Isothermal transformation behavior of thermally-grown wüstite

The transformation behavior of wüstite (FeO) below the FeO→ Fe+Fe3O4 eutectoid temperature of about 570°C was studied using laser Raman spectroscopy in conjunction with optical microscopy, backscattered electron microscopy, and electron probe microanalysis. A thermally-grown wüstite layer was initially formed by oxidizing a mild steel in air at 900°C for 100 s. The thermally-grown layer was then isothermally annealed at 100–500°C for up to 6 hours. The transformation behavior was found to be very dependent on degree of undercooling below the eutectoid temperature. At temperatures down to about 350°C, transformation was initiated by Fe3O4 precipitation and then followed by lamellar Fe+Fe3O4 growth. However, at a temperature of 270°C, the FeO transformed directly to a fine, granular mixture of Fe+Fe3O4. No transformation was observed at 200°C and lower. The kinetic results were used to construct isothermal transformation diagrams.