Transition from regular oscillations in flow stress to single peak during high-temperature torsional deformation of a low-alloy steel

The high-temperature deformation of a low-alloy steel has been studied by torsion testing at temperatures in the range 850–1150°C and at constant strain rates in the range 0·6 × 10⁻³ to 5·3 S⁻¹. The stress-strain curves obtained were characteristic of the occurrence of dynamic recrystallization over the entire range of testing conditions. Analysis of flow-stress data before the peak, at the peak, and during steady state gives a constant activation energy for deformation of 315 kJ mol- 1, indicating that the activation energies for dynamic recovery and dynamic recrystallization are the same. Correlations of the strains to the peak flow stress and to steady state and also of the dynamically recrystallized grain size with the Zener-Hollomon parameter and original grain size have been obtained. By using these correlations the interrelating parameter between the strain to the peak flow stress and the critical strain for the onset of recrystallization has been obtained. It is shown that the ‘relative grain size model’ for the transition from regular oscillations in the flow stress to a single peak is a reflection of the ‘critical strain condition’.