Ultra-fine bainite structure in hypo-eutectoid steels

Ultra-fine, carbide-free bainitic structure of plate thickness between 34 and 116 nm has been obtained by low temperature austempering process of two hypo-eutectoid steels with 0.42 and 0.57% C. Decreasing the carbon content results in accelerating the bainite transformation reaction together with decreasing the retained austenite content, which is known to be detrimental to the mechanical properties. Furthermore, lowering the carbon content below the eutectoid composition allowed intercritical annealing of the material which resulted in a wider window for heat treatment parameters and consequently in a spread field for mechanical properties. Dilatometric measurements were used to design the suitable heat-treatment parameters including an estimation of the required time frames for the cessation of the bainitic reaction. The structure was characterized using light optical microscopy (LOM), scanning electron microscopy (SEM) and X-ray diffractometry. In order to investigate the effect of the microstructure parameters on the materials mechanical properties, compression tests had been conducted at room temperature.