In situ SEM analysis of surface oxidation mechanisms in carbon steel during vacuum heat treatment

Understanding surface effects, such as oxidation, that occur during the heat treatment of steels is vital in producing desirable mechanical properties. To mitigate against thermal oxidation, heat treatments are often conducted in a ‘fine’ vacuum (∼10−4 mbar); however, studies have shown that even at these vacuum levels oxidation occurs. To further understanding of this behaviour, an alternative in situ Scanning Electron Microscopy approach, facilitated by a novel heat stage, has been used to study the surface morphology of carbon steel during thermal oxidation. The data provide insight into the surface formation of oxidation under fine vacuum conditions, demonstrating that the process begins with the generation of thermally etched grain boundaries, where initial oxidation is focussed, followed by the formation of oxide scales across individual grains. Eventually these individual scales agglomerate to form a continuous oxide layer. The data further suggest that these oxide layers are likely to be predominantly wustite. The surface data also indicate an absence of blistering, commonly observed on the steel surface at the these temperatures and timescales. The lack of blisters both in situ and ex-situ is thought to be attributed to an absence of a gaseous environment preventing blister formation.