2019 Volume 105 Issue 2 Pages 155-162
Fully pearlitic steel was wire-drawn up to the strain of 2.2. Torsion tests were performed using two types of specimen; one was as-drawn specimen, and the other was aged at 423 K for 3.6 ks. A delamination crack propagated along the longitudinal direction of the wire in the aged specimen while it showed normal fracture perpendicular to the longitudinal direction in the as-drawn specimen during the torsion tests. Backscattered electron images indicated that the cementite lamellae beneath the delamination crack had vanished while cementite lamellae beneath the normal fracture surface had rotated until the fracture in the as-drawn specimen. Torsion tests with different stain rates indicated the inverse strain-rate dependence of the onset of the delamination. Those suggest that the plastic deformability of ferrite and the existence of the thermally activated process which controls the cementite dissolution are the points for the onset of the delamination. In the present study, the effect of aging and deformability of ferrite on delamination is discussed, suggesting that the delamination crack propagates as the result of the local plastic instability in the scale of several microns.