Bainite Formation in High Carbon Copper Bearing Steels

Abstract

The bainite reaction has been studied extensively in steels and other non-ferrous alloys. However up to present a significant disagreement exists regarding the nature and the mechanism of this phase transformation. During the isothermal bainitic transformation in hypereutectoid carbon steels alloyed with copper or copper and nickel, it was found that at all the transformation temperatures studied, the formation of equally copper supersaturated bainitic ferrite and cementite always occurred. This observation implies the formation of bainitic ferrite and cementite from the parent austenite phase without redistribution of the alloying elements, since the solubility of copper in cementite is negligible and very low in bainitic ferrite. By carefully designing suitable tempering treatments, it is possible to produce copper precipitation within tempered bainitic ferrite in both types of steels using low tempering temperatures. In addition, copper precipitation within the tempered bainitic cementite of the copper steel has been found to occur within a narrow tempering temperature region. However, the orientation relationship of the copper precipitation, within the tempered bainitic cementite and within the tempered bainitic ferrite has been found to be different. The interpretation of these experimental data strongly supports the theory promoting formation of the bainite through a shear controlled type of mechanism.