Defect Recovery in Severely Deformed Ferrite Lamellae During Annealing and Its Impact on the Softening of Cold-Drawn Pearlitic Steel Wires

Cold-drawn pearlitic steel wires with a drawing true strain of 3 were annealed at temperatures (T_ann) ranging from 423 K to 723 K (150 °C to 450 °C) with an interval of 50 K. Recovery of the lattice defects in the severely deformed ferrite lamellae were characterized by means of high-energy X-ray diffraction and positron annihilation techniques (including positron annihilation spectroscopy and coincidence Doppler broadening spectroscopy). Accordingly, the impact of defect recovery on the softening of the annealed wires was investigated. It is found that at low temperatures [T_ann ≤ 523 K (250 °C)], the recovery of the lattice defects in ferrite lamellae is dominated by the agglomeration and annihilation of vacancy clusters, while at T_ann > 523 K (250 °C), the recovery process is controlled by the annihilation of dislocations. Further analyses on the softening of the annealed wires indicate that the evolutions of dislocation density and concentration of vacancy clusters, and the strain age hardening in ferrite lamellae play important roles in changing the strength of the wires. The strain aging hardening leads to a maximum strength at 473 K (150 °C). Above 523 K (250 °C), the annihilations of vacancy clusters and dislocations in ferrite lamellae cause a continuous softening of the wires, where the decrease in dislocation density plays a major role.