belgium. Contact e-mail: annelies.malfl[email protected] F. MOMPIOU, PhD, is with CEMES, Centre d’Elaboration des Mate´riaux et d’Etude Structurale, 31055 Toulouse, Cedex 4, France. F. CHASSAGNE, Senior Metallurgist, and J.-D. MITHIEUX, Research Engineer in Metallurgy, are with APERAM Isbergues Research Centre (formerly ArcelorMittal Isbergues Research Centre), 62330 Isbergues, France. Manuscript submitted November 16, 2010. Article published online June 28, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
precipitates in Nb-stabilized ferritic stainless steel grades. In general, in a Fe-Cr-Nb-C-N steel, three kinds of Nb precipitates are observed, i.e. hexagonal Fe2 Nb, face-centered-cubic Nb(C,N), and a cubic Fe3 Nb3 C phase with prototype Fe3 W3 C. According to the work of Sim et al.,[5] Nb(C,N) and Fe2 Nb precipitates coexist in an industrial Fe-15Cr-0.38Nb-0.01C steel after 2 hours heat treatment at 973 K (700 C). After 1000 hours, Fe3 Nb3 C precipitates are observed. A linear relationship between proof strength and Nb content in solid solution was observed. It is stated that the precipitation of Nb(C,N) decreases slowly the hightemperature strength, whereas the faster coarsening of Fe2 Nb precipitates abruptly decreases this strength. Fujita et al.[6] determined the solubility product of the Fe3 Nb3 C phase in different Fe-Cr-Nb-C-N ferritic stainless steel grades between 973 K and 1273 K (700 C and 1000 C). Their results indicate that the amount of Fe3 Nb3 C increases with ageing time and temperature and that Cr enhances the formation of this Fe3 Nb3 C phase. Also, according to Fujita et al.,[3] the reduction in proof strength at 1173 K (900 C) of a steel with 0.5 wt pct Nb after 500 hours at 1173 K (900 C) with respect to the steel in its initial condition is explained through the formation and coarsening of Fe3 Nb3 C precipitates, resulting in a loss of Nb in solid
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