**3.1. Microstructure characterisation**

magnetic properties of the oxide because hematite is an n-type (n = negative carrier) semi-

In a word, these iron oxides have the different crystal symmetries representing by a different space group: ferrite, *Im-3m*; wustite, *Fm-3m*; magnetite, *Fd-3m*; hematite, *R-3c*, and different lattice parameters: ferrite, wustite and magnetite in cubic symmetry with lattice parameters (*a*) of 0.287, 0.431 and 0.840 nm, respectively, and hematite in a trigonal structure with

Due to the presence of alloying elements and impurities in Fe-C steel alloys, the oxidation rates, phase development and morphologies of oxide scale are dramatically different from pure iron under various processing parameters. The reader can be referred to some published books [1], with regard to these four factors, alloying elements, oxidation kinetics, resulting oxides and their microstructure. More attention will be given here to the relationships

The oxide scale formed on low carbon steels generally consists of a typical three-layered

This three-layered microstructure remains until a eutectoid point of the Fe-O phase equilibrium diagram is reached if oxygen is available at a temperature of 570°C. When the temperature drops below 570°C, the wustite phase becomes unstable and will decompose into eutectoid products of magnetite and ferrite. During the cooling from high temperature, the morphology and composition within oxide scale will be changed significantly. This variation

Analogous to the alloying strength in steel substrate, the alloying additions modified the oxidation quite dramatically. Normally, the amount of silicon and chromium aims to form a protective oxide scale, whereas small additions of nickel, copper, niobium, molybdenum and vanadium led to a greatly increased adherence of oxide scale. For instance, silicon generally enriches and forms an anchor-like morphology at the oxide scale/steel interface or grain boundaries of the oxides. Manganese is normally used as a solvent, while the transport of carbon is via defects such as pores rather than lattice or grain boundary diffusion. Because manganese has a stronger affinity for oxygen than iron does, manganese is normally spread sparingly over the entire oxide layer in the cross-sectional direction [3]. In addition to alloying elements, the atmosphere of gas as it interacted with oxides, especially water vapour, has made our understanding of the overall situation elusive [5], while the different behaviour of

mainly depends on heat treatments, atmosphere of the gas and chemistry of steels.

steel alloys in air-moisture mixtures has further complicated the set of observations.

The thickness or weight change in oxide scale with time is generally used to assess oxidation rates of the metallic alloys. In a linear or parabolic growth rate, the mixed para-linear kinetics is widely accepted to deal with the short-time growth of oxide scale during high-temperature metal processing. With the oxidation of steels below 727°C (the eutectoid point of the Fe-C system [4]), the oxidation kinetics is similar to pure iron due to without decarburisation at

O3

), an intermediate layer of magnetite

*x* = 0.84–0.95) adjacent to the steel substrate [4].

conducting oxide in which the diffusion of anions is dominant.

between theories associated with grain boundaries of oxide scale.

microstructure with a thin outer layer of hematite (Fe<sup>2</sup>

) and an inner layer of wustite (Fe1−xO, 1−

*a* = 0.504 nm and *b* = 1.377 nm [3].

**2.2. Oxidation of steel alloys**

62 Study of Grain Boundary Character

(Fe<sup>3</sup> O4

> **Figure 2** shows the oxidised samples in the cross-sectional or thickness direction parallel to the direction of oxide growth and from the top surface. Electron backscattered diffraction (EBSD) phase mapping shown in **Figure 2a** and **b** indicates a columnar-shape microstructure between the outer granular grains and the globular inner layer [7]. The oxide scale is composed of a thin outer layer of hematite and the inner duplex magnetite layers. The outer layer is columnar in structure, whereas the inner layer is much finer grained and the grains are equiaxed. The grains of magnetite have granular shape with the grain size around 3 µm in the outer layer of oxide scale. In addition, hematite near the surface gradually penetrates into the cracks within the oxide scale.

> Grain shape and size highly influence the oxidation of pure metals and their alloys at high temperature. Oxidised scale shows the rough microstructure with valleys around grain boundary (**Figure 2c**, **d** [8]). This indicates that the transport of cations along grain boundaries is the dominant mechanism for outer scale growth. The diffusion of metal ions can result in vacancies and cavity to facilitate the formation of local pores [8]. Therefore, the grain-refined metal substrate can enhance the grain boundary diffusion at high temperature.
