Hot Deformation and Recrystallization of Austenitic Stainless Steel: Part I. Dynamic Recrystallization
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DYNAMIC recrystallization (DRX) is the most important restoration mechanism during hot deformation of austenite; affecting the final microstructure and, therefore, the mechanical properties of the deformed material. Extensive research has been performed on the mechanisms and the variables involved in this phenomenon.[1–5] These mechanisms are believed to vary depending on the steel composition and deformation conditions. However, in commercial low-carbon steels, transformation usually restricts the study of DRX of austenite at lower temperatures and, hence, limits the study of this phenomenon over a wide temperature range. Austenitic stainless steels, which do not undergo phase transformation over a wide temperature range, can be used as model alloys to investigate recrystallization. During deformation, the pre-existing grain boundaries elongate along the deformation direction, grain boundary serration appears, and then new DRX grains nucleate at the serrated grain boundaries. The general descriptive model for DRX is that nucleation occurs at the serrated pre-existing grain boundaries and increases until a layer of DRX grains covers these boundaries. Then the recrystallization reaction proceeds via nucleation at the interfaces between the recrystallized and nonrecrystallized material, until new grains consume the structure. This type of DRX structure has been known as a necklace structure. Once the necklace structure is A. DEHGHAN-MANSHADI, formerly with the Centre for Materials and Fibre Innovation, Deakin University, Geelong, is with the Faculty of Engineering, University of Wollongong, Wollongong, NSW, Australia. Contact e-mail: [email protected] M.R. BARNETT and P.D. HODGSON are with Centre for Materials and Fibre Innovation, Deakin University, Geelong, VIC, Australia. Manuscript submitted November 29, 2007. Article published online April 1, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
completed, a steady state is reached; continuing nucleation and growth maintain the structure (with equiaxed grains) at a constant stress. Despite this simple explanation for DRX phenomenon (i.e., necklacing), its evolution during hot deformation of different materials is not simple and different parameters, such as strain rate, temperature, and initial grain size can affect the necklace structure.[6–10] Although this structure is observed during hot deformation of many materials and under different deformation conditions,[11,12] there are still many issues regarding the progress of the DRX microstructure based on this phenomenon and especially the contribution from other DRX mechanisms, such as grain boundary sliding and continuous DRX. In this study, the effect of thermomechanical parameters including strain, strain rate, and temperature on both the mechanical and microstructural aspects have been investigated. Also, constitutive equations necessary to predict the flow behavior of this steel during hot deformation have been derived. The main focus has been on the evolution of the DRX microstructure as a function of strain under
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