On the High Temperature Deformation Behaviour of 2507 Super Duplex Stainless Steel M.K. Mishra, I. Balasundar, A.G. Rao, B.P. Kashyap, and N. Prabhu (Submitted October 6, 2016; in revised form December 22, 2016; published online January 11, 2017) High temperature deformation behaviour of 2507 super duplex stainless steel was investigated by con- ducting isothermal hot compression tests. The dominant restoration processes in ferrite and austenite phases present in the material were found to be distinct. The possible causes for these differences are discussed. Based on the dynamic materials model, processing map was developed to identify the optimum processing parameters. The microstructural mechanisms operating in the material were identified. A unified strain-compensated constitutive equation was established to describe the high temperature defor- mation behaviour of the material under the identified processing conditions. Standard statistical parameter such as correlation coefficient has been used to validate the established equation. Keywords constitutive equation, hot deformation, processing map, super duplex stainless steel 1. Introduction Super duplex stainless steel (SDSS) belongs to the family of stainless steel and has two-phase microstructure viz. ferrite and austenite (Ref 1). The presence of these two phases in the microstructure enables SDSS to possess the properties of both ferritic and austenitic stainless steels. SDSS is distinct from other grades of duplex stainless steel (DSS) due to its better corrosion resistance (pitting resistance equivalent number greater than 40) (Ref 2). Since SDSS has excellent combination of mechanical properties and corrosion resistance, it is widely used in petrochemical, nuclear and marine industries (Ref 3). The high temperature deformation behaviour of ferritic (Ref 4-6) and austenitic (Ref 7, 8) stainless steels has been discussed in detail by various researchers. It is well established that ferrite is characterized by high stacking fault energy (SFE) which enables easy climb and/or cross-slip of dislocations during high temperature deformation resulting in dynamic recovery (DRV) (Ref 9). Austenite is characterized by low SFE, where the mobility (cross-slip) of the dislocations is restricted, and hence, the dislocation density increases continuously during high temperature deformation. As the internal energy due to accumulation of dislocations attains a threshold value, dynamic recrystallization (DRX) takes place where the deformed grains are replaced with new strain-free grains (Ref 8-11). When both ferrite and austenite are deformed simultaneously as in the case of SDSS, the material behaves differently, compared to, a single-phase ferritic or austenitic stainless steel. This can be attributed to the individual and synergistic response of the ferrite and austenite phases to the imposed processing condi- tions. Further, the deformation behaviour of two-phase material such as SDSS also depends on the volume fraction, morphol- ogy and topology of the phases involved (Ref 12). It is well known that stored energy is the driving force for the occurrence of softening phenomena. At high temperature, since ferrite is soft compared to austenite, it is expected that the deformation would be accommodated by the ferrite phase. Hence, in duplex stainless steel, restoration processes occur in the ferrite phase earlier than in the austenitic phase. At present, there seems to be a controversy about the dominant softening processes occurring during hot deformation in DSS. A recent work maintains that the mechanism of softening in ferrite and austenite is DRV and DRX, respectively (Ref 13), while other researchers propose that the mechanism of softening in ferrite and austenite is DRX and DRV, respectively (Ref 14, 15). Hence it is important to investigate and determine the restoration mechanisms operating in the constituents phases of SDSS. Processing map based on dynamic materials modelling (DMM) is widely used to evaluate hot working mechanism and to predict optimum processing parameters. Processing map identities ‘‘safe’’ and ‘‘unsafe’’ domains for processing of materials (Ref 16, 17). Chen et al. (Ref 13) studied the hot working characteristics of 2205 DSS using the processing maps and reported both stable and unstable regions of deformation. Based on the microstructural observations, they found that instability is associated with twinning, flow localization and wedge cracking. Ma et al. (Ref 18) developed processing map for DSS with two different compositions—one with rare earth element (Ce and La) addition and the other without rare earth addition. Based on this study, they reported that the rare earth containing DSS exhibits a narrow flow instability region and improved hot workability. The hot working procedures adopted for DSS are usually complex due to high risk of edge-crack formation. The objective of the current work is twofold; first, to systematically evaluate the high temperature deformation behaviour of 2507 SDSS and identify the corresponding restoration mechanisms operating in both the ferrite and austenite phases present in the material. The second objective is to develop a strain-dependent constitutive equation that can M.K. Mishra, B.P. Kashyap, and N. Prabhu, Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai 400076, India; I. Balasundar, Near Net Shape Group, Aeronautical Materials Division, Defence Metallurgical Research Laboratory, Hyderabad 500058, India; and A.G. Rao, Naval Materials Research Laboratory, Shil-Badlapur Road, Ambernath 421506, India. Contact e-mail: manjeshkumar.mishra@gmail.com. JMEPEG (2017) 26:802–812 ÓASM International DOI: 10.1007/s11665-017-2508-y 1059-9495/$19.00 802—Volume 26(2) February 2017 Journal of Materials Engineering and Performance