Work hardening behavior of prestrained steel in tensile and torsion tests

Abstract

The work hardening behavior of metals subjected to strain path changes is different from that in monotonic deformation. Such changes can also lead to transients in flow stress and in the strain hardening rate. The origin of these phenomena is the instability of the dislocation structure produced during the initial straining. Subsequent deformation under a different path involves a restructuring of the dislocations [1]. The effect of a change in the strain path on the behavior of prestrained aluminum alloys was studied by Wilson et al. [2]. Decreased or enhanced work hardening rates were observed at the beginning of the second stage of deformation. These effects were associated either with the reorientation of internal stresses or to the dissolution of the original dislocation structure and the formation of a new one. Vieira and Fernandes [3] studied the influence of a double change in the strain path on the flow curve of copper sheets. The overall behavior depended more on the orientation relationship between the previous and the subsequent stage and less on the sequence of deformation. A high initial flow stress was usually followed by a low work hardening rate. The dislocation structure tended to correspond to the last mode of deformation. A transient flow behavior can also be caused by a change in the strain rate. The association of changes in the strain path and in the strain rate was investigated by Bate [4]. The effect of a strain rate change on the work hardening of aluminum samples was cancelled when a large change in the deformation mode was imposed. The elimination of the rate sensitivity was associated with the disruption of the cell walls due to a change in the strain path. The effects of a change in the strain path were evaluated in tension and torsion tests in this paper. The experiments were conducted in two and three stages and the results were compared with the flow curves from monotonic experiments. The material was a low carbon steel (0.245%C; 0.407%Mn; 0.155%Si; 0.0076%S). The torsion and tension specimens, 3.10 mm in radius (R) and 44.70 mm in length (L), were annealed in vacuum at 1000 ◦C for 20 min and furnace cooled to room temperature. The tests were performed at room temperature in a MTS servo hydraulic testing machine. The initial strain rate used in all experiments was 6.34× 10−2 s−1 [4, 5]. The samples were stored at∼−5 ◦C between two stages of deformation. Load (Q) and elongation (1L) were measured in tensile tests and torque (T ) and angle of twist (θ ) were measured in torsion tests. These data were converted into effective stress (σ ) and strain (e) using the following equations [6, 7]: Tension (uniform strain range):