**3.3. Character distribution of grain boundaries**

Effects of grain orientation and grain boundary characters on the elevated temperature oxidation behaviour demonstrate the role of grain boundaries in enhancing high-temperature oxidation resistance of various polycrystalline steel alloys. Overall surface energy and tribological behaviour can also be enhanced by grain boundary engineering.

### *3.3.1. Grain boundaries*

Grain boundaries can be classified geometrically in terms of the relative misorientation between the neighboured grains. This relative misorientation can be defined by misorientation axis and angle. For instance, 2° ≤ *θ* < 15° misorientations are defined as low-angle grain boundaries (LAGBs), whereas the high-angle grain boundaries (HAGBs) are ≥15°. Certain specific combinations resulting in a coincidence site lattice (CSL), the degree of coincidence is represented by the reciprocal density of common lattice points, denoted as the Σ number. As such, special grain boundaries refer to the low ∑ (∑ ≤ 29) CSL boundaries, even though there is no physical basis for this assumption [14].

In oxidation and corrosion, it is widely believed that HAGBs have undergone hot corrosion and substantial depletion/segregation of alloying elements through the entire cross section. Distribution of grain boundaries in surface layer of oxide scale reveals that the misorientation tends to be large near grain boundaries, particularly at the oxide-substrate interface, where the high fraction of small magnetite grains is accumulated.

#### *3.3.2. Special grain boundaries*

Low-energy CSL boundaries with higher mobility can enhance the resistance of cracking or oxidation [15, 16]. CSL boundaries with low Σ orientation (Σ ≤ 49) display improved physical and chemical properties relative to general or high CSL boundaries (Σ > 49) [17]. Some studies [18] reported that the resistance to intergranular oxidation of Ni-Fe alloy increased upon increasing the fraction of special boundaries. The extent of oxidation of individual Σ boundaries in Ni-Fe alloys is based on morphological observations. It found that Σ3, Σ11 and Σ19 were more resistant to oxidation than other Σ boundaries [19].

In a oxidised microalloyed low carbon steel [12], a high proportion of low-angle and low-ΣCSL boundaries, magnetite for 60°/<111> (Σ3), and hematite for 57.42°/<1-210> (Σ13b) and 84.78°/<0-110> (Σ19c) can be found. Misorientation peaks occur in α-Fe<sup>2</sup> O3 for axes near <0001> in the angle range of 27°–63° and <102> in the angle range of 63°–83°. For α-Fe<sup>2</sup> O3, the relatively high densities correspond to 57.42°/<110> (Σ13b) and 84.78°/<010> (Σ19c).

Furthermore, CSL boundaries distributions in **Figure 4** [17] reveal that Fe<sup>3</sup> O4 carries a high proportion of Σ3, Σ5 and Σ7, whereas α-Fe<sup>2</sup> O3 has a profound fraction of Σ7, Σ13b and Σ19c. It is noted that coherent twins have been excluded from this analysis, which results in a significantly lower fraction of Σ3 boundaries. In any case, it becomes clear that these low CSL grain Grain Boundary in Oxide Scale During High-Temperature Metal Processing http://dx.doi.org/10.5772/66211 67

**Figure 4.** Histogram plots of CSL boundary distribution for (a) Fe<sup>3</sup> O4 and (b) α-Fe<sup>2</sup> O3 , of the samples with different thickness reductions (TRs) and cooling rates (CRs) of oxide scale formed on a microalloyed low carbon steel [17].

boundary characteristics in Fe<sup>3</sup> O4 and α-Fe<sup>2</sup> O3 can be used to enhance crack resistance and further improve tribological properties of oxidised steels during high-temperature processing.
