Failure of scales formed on Cu-containing low carbon steels during cooling

Low carbon steels containing 0 to 1.5 mass% Cu were oxidised in air at 1 073, 1 150, 1 273 and 1 386 K for up to 3.6 ks. An acoustic emission (AE) technique was used to assess the temperature at which mechanical failure of the scale takes place during cooling in static air. The conventional metallographic examinations revealed that the scales consist mainly of two FeO layers; the inner layer is porous FeO and the outer layer is dense FeO for oxidation temperatures of up to 1 273 K. Cu and Si are enriched in the porous FeO layer at the interface of the two FeO layers. At 1 386 K an FeO layer containing Fe2SiO4 (fayalite) grains is formed instead of the inner porous FeO layer. Cu is enriched at the scale/substrate interface, while Si is enriched as Fe2SiO4 grains in the inner FeO layer near the both interfaces. The enrichment of Cu is unrecognisable for the Cu content less than 0.5%. The modes of scale failure observed are(a) partial separation at the two FeO layers, (b) partial separation at the scale/substrate interface, (c) through-scale crack almost normal to the substrate surface, (d) shear crack in the inner FeO layer at about 45 degrees to the substrate surface and (e) blistering of a thin Fe2O3 layer over a small area. The frequency is in this order; i.e. (a) is most frequently observed and (e) is very rare. The values of Delta T (oxidation temperature-failure temperature) detected by the AE measurement were converted into the apparent thermal stresses to cause the scale failure. The stress increases in a range 0.5 to 1 GPa as the oxidation temperature rises up to 1 273 K. A further temperature rise slightly decreased the stress. This is attributable to the formation of Fe2SiO4 grains in the FeO layer. The increase in the Cu content slightly increases the stress.