Hot workability of 2304 and 2205 duplex stainless steels

The duplex stainless steels 2304 and 2205 were subjected to torsion testing over the range of temperatures from 1000 to 1200 degrees C and strain rates from 0.1 to 5 s(-1) to characterize their hot working behaviour. The flow curves exhibit a peak followed by a decline towards a steady state that is attained at high temperatures and low strain rates. The peak stresses are fitted to a sinh-Arrhenius constitutive equation with activation energies of 536 and 406 kJ/mol, respectively. The optical microstructure shows elongated austenite regions in a ferrite matrix that increase in volume fraction with a rising temperature. The austenite phase becomes more elongated as the fracture strain rises indicating that both phases codeform in the temperature regime studied. During hot deformation, the ferrite effectively undergoes dynamic recovery, thus its substructure consists of well polygonized subgrains. In contrast, dynamic restoration processes are much suppressed in the austenite phase. The austenite substructure contains dense irregular dislocation networks and the dynamic recrystallization is observed only at very high temperatures. The hot ductility of the steels is limited by the cracking at the ferrite/austenite interphase boundaries; however, it improves as the temperature rises and the strain rate declines reaching maximum true strains of about 1.2 and 3, respectively. Present results indicate that the hot workability of 2304 and 2205 duplex stainless steels can be improved modestly by multistage testing. This is attributed to the static restoration processes (static recovery of ferrite and static recrystallization of austenite) which reduce internal stress concentrations but can not inhibit interphase boundaries from nucleating the cracks.