Rapid transformation of scale on steel sheets by laser irradiation

Scale, iron oxide on hot rolled steel sheets, mainly consists of several kinds of oxides; hematite (Fe₂O₃), magnetite (Fe₃O₄) and wustite (FeO), as well as iron (Fe) itself. In this study, controlling of scale properties was studied using laser irradiation, which realizes heating only thin scale surface layer without changing properties of steel substrate. In our experiment, diode laser beam up to 3 kW power was scanned on scale surface at 0.2 m/sec. The composition analysis revealed that the following reaction takes place in the scale: Fe₃O₄ + Fe → 4FeO. It is known that the reaction occurs at above 580 °C under thermal equilibrium, but the result shows that heating time of just several milliseconds over 580 °C is enough for the reaction. The mechanism investigation is also presented.