Microstructural characterization of cobalt-containing super-bainitic steels

Adding ~2wt% silicon to the carbon steel can produce carbide-free bainitic steels under proper heat treatments, which consists of a mixture of bainitic ferrite and carbon-enriched residual austenite. This kind of steel has excellent properties in tensile strength and toughness due to ultra-fine grain and high dislocation density, so it is called “super-bainite”. The heat treatment for super-bainite is very simple, which conducts isothermally at low temperature but very long processing time. That means more energy and money will be wasted and can’t be applied in industrial processes.
In this research, the topic is to study the method about accelerating bainite transformation by adding ferrite-promoting elements, cobalt. We expect the whole transformation time would be shortened by cobalt additions which lower the free energy change of ferrite transformation, and also increase the volume fraction of bainite sub-unit. The experiments results show that the cobalt additions exactly accelerate the transformation but also induce pearlite nucleation and growth. The pearlite structure can be observed along the grain boundary which might influence the mechanical properties. The oil bath treatment was chosen to cool samples from austenite temperature for preventing the transformation of pearlite. This result showed that the existence of pearlite would shorten the bainite transformation time. The pearlite transformation would create carbon-depletion regions nearby so that they would be superior nucleation sites of bainite. Since the super-bainite transformation rate is nucleation control, the additional nucleation sites from pearlite transformation will also accelerate bainite transformation.
By XRD analysis, it can be seen that the cobalt additions increase the volume fraction of bainite sub-unit about 3%. The increase of bainite phase effectively strengthened the matrix, and final hardness was lifted up about 20 Hv.