In-situ Observation of Cracking and Healing Behavior of High-carbon Steel during Oxide Scale Growth

This study investigates cracking and healing behavior during the oxide scale formation on high-carbon steel. The steel was heated to 1000°C at a rate of 2°C/s in the air atmosphere. The surface morphology of the oxide scale was monitored in situ using Laser Scanning Confocal Microscopy (LSCM) and oxide phases in the scale were analyzed by X-ray diffraction (XRD). The microstructures of scale cross-sections were analyzed by Electron Probe Micro-analysis (EPMA). Based on the surface morphology of the oxide scale at different temperatures, we established a relationship between blistering and the “state” of the oxide scale. Blistering occurs during the scale growth due to the decarburization process, since evolved gaseous products cause swelling and eventual rupturing of the oxide scale, forming cracks on the surface. Thermal stress induced by the difference in FeO/Fe3O4 thermal expansion coefficients causes crack expansion. Subsequent crack healing consists of the following steps: (1) oxygen penetrates the oxide scale through the cracks and reacts with FeO to form Fe3O4. This reduces the difference in the thermal expansion coefficients and decreases the thermal stress, restraining the crack expansion; (2) at a certain temperature, the oxidation rate increases and oxygen reacts with the substrate to form new oxides, which fill in the formed blisters. The mechanism of crack expansion and healing was developed by monitoring the evolution of oxide scale morphologies during the blistering process.