Effect of Thermomechanical Rolling of the Induction Hardenability of a Micro-alloyed 1045 Steel

A micro-alloyed 1045 steel was commercially rolled into 54 mm diameter bars by conventional hot rolling at 1000 ^oC and by lower temperature thermomechanical rolling at 800 ^oC. The lower rolling temperature refined the ferrite-pearlite microstructure and influenced the microstructural response to rapid heating at 200 ^oC/s, a rate that is commonly encountered during single shot induction heating for case hardening. Specimens of both materials were rapidly heated to increasing temperatures in a dilatometer to determine the Ac₁ and Ac₃ transformation temperatures. Microscopy was used to characterize the dissolution of ferrite and cementite. Continuous cooling transformation (CCT) diagrams were developed for rapid austenitizing temperatures 40 ^oC above the Ac₃ determined by dilatometry. Dilatometry and microstructure evaluation along with hardness tests showed that thermomechanical rolling reduced the austenite grain size and lowered the heating temperature needed to dissolve the ferrite. With complete austenitization at 25 ^oC above the Ac₃ there was little effect on the CCT behavior.