Effect of Prior Microstructure on Mechanical Properties of Medium Carbon Pearlitic Steels After Spheroidizing Annealing

Machinery parts made from medium carbon steels are sometimes formed after spheroidizing annealing (SA). In order to improve the formability, it is necessary to reduce the yield strength and increase the reduction of area to fracture. In the present study, changes in the mechanical properties of medium carbon steel wire rods after SA were precisely investigated in relation to prior microstructure. In the steels with pearlite, the yield strength was drastically reduced and the reduction of area was raised after SA because of microstructural changes from pearlite to ferrite with dispersed spheroidized cementite. It was also found that the yield strength of ferrite with spheroidizing cementite depended sensitively on cementite-particle spacing as well as ferritic grain size. The reduction of area was, however, reduced as cementite size increased. The yield strength of the rod drawn in advance rapidly dropped after SA over 958 K, which was lower than that of ferrite steels with pearlite. Such changes in the mechanical properties after spheroidizing annealing could be reasonably understood from the difference in distribution of spheroidized cementite particles.