Use of EBSD to characterise high temperature oxides formed on low alloy and stainless steels

Abstract

Exposure of steel to high temperatures in air leads to the formation of an oxide scale, the composition and structure of which depends sensitively on the oxidation conditions and the alloying elements contained within the steel. In this paper, the oxide scale structures formed on low alloy and stainless steels are characterised using electron backscatter diffraction (EBSD). In low alloy steels, this crystallographic information obtained using EBSD enables both the phases within the scale (i.e. haematite, magnetite and wüstite) and orientation relationships between them to be established. This showed that both strong preferred growth within the phase layers and orientational relationships between phase layers, can occur depending on the composition and oxidation conditions. For the scales on stainless steels, the technique enabled the two crystallographic structures, corundum and spinel to be isolated. These structures can be easily differentiated using the EBSD data alone, but the individual phases within them can only be distinguished using the chemical data, collected simultaneously with the EBSD data, because of their crystallographic similarity. This technique revealed two discrete phases for each structure within the oxide scales. For the spinel structure, this consisted of a predominantly chromium and iron containing layer beside the substrate below a coarse grained phase composed of nickel and iron. Meanwhile, an iron rich (haematite) layer at the upper scale surface and a thin chromium rich phase that exists within the fine grained lower scale both possessed the corundum structure.