Effect of microstructural features on the hot ductility of 2.25Cr-1Mo steel

Some factors contributing to the hot ductility losses of a 2.25Cr-1Mo steel were identified over the temperature range 750-950 °C, after the specimens were austenitized at 1000 °C, furnace cooled to different temperatures, and held there for sufficient periods of time, followed by tensile testing. There were two types of ferrite present in the microstructure, namely, pro-eutectoid ferrite and deformation-induced ferrite. The pro-eutectoid ferrite was only formed below Ar3 (∼825 °C), which was nucleated on the inclusions and distributed uniformly. Nevertheless, the deformation-induced ferrite was formed in a much wider temperature range. It was distributed mainly along austenite grain boundaries above Ar3, and around the pro-eutectoid ferrite below Ar3. The deformation-induced ferrite had a primary effect on the hot ductility, which was mainly responsible for a hot ductility trough. There was a peak in the quantity of deformation-induced ferrite between 800 and 900 °C, which was just corresponding to the hot ductility trough. The morphology of ferrite was also essential. The net-like structure of ferrite formed along austenite grain boundaries was the most deleterious to the hot ductility.