METAL 2005 24.- 26.5.2005, Hradec nad Moravicí ___________________________________________________________________________ 1 Microstructural investigation of the oxide scale on low carbon steel S. Birosca, G. D. West and R. L. Higginson IPTME: Institute of Polymer Technology and Materials Engineering, Loughborough University, Loughborough, Leicestershire LE11 3TU,UK. Abstract There has been a long history of studies on the oxidation behaviour of carbon steel at high temperatures. The oxidation behaviour and the scale structure developed are complex. The microstructure and the ratio of the three classical layers of wüstite, magnetite and haematite may be difficult to identified by conventional techniques such as optical, standard Scanning Electron Microscopy (SEM) or X-ray diffraction. An unambiguous characterisation of the scale and the correct identification of the phases within the scale are difficult unless the crystallographic structure for each phase in the scale is considered and a microstructure- texture analysis is carried out. Throughout this work, Electron Backscatter Diffraction (EBSD) has proved to be a powerful technique for identifying the individual phases in the oxide scale accurately in order to obtain a better understanding of the mechanism of the oxide scale growth on steel and how scale microstructures develop. The results have shown how oxidation conditions such as temperature and time of exposure effect the grain size and microstructural development of the oxide phases. 1. INTRODUCTION Steels subjected to high temperatures in an oxidised environment leads to the formation of oxide layers on their surfaces. This oxide can influence the friction conditions; heat transfer and tool wear during deformation and ultimately influence the surface finish of the final product [a, b]. The microstructures of scales can be highly complex, however, the scales on iron and steel are generally characterized by a three layer model. The innermost layer, closest to the substrate, with the lowest oxygen content is wüstite (FeO); there is an intermediate magnetite (Fe 3 O 4 ) layer and a final thin oxygen rich haematite (Fe 2 O 3 ) layer. This classical view of the oxide layers is however, complicated by the heat treatment cycle, oxidation environment, and alloying elements in the steel. According to most of the published literature, as the oxidation temperature increases the thickness of the scale also increases due to faster diffusion of iron and oxygen ions [c, d, e]. The diffusion of ions is also affected by other factors for example the porosity of the scale, cracks that may develop and the scale adhesion to the substrate. Such factors are particularly important where the steel is subjected to mechanical work. This can cause the opening of cracks and defects allowing increased diffusion of oxygen to the surface of the substrate in localized regions. The temperature also influences the fraction of the three phases within the scale [f]. At low temperatures (<650°C) the magnetite phase dominates the scale. At higher temperatures the wüstite phase becomes more dominant with the haematite layer remaining low at all temperatures. Work has shown that during cooling the wüstite phase is not stable and can transform to magnetite [c, e]. Magnetite precipitates on the wüstite due to a saturation of oxygen during cooling from the oxidation to room temperature. This precipitation can further complicate the distribution of the phases within the scale.