Hot Deformation and Recrystallization Mechanisms in a Coarse-Grained, Niobium Stabilized Austenitic Stainless Steel (316
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TRODUCTION
DUE to their excellent corrosion resistance and appropriate combination of mechanical properties at high temperatures, austenitic stainless steels are widely used in chemical, oil and nuclear industries.[1,2] Ni-Cr-Mo AISI 316-type austenitic stainless steel grades are of particular use in vessels under high operating temperatures and pressures.[2] Obtaining a homogeneous microstructure in heavy components requires deep understanding of their hot deformation behavior and underlying physical mechanisms. The hot workability of these steels is governed by the competition between work hardening, dynamic recovery (DRV), and dynamic recrystallization (DRX).[3] Post-dynamic recrystallization (post-DRX) and static recrystallization (SRX) can induce further metallurgical evolution during
A. HERMANT is with MINES ParisTech, PSL Research University, MAT-Centre des Mate´riaux, UMR CNRS 7633, BP 87, 91003 Evry Cedex, France and also with CEA-Centre de Valduc, 21120 Is sur Tille, France. E. SUZON is with CEA-Centre de Valduc, 21120 Is sur Tille, France. P. PETIT, J. BELLUS, and E. GEORGES are with Aubert & Duval-Usine des Ancizes, 63770 Les Ancizes, France. F. CORTIAL is with Naval Group Research-CESMAN, 44340 Bouguenais, France. M. SENNOUR and A.-F. GOURGUESLORENZON are with MINES ParisTech, PSL Research University, MAT-Centre des Mate´riaux, UMR CNRS 7633, BP 87, 91003 Evry Cedex, France. Contact e-mail: [email protected] Manuscript submitted July 4, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
cooling and further annealing, respectively.[4] Competition between all of these mechanisms strongly relies on the particular steel chemistry and on deformation conditions. The stacking fault energy (SFE) governs the dislocation mobility and recrystallization mechanisms.[5–7] At room temperature, Mo-containing 316-type austenitic stainless steels present relatively high SFE > 50 mJ m2.[6] In such medium-SFE materials, recovery is promoted prior to recrystallization, by dislocation climb and cross-slip leading to dislocation annihilation or subgrain formation.[8,9] Annihilation and spatial re-arrangement of dislocations decrease the energy stored in the deformed material. The onset of recrystallization is therefore delayed.[8] Conversely, austenitic stainless steels with lower SFE, such as Mo-free AISI 304 (SFE ~ 20 mJ m2[6]), show little recovery due to highly dissociated dislocations.[8,10,11] DRX and SRX mechanisms by nucleation and growth of new dislocation-free grains are therefore dominant in these steels.[12,13] Although this is scarcely documented in literature, it is worth noting that the SFE generally increases with temperature,[14–17] which could affect the competition between recovery and recrystallization. Besides their effect on the SFE, solute elements such as Mo affect recrystallization mechanisms in AISI 316 austenitic stainless steel.[18,19] Carbide-forming Mo may induce a solute drag effect which clearly delays the DRX and SRX kinetics in comparison to Mo-free AISI 304 steels.[18,
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