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Abstract: We present the structure-property relationships of oxide scales and interfaces formed on carbon steels. We select four different steels having the same alloying elements, but different compositions. Using thermodynamic calculations, the phases of the scales formed at a fixed temperature are predicted as a function of oxygen partial pressures (). The model steels are oxidized under the same condition to the calculations for a fixed time. The scale microstructures are analyzed by electron probe micro-analysis (EPMA) and transmission electron microscopy (TEM). The mechanical strength and fracture patterns of the scales are analyzed by tension test. According to the analysis and test results, the adhesion strength of the scales are determined by the contents of reactive elements such as Si and Cr that form continuous oxide layers along the interfaces. The overall structure of the scales and the cohesion strength depend on O concentration in the scales which is controlled by C content. gradient in the layers of mostly Fe oxides becomes greater with more C for a fixed Si concentration increasing the fracture resistance. The predicted phases agree well with the microstructural analysis results. [Copyright &y& Elsevier]

Abstract: In this study, we investigated the influence of the Cr content on the microstructure and interfacial adhesion of the oxide scale formed on steel alloys. The four different alloys selected for this study are stainless steels and low- and medium-carbon steels with different Cr and Si contents. The model steels were oxidized at 1100–1200°C for 4–5h. The types of oxide phases in the scale were analyzed by X-ray diffraction (XRD), and the results were compared with the phases predicted by thermodynamic calculations. The cross-sectional scale microstructures were analyzed by electron probe micro-analysis (EPMA). The interfacial adhesion strength of the scale with the model alloys were analyzed by a uniaxial tension test. According to our results, continuous Cr-oxide layers are formed along the interfaces; the thickness of these layers proportionally increases with Cr, while the total thickness of the scale is inversely proportional to the Cr content. The thick Cr-oxide layers seem to hinder the interdiffusion of Fe and O between the scale and the substrate, which decreases the growth rate of Fe-oxides on the top surface. The predicted phases in the scale and the minimum oxygen partial pressure at which each oxide phase is formed agree well with the microstructural analysis results. Our results also revealed that the Cr-oxide layers survived the tension test for up to 5% of the strain, whereas the scale on top of the Cr-oxides cracked and delaminated. Based on these results, the Cr content determines the adhesion strength of the scale. [Copyright &y& Elsevier]