Dependency of Recrystallization Mechanism to the Initial Grain Size
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THE objective for thermomechanical processing of steels is generally to improve the final mechanical properties by refining the room-temperature grain size. In the steel industry, this achieved through austenite conditioning, where the large initial, as cast or reheated, grain size is refined through recrystallization during or after deformation. In structural steels, there is also transformation from austenite to ferrite. Insight into the hot-working behavior of steels is often obtained through the use of alloys that do not transform during quenching to room temperature. This includes the use of conventional stainless steels or Fe-Ni model alloys to study the dynamic and static recrystallization reactions. The characteristics of dynamic recrystallization (DRX) and post-DRX in austenite at a constant initial grain size and under different deformation conditions have been studied by numerous authors.[1–8] The initial grain size has a strong influence on the kinetics of recrystallization and microstructure due to the change in the grain-boundary surface area, which is the principal nucleation site. It is well known that in hot deformation, a decrease in the initial grain size will accelerate the onset of DRX, increase DRX kinetics and also affect the microstructural and mechanical characteristics of the deformed structure.[5,9,10] While for a given deformation condition, the effect of initial grain size on DRX final grain size and steady-state stress is less pronounced, a transition from single- to multiple-peaks behavior is reported in several materials when the initial grain size is decreased below a critical value.[5,11,12] A. DEHGHAN-MANSHADI, Research Fellow, is with the Faculty of Engineering, University of Wollongong, NSW, 2522, Australia. P.D. HODGSON, Professor, is with the Centre for Material and Fibre Innovation, Deakin University, Waurn Ponds, VIC, 3217, Australia. Contact e-mail: [email protected] Manuscript submitted April 21, 2008. Article published online October 2, 2008 2830—VOLUME 39A, DECEMBER 2008
Conventional DRX of austenite usually occurs through a discontinuous mechanism, involving nucleation and growth of strain-free grains,[2,13] i.e., by serration, bulging, and then migration of original highangle grain boundaries (HAGB). However, under certain test conditions or initial microstructures, a transition from discontinuous recrystallization (DDRX) to continuous recrystallization (CDRX) has been reported in austenite.[11,14,15] For example, a transition from DDRX to CDRX was observed in 316L austenitic stainless steel with a decrease in the deformation temperature (starting with a similar initial grain size and at similar strain rates).[15] In other research,[11] decreasing the initial grain size to a very small value of 2.8 lm in 304 austenitic stainless steel led to a continuous type of DRX based on grain-boundary sliding. Continuous recrystallization, which also has been termed ‘‘rotation recrystallization,’’ ‘‘in-situ recrystallization,’’ or ‘‘extended recovery,’’ is accompanied by a continuous
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