Phase transformation behavior of Grade 91 ferritic steel

The phase transformation behavior of Grade 91 steel, which is an advanced ferritic steel, has been investigated using differential thermal analysis (DTA). In our DTA experiments, disk-shaped samples were normalized at 1080 °C, and then tempered at temperatures between +10 °C and −40 °C of the ferrite to austenite transformation temperature (Ac1 temperature) determined during normalizing, at a heating and cooling rate of 30 °C/min. The DTA curves on heating during normalizing showed that a magnetic transition and the Ac1 transformation temperatures occurred at 744 °C and 847 °C, respectively. Two overlapping exothermic peaks were observed in the temperature range 770–700 °C in the DTA cooling curves after tempering at temperatures between +10 °C and −30 °C from the Ac1 temperature. A partial austenitization seemed to have occurred, even when holding the tempering temperature below the Ac1 temperature and, thus, the high temperature peak was due to the austenite to ferrite transformation. The low temperature peak corresponded to a magnetic transition. The formation of ferrite during cooling after tempering can be considered to arise from the heat evolved from the magnetic transition, which influences the localized cooling of the sample near the Curie temperature.