Effect of Cobalt Addition on Transformation Behavior and Drawability of Hypereutectoid Steel Wire

The grain boundary cementite and Widmanstätten cementite consisted in a hypereutectoid steel have been considered to cause the brittle fracture. In this study, the effect of cobalt addition on the transformation behavior of the hypereutectoid steel has been examined in order to control the grain boundary cementite precipitation. It was found that in the case of hypereutectoid steels containing carbon content less than 1.3 wt%, the grain boundary cementite precipitation could be suppressed with the cobalt addition and patenting treatment. By suppression of grain boundary cementite, the hypereutectoid steel showed good drawability comparable with that of eutectoid steel and the increase of tensile strength comparing with eutectoid steel. The analysis using Embury-Fisher relation showed that the increase of workhardening rate was due to the refinement of lamellar spacing, which was confirmed by microstructural observation. Furthermore, it was found that refinement of lamellar spacing was attributed to increased carbon content in the case of fully pearlitic microstructure. The effect of cobalt addition on the transformation behavior was also discussed from CCT curves.