Abstract
The super-duplex stainless steel UNS S32750 consists of two main phases, austenite and ferrite, which differ not only by their morphology, physical, and mechanical properties, but also by their deformation behaviour. A heterogenous deformation can be obtained during thermomechanical processing, generating internal stresses and sometimes fissures or cracks on sample lateral surfaces, due to ferrite’s phase lower potential of plastic deformation accommodation in comparison with austenite phase. The research objective is to determine the optimum range of the applied deformation degree, during hot deformation processing by upsetting of the super-duplex steel (SDSS) UNS S32750. In the experimental program several samples were hot deformed by upsetting, by applying a deformation degree between 5–50%, at 1050 °C and 1300 °C. The most representative hot-deformed samples were selected and analysed by scanning electron microscope-Electron Backscatter Diffraction (SEM-EBSD), to determine the main microstructural characteristics obtained during thermomechanical processing. When considering the experimental results, the influence of the applied deformation degree on the microstructure has been evaluated. Microstructural features, such as nature, distribution, morphology and relative proportion of constituent phases, Grain Reference Orientation Deviation (GROD), and recrystallization (RX), were analysed, in correlation with the applied deformation degree. Finally, it was concluded that the UNS S32750 alloy can be safely hot deformed, by upsetting, at 1050 °C and 1300 °C, with a maximum applied deformation degree of 20% at 1050 °C and, respectively, by 50% at 1300 °C.
Highlights
Super Duplex Stainless Steels (SDSS) are defined as a class of stainless steels with a microstructure consisting of two phases: ferrite δ and austenite γ, in approximately equal proportions, containing enough Cr, Mo, and N to provide high resistance to pitting corrosion
10,illustrates x FOR PEER a of representative scanning electron microscope (SEM)-EBSD composite images, resulting from 5 of 13the microstructural analysis that was performed for the super-duplex steel in the initial structural state
When the UNS S32750 SDSS alloy is hot deformed, by upsetting, at 1050 °C, the weight fraction of ‐phase RX grains shows a continuous increase up to a deformation degree of 20%, when it reaches a value that is close to 10.8%
Summary
Super Duplex Stainless Steels (SDSS) are defined as a class of stainless steels with a microstructure consisting of two phases: ferrite δ and austenite γ, in approximately equal proportions, containing enough Cr, Mo, and N to provide high resistance to pitting corrosion. SDSS successfully combine the properties of ferritic and austenitic stainless steels, possessing a good combination of strength, ductility, and corrosion resistance in different corrosive environments [1,2,3,4]. They represent a very small percentage from the total of stainless steels The hot workability of a metallic material depends in a very complex manner on the material composition and on the thermomechanical processing parameters These two categories of factors cannot be treated independently, as they are closely interconnected, since process parameters can induce important changes in properties. The ability of the material to withstand deformation, Metals 2020, 10, 673; doi:10.3390/met10050673 www.mdpi.com/journal/metals
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