The effect of cyclic torsion on the hot dynamic restoration of interstitial free steel in the austenitic range

Abstract

The microstructures developed in hot and warm working are profoundly affected by variables such as the prevailing temperatures, strain rates and strains. Another variable that has been receiving attention is the “strain path” followed during processing. Cyclic cold straining of metals leads to lower work hardening than monotonic deformation. The degree of hardening decreases as the strain amplitude (Δ ε) is lowered. It has also been shown that the flow stress of copper in hot cyclic torsion is substantially lower than under monotonic torsion. The results for the warm cyclic stress–strain curve for an interstitial free steel (IF steel) processed in the ferritic (BCC) range were similar to those for copper. Cyclic straining led to steady-state flow stresses below those obtained in monotonic torsion for strain amplitudes (Δ ε) up to about 0.6. Lower amplitudes corresponded to decreasing steady-state stresses. The objective of the present paper is to investigate the effect of strain path changes, in cyclic torsion of an IF steel in the austenitic range. The testing temperature was 1223 K (950 °C) and the strain rate was 0.1 s −1. The flow curves in the monotonic torsion displayed stress peaks typical of dynamic recrystallization (DRX). The flow curves in the cyclic torsion with lower amplitudes were below those under monotonic torsion. A composite test was carried out, where the sample was initially deformed under monotonic straining, then under cyclic loading (Δ ε=0.03), and finally under monotonic loading again. The results can be interpreted in terms of dislocation rearrangements between configurations typical of monotonic and cyclic straining.