Selective oxidation of Al rich Fe–Mn–Al–C low density steels

Abstract

The surface quality of steels containing solutes that have a strong affinity for oxygen can be markedly compromised during processing at elevated temperatures. Here, we examine a class of steels that are being developed for low density applications and hence, have relatively large concentrations of aluminium and manganese. These two elements compete for what little oxygen is available in the predominantly hydrogen–nitrogen mixture used to protect the steel during heat treatment. It is found that although the general sequence of oxidation does not seem to vary as the aluminium concentration is increased, the stoichiometry of the oxides changes. Manganese rich oxide is always the first to form for all the dew points examined, but is rapidly overtaken by aluminium oxide when the dew point is kept below −30°C. In the latter case, the eventual formation of an alumina film also halts the internal oxidation of aluminium. The results obtained using a variety of high resolution microscopy and analytical techniques have been analysed by calculating phase diagrams and by creating a finite difference model to reveal aspects of the internal oxidation problem.