Effect of annealing on microstructure and mechanical properties of cold-drawn pearlitic steel wires

The effects of low-temperature annealing on microstructure and mechanical properties of cold-drawn pearlitic steel wires were investigated. The tensile strength initially increases, peaking after 1 min of annealing, then decreases with annealing at 400 degrees C for longer durations. The initial strength increase is deemed to result from age hardening during short-time annealing at 400 degrees C, while the subsequent decrease in strength over longer times at this temperature is attributed to age softening. Age hardening is influenced by two factors: the diffusion of dissolved carbon atoms in lamellar ferrite and the subsequent pinning of dislocations, which includes the dissolution of about 0.022% wt.% carbon in the iron lattice when annealed at 400 degrees C for 1 min; and the dislocations pinned by newly formed nano-sized cementite particles. The occurrence of age softening is attributed to the spheroidization of lamellar cementite and the decreased dislocation density through the recovery of ferrite. When the annealing temperature is increased to 450 degrees C and 500 degrees C, the tensile strength gradually decreases over the annealing time. The dissolved carbon in the iron lattice is small or negligible, and the carbide growth is more pronounced at higher temperatures, resulting in less significant effects of aging hardening.