The effect of alloy compositions on the microstructure and the mechanical strength of oxide scales on four selected steels

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 (pO2). 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. pO2 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.