Influence of microstructure on the flow behavior of duplex stainless steels at high temperatures
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I. INTRODUCTION
THE need for high strength and corrosion resistance in chloride-containing media has made duplex stainless steels an attractive alternative to the single-phase austenitic and ferritic grades.[1,2] Although advantageous for many industrial applications, the processing of these materials requires special care due to the existence of low ductility regions within the hot working range.[3] In a general sense, the presence of a massive second phase during processing increases the complexity of deformation, with the result that the microstructure can limit the attainable strain. Along with the softening mechanism operating in each phase, the grain and interphase boundaries play important roles since accommodation of the deformation depends, not only on the plasticity characteristics of the two phases, but also on how these are affected by the presence of the interfaces.[3,4] It has been well established that, during high-temperature deformation, after some amount of work hardening, singlephase ferritic stainless steels soften by intense dynamic recovery, while austenitic stainless steels, with relative low stacking fault energies, soften by dynamic recrystallization.[5–8] When straining is performed at a constant temperature and strain rate, the flow stress rises in the initial workhardening regime and then becomes constant in the ferritic grades; by contrast, it experiences a maximum before dropping to the steady state in the austenitic steels. When the two phases are deformed together, the strain distribution is no longer uniform. In duplex stainless steels, when the ferrite O. BALANCIN, formerly Visiting Professor, Department of Metallurgical Engineering, McGill University, is Professor, Departamento de Engenharia de Materiais, Universidade Federal de Sa˜o Carlos. W.A.M. HOFFMANN, Research Associate, is with the Departamento de Engenharia de Materiais, Universidade Federal de Sa˜o Carlos, Sa˜o Carlos, SP, Brazil. J.J. JONAS, Professor, is with the Department of Metallurgical Engineering, McGill University, Montreal, PQ, Canada H3A 2B2. Manuscript submitted March 1, 1999.
METALLURGICAL AND MATERIALS TRANSACTIONS A
phase constitutes the matrix, there are strain concentrations in the softer alpha phase at the commencement of straining. As deformation proceeds, the strain gradients can decrease as a result of accommodation mechanisms such as recovery and recrystallization, as well as of interphase and grainboundary sliding. In spite of the complex deformation behavior of the duplex stainless steels, it has been found that the ferrite phase continues to exhibit intense dynamic recovery while the austenite phase undergoes dynamic recrystallization. For example, by subjecting samples of 21Cr 2 10Ni 2 3Mo duplex stainless steels to hot torsion tests, Cizek and Wynne[9] found that the ferritic matrix softens by extended dynamic recovery (or continuous dynamic recrystallization) and the flow curve is characterized by a mild stress peak, near which small amounts of dynamically recrystallized grains were observe
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