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Microstructural observations of the oxide scale growth during short-term heat treatment of cast irons

Commercial iron pipes are manufactured by centrifugal casting in a water-cooled steel mould. Coarse eutectic cementite is usually present in the as-cast tubes. Therefore, iron pipes must be subjected to a graphitisation heat treatment at about 950° C. The long-term adhesion and the durability of anticorrosion epoxy coatings applied in a final stage to the tubes depend on the nature and adherence of the oxide scale grown during this graphitisation heat treatment. Microstructural characterisations of the oxide scale, decarburised zone and internal oxidation were performed on several samples cut from different parts of heat-treated pipes. Attempt was made to relate the observed microstructural features of oxide scales and underlying material to the adhesion of the external oxide scales, and to characterize the relationships between decarburisation, internal oxidation and development of oxide scales. The major importance of the extent of internal oxidation on the apparent adherence of oxide scales is emphasized.