## 3 Results and Discussions
### Effect of Process Parameter on Microstructural Evolution
#### 3.1.2 Ultra-Fast Heating Treatment  
The effect of UFH conditions is summarized in Table 4 in terms of average grain size and Vickers hardness values as a function of different HR and T\({}_{\text{peak}}\) (see the treatment conditions in Table 2). Moreover, in Figures 3 and 4, the microstructure evolution and frequency distribution of the grain size are reported, respectively, for the inductor power of 80% and 90%. All samples exhibit a fully recrystallized microstructure and as expected, and as evidenced by the graph shown in Figure 5, an increase in peak temperature results in a linear increase in the average grain size. For each sample, the microstructure appears to be homogeneous over the entire section of the sample, confirming the uniformity of temperature through the sheet thickness during UFH treatment. Figure 6 compares the microstructures obtained by conventional annealing and UFH (P = 90%, HR = 264 \({}^{\circ}\)C/s, T\({}_{\text{peak}}\) = 1038 \({}^{\circ}\)C and P = 80%, HR = 206 \({}^{\circ}\)C/s, T\({}_{\text{peak}}\) = 1085 \({}^{\circ}\)C) with that of a conventional industrially produced material of grade M310-50A. The relative average grain sizes and Vickers hardness values are shown in Table 5. \begin{table}
\begin{tabular}{c c c c c} \hline
**P\%** & **T\({}_{\text{peak}}\)** (\({}^{\circ}\)C) & **HR (\({}^{\circ}\)C/s)** &
\begin{tabular}{c} **Average Grain** \\ **Size d (\(\upmu\)m)** \\ \end{tabular} & **Std. Dev. (\(\upmu\)m)** & **HV\({}_{\text{1}}\)** \\ \hline
80 & 930 & 213 & 30.8 & 16.5 & 197.8 \(\pm\) 6.3 \\
80 & 1014 & 205 & 46.2 & 22.6 & 200.1 \(\pm\) 0.8 \\
80 & 1085 & 206 & 62.5 & 32.3 & 201.6 \(\pm\) 1.3 \\
80 & 1167 & 229 & 68.2 & 40.4 & 200.3 \(\pm\) 1.2 \\ \hline
90 & 953 & 267 & 40.8 & 22.3 & 196.6 \(\pm\) 2.4 \\
90 & 1000 & 254 & 43.7 & 22.8 & 192.6 \(\pm\) 2.6 \\
90 & 1038 & 264 & 56.2 & 30.6 & 199.2 \(\pm\) 5.5 \\
90 & 1090 & 265 & 58.0 & 32.7 & 203.1 \(\pm\) 5.1 \\ \hline \end{tabular}
\end{table} 
Table 4: Average grain size and Vickers hardness values of UFH samples. \begin{table}
\begin{tabular}{c c c c c} \hline
**Soaking Time** & **Maximum** & **Average Grain** & **Std. Dev (\(\upmu\)m)** & **HV\({}_{\text{1}}\)** \\
**(s)** & **Temperature (\({}^{\circ}\)C)** & **Size d (\(\upmu\)m)** & **Std. Dev (\(\upmu\)m)** & **HV\({}_{\text{1}}\)** \\ \hline
10 & 631 & - & - & 279.9 \(\pm\) 3.5 \\
25 & 965 & 26.0 & 13.0 & 196.4 \(\pm\) 2.9 \\
60 & 1050 & 87.9 & 43.6 & 193.8 \(\pm\) 3.0 \\
90 & 1050 & 120.5 & 70.4 & 192.7 \(\pm\) 2.7 \\ \hline \end{tabular}
\end{table} 
Table 3: Average grain size values of conventional annealed samples through intercept method. Figure 3: Full thickness microstructures and grain size distributions of NGO grade after UFH at P = 80%. Specim number and annealing treatment conditions according to Table 2.  
Figure 4: Full thickness microstructures and grain size distributions of NGO grade after UFH at P= 90%. Specim number and annealing treatment conditions according to Table 2.  
Figure 5: Average values of grain size vs. peak temperature after ultra-fast heating treatment. Figure 6: Comparison of full thickness microstructures: (**a**) conventional annealing after 30 s of soaking time at 1050 \({}^{\circ}\)C; (**b**) reference NGO material M310-50A that has undergone industrial annealing process; (**c**) ultra-fast heating with peak temperature of 1038 \({}^{\circ}\)C (HR = 264 \({}^{\circ}\)C/s); (**d**) ultra-fast heating with peak temperature of 1085 \({}^{\circ}\)C (HR = 206 \({}^{\circ}\)C/s). By comparing the material behavior, it is evident that the mean grain size, as well as hardness values for the conventional annealing material and M310-50A material, are very similar. This demonstrates the ability and reliability of the annealing treatment in a laboratory tube furnace in the reproduction of industrially produced microstructures. Instead, the microstructures obtained after UFH are characterized by an average grain size, which is about one half smaller than that obtained by the conventional CA treated.