2014 Volume 100 Issue 3 Pages 352-358
Blistering is a phenomenon in which oxide scale swells during oxidation at high temperatures. Blistered scale causes surface defects in steel products during hot-rolling processes. The present study investigated the effect of oxygen and the scale layer structure on blister initiation when steel is oxidized at high temperatures, and the mechanism of blister formation is discussed. The following conclusions are drawn. Blisters are not formed when the scale is a wustite mono-layer, but start to nucleate when the scale layer structure changes from the wustite mono-layer to a three-layered scale comprised of hematite (Fe2O3), magnetite (Fe3O4), and wustite (FeO). The compressive stress in the oxide scale that is applied due to the oxide scale growth is largely released when scale layer structure changes from the wustite mono-layer to the three-layered scale. This result suggests that the compressive stress is not the main factor for blister initiation. It is deduced that the pressure of the CO, CO2, and N2 gases generated at the scale/steel interface is the main factor for blister formation. The hematite layer on the top surface of scale presumably acts as a barrier of gas permeability.