Abstract
The need for materials with higher strength and corrosion resistance in corrosive environments, such as in the oil extraction in saline media, has led to the use of super duplex stainless steels in projects such as the Pre-sal. The manufacture of these materials involves the step of thermomechanical processing, whose performance depends on the workability of the material. Processing conditions in which the super duplex stainless steel UNS S32760 can be worked safely and in which the material can fail were investigated in this presentation. The physical simulation was performed by means of hot torsion testing. The tests were performed at temperatures ranging from 900°C to 1200°C and strain rates of 0.01s-1 to 10s-1. The evolution of strain rate sensitivity of flow stress (m) for deformation of 0.5 at all temperatures investigated here was determined. After attaining the values of m for each deformation condition, the values of the power dissipation efficiency (η) were calculated, an instability criterion (ξ) was applied, and processing maps were constructed. Using these maps, the effects of deformation conditions on the power dissipation efficiency and the material plastic instability were discussed. The domains of processing maps, the observed microstructures and the shape of plastic flow stress curves were associated.
Highlights
The austenitic-ferritic stainless steels solidify with ferrite structure at temperatures close to 1430°C and becomes dual phase with the α → γ transformation by solid state reactions on cooling
Beside the hardening and softening mechanisms required to process each one of these phases, grain and phase boundaries have an important role in the deformation of duplex microstructure, since the deformation accommodation depends on the macroscopic plastic behavior and fracture characteristics of the phases and interfaces presents (Balancin, Hoffmann & Jonas et al, 2000; Gutiérrez, Iza-Mendia & Pinõl et al 2000)
The flow stress curves have the characteristic shape of the hot plastic behavior of metallic materials: the stress increases with the strain to a maximum and decreases
Summary
The austenitic-ferritic stainless steels solidify with ferrite structure at temperatures close to 1430°C and becomes dual phase with the α → γ transformation by solid state reactions on cooling. Thermomechanical processing of these steels consists of reheating to 1250°C and deformation schedules by rolling or forging with temperature decreasing to levels close to 1000°C In this temperature gap the phase transformation with the nucleation and growth of austenite particles at boundaries and in the ferrite matrix occurs. Prasad et al (Prasad & Seshacharyulu et al 1998; Doraivelu, Gegel, Malas & Prasad et al 1984) proposed to build maps using the Dynamic Materials Model In this technique, the processing map is constructed by plotting the power dissipation in the workpiece (η) due to microstructural changes as a function of temperature and strain rate, and (η) given by:. We do not have information about studies on thermomechanical processing of super duplex stainless steels These steels with a more complex chemical composition are characterized by higher strength and corrosion resistance in saline environments. The workability of these steels using processing maps was studied
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