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DUPLEX stainless steels (DSS) basically show a dual-phase microstructure consisting of approximately equal volume fractions of BCC ferrite (a) and FCC austenite (c). For the industries, the duplex steels have attracted great attention due to their lower price, higher strength as well as better corrosion resistance than the traditional AISI 304 austenitic stainless steel. Recently, these steel grades are being increasingly employed in chemical, petro-chemical, nuclear, and energy application fields.[1–3] These stainless steel grades can be processed by different routes, i.e., casting, forging, extrusion, or rolling. Such forming operations are usually performed at high temperature, at least in the earlier manufacturing stages. However, different mechanical responses of the austenitic and ferritic phases under hot working conditions could lead to defects in microstructure and deteriorated properties after hot deformation.[4] Thus, well-developed microstructures and satisfied product performances are strongly controlled by hot forming process parameters.

SARANYA KINGKLANG, Master Student, and VITOON UTHAISANGSUK, Lecturer, are with the Department of Mechanical Engineering, Faculty of Engineering, King Mongkut’s University of Technology Thonburi, 126 Pracha Uthit Road, Bang Mod, Thung Khru, Bangkok 10140, Thailand. Contact email: vitoon.uth@ kmutt.ac.th Manuscript submitted December 20, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A

To investigate hot deformation behavior of these steel grades, two approaches for establishing constitutive relationships have been usually applied. The first one is physical models. Principally, the physically based model is related to various basic physical parameters like grain size and dislocation density. Thus, such model can more reliably describe material strain hardening with regard to microstructure evolution, deformation mechanisms, softening mechanisms (recovery and recrystallization), or grain boundary mobility. Therefore, this model can be used to predict material behavior under different conditions. However, for the physical models large amounts of extensive and costly experiments are required in order to identify their material parameters. For example, a composite model was proposed,[3] in which austenite and ferrite were considered as hard and soft phases. At an early stage of deformation, strain distribution was inhomogeneous, because strain accumulated in ferritic regions, while austenite was not yet deformed. In Reference 5, the flow curves of the ferritic steels exhibited typical dynamic recovery and thus were modeled by a dislocation density evolution. In case of the second approach, namely phenomenological model, empirical models as a mathematical representation were used to describe the correlation between flow stresses of materials and strain, strain rate, and deformation temperature under a wide range of working conditions.[6,7] Generally, material parameters for such models can be just determined from experimental flow stress curves.[4] The advantages of this model are t