Characteristics of scale/substrate interface area of Si-containing low-carbon steels at high temperatures

Low-carbon steels containing Si of up to 1.5 mass% were oxidised in air at 1416 and 1476 K, i.e. below and above the eutectic temperature of FeO and Fe2SiO4. An acoustic emission technique was used to assess the temperature at which a major scale spallation takes place during cooling in static air. This allowed evaluation of the thermal stress. The stress for the scale spallation during cooling from 1416 K increases as the Si content increases, while it is almost constant for cooling from 1476 K. This difference is attributable to the formation of liquid phase in the latter case. The conventional metallographic examinations revealed that scales formed at 1416 K consist mainly of two FeO layers except for 0.01%Si steel; the inner layer is porous FeO and the outer layer is mainly dense FeO. The porous layer consists of a mixture of FeO and Fe2SiO4 grains. A row of large micropores separates the dense FeO layer. These micropores become larger as the Si content increases. At 1476 K the eutectic liquid phase is formed between the scale and the substrate, and penetrates very deep along grain boundaries of the FeO and slightly to the substrate.