Effect of thermal etching and oxidation on in situ characterisation of grain growth in carbon steel

Abstract

This paper presents the results from a recent investigation into the impact of thermal etching and oxidation on grain growth across the surface and bulk of carbon steel during heat treatment. The study used in situ high temperature Scanning Electron Microscopy imaging, facilitated by a novel heat stage, to capture microstructural changes during heat treatments between 800 and 920 °C. The key observations made using this surface imaging included oxidation, the formation and development of thermal etching, and grain boundary movement. In situ data were further supported by ex situ compositional and optical microstructural data obtained by first sectioning the bulk material. Comparison between the results highlighted a significant discrepancy between the surface and bulk grain growth of the specimens during heat treatment at all temperatures. Further investigation concluded that the combination of thermal etching and oxidation lead to the retardation of grain growth on the surface of the carbon steel, but that grain growth in the bulk specimen appeared to be unaffected. The mechanism that causes the grain retardation is not dissimilar to that of Zenner pinning, where in this case oxide particles pin the newly exposed etched grain boundaries. Hence, oxidation formation within the boundaries decreases the energy of the overall system resulting in movement of the grain boundary becoming less energetically favourable. It is anticipated that these findings will be used to improve understanding of the surface effects that occur during the heat treatment of carbon steel in a vacuum environment.