Dynamic Restoration Processes in a 23Cr-6Ni-3Mo Duplex Stainless Steel: Effect of Austenite Morphology and Interface Cha
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INTRODUCTION
DUPLEX stainless steels (DSS) containing ~50 to 50 phase fractions of austenite and ferrite offer an optimum combination of mechanical properties and corrosion resistance, which makes them suitable candidates for numerous applications, particularly in the marine and petrochemical industries.[1] Nevertheless, manufacturing of these steels is rather challenging, namely due to the complexities associated with their hot working. Complex high-temperature workability of DSS, originating from the presence of two phases having different characters and being separated by varied interfaces, has been a subject of intense study in recent years.[2–9] It has been shown that the markedly different restoration
N. HAGHDADI, P. CIZEK, H. BELADI, and P.D. HODGSON are with the Institute for Frontier Materials, Deakin University, Geelong, Victoria, 3216, Australia. Contact e-mail: [email protected] Manuscript submitted December 9, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
behavior of the constituent phases, strain partitioning, elemental segregation, and the character of different interfaces, particularly austenite–ferrite and austenite–austenite boundaries, would highly affect the behavior of the above steels during hot deformation. Hot deformation and restoration behavior of austenite and ferrite shows distinctly different characteristics. Austenite possesses a low stacking fault energy (SFE), which makes it resistant to dynamic recovery (DRV) and prone to discontinuous dynamic recrystallization (DDRX).[10,11] The latter process consists of nucleation and growth of new grains and its progress with strain results in gradual softening in the stress–strain curve. Recrystallized grains in single-phase austenite have been reported to form through strain-induced boundary migration (SIBM), i.e., bulging of the preexisting high-angle grain boundaries, commonly accompanied by (multiple) twinning.[10,12–17] Duplex steels typically contain a limited amount of the original austenite–austenite high-angle boundaries, which (together with the limited strain usually transferred to austenite, as discussed further below) generally leads to the
marked suppression of the DDRX process.[2,18] In this context, Cizek[7] has recently reported that under high-temperature and low-strain rate conditions, austenite might tend to dynamically soften through a large-scale subgrain coalescence mechanism rather than via DDRX. It should be noted that the extent of DDRX and its mechanism in DSS under different hot deformation conditions is currently poorly understood, and more detailed efforts are needed to extend the understanding of this issue. In contrast to austenite, ferrite having a high SFE value and multiple active slip systems at high temperatures recovers readily. Under certain hot deformation conditions, which can generally be characterized by the corresponding Zener–Hollomon parameter, Z representing the temperature-compensated strain rate,[10] DRV might gradually evolve with increasing strain into the process frequently termed conti
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