Effects of the residual stress, interfacial roughness and scale thickness on the spallation of oxide scale grown on hot rolled steel sheet

Failure at the interface between a steel substrate and oxide scale was investigated by semi-analytical and finite element numerical simulations. Two major stress components along the interface, i.e., tensile normal stress at the peak and shear stress at the inflection point of the idealized undulated interface, were calculated and used for the failure analysis. The mechanical properties of the oxide scale and steel substrate were experimentally measured by the indentation and uniaxial tensile tests, respectively. Growth and thermal residual stresses accumulated at the oxide scale during cooling were calculated by a coupled numerical-analytical method, which was experimentally validated by measuring the residual stresses in the oxide scales with different thicknesses. The finite element simulation of a four-point bending test was conducted in order to reproduce uncoiling process during which spallation of oxide scale may occur. From the analysis of the simulation results, the two major stress components turned out to be amplified by the roughness of the interface and the residual stresses generated by the growth stress of oxide scale and the thermal mismatch between the oxide and steel substrate during cooling. In addition, the shear stress proved to be a significant factor for the spallation behavior. The effect of scale thickness on the spallation was discussed by using the concept of failure map based on interfacial fracture energy.