## 3 Results  
### Flow Stress-Strain Curves  
Flow stress-strain curves provide information on metal deformation behaviour during forging. However, the accuracy of the curves is affected by the interfacial friction between the tool and the workpiece as it modifies the force required during deformation [33]. At the sample and the die end, graphite foil and nickel paste were placed to reduce the effect of friction during deformation. At high strain, however, the friction effect is more significant, causing inhomogeneous deformation and hence contact between the samples and die surface. This is known as bartelling, and the flow stress-strain curves have to be corrected for friction before further analysis [33; 34]. The bartelling coefficient was calculated using the sample geometry size according to [PERSON] et al. [35]. Equation (1) was used to determine the bartelling coefficient, \(b\), with the allowable range of bartelling coefficients as 1 < \(b\)\(\leq\) 1.1 [36]. Friction within this range is negligible and only stress-strain curves of samples with a bartelling coefficient beyond this range were corrected for friction [37]. Figure 4 shows plots of uncorrected and friction-corrected flow stress curves of the two P91 steels at a temperature of 950 \({}^{\circ}\)C and strain rate of 0.01 s\({}^{-1}\), 0.1 s\({}^{-1}\), 1 s\({}^{-1}\), and 10 s\({}^{-1}\). The results show that the measured flow stress values in the stress-strain curves were higher than the friction-corrected values. This variation may be due to the interfacial friction effect experienced at the test sample and anvil interface.  
\[b=\frac{h_{f}d_{f}^{2}}{h_{o}d_{o}^{2}} \tag{1}\]  
where \(h_{o}\) and \(h_{f}\) are the initial and final height of the sample and \(d_{o}\) and \(d_{f}\) are the initial and final diameter of the sample.  
Figure 5 (steel A) and Figure 6 (steel B) show friction-corrected flow stress-strain curves at various deformation conditions. The flow stress-strain curves show that the flow stress depends on the strain rate and deformation temperature. For a given strain rate, flow stresses decrease with an increase in the deformation temperature of the two steels investigated. These metal flow patterns may be due to the increase in the mobility of grain boundaries as deformation temperature increases [38]. At a given temperature, flow stress increases with an increase in the strain rate. The flow stress-strain curves  
Figure 4: Plots of uncorrected and corrected flow stress–strain curves.  
(Figures 5 and 6) show that at the beginning of the deformation process, the flow stress increases rapidly up to a strain of \(\sim\)0.2. This characteristic flow behaviour occurs due to the rapid generation of dislocation density resulting in a work-hardening deformation mechanism [39]. At higher strain (>0.2), most flow stress-strain curves for the two steels reached a steady-state condition. This characteristic behaviour of flow curves indicates a balance between work-hardening and -softening mechanisms, especially dynamic recovery (DRV) for these steels [40]. The flow stress-strain curves for the two steels studied did not exhibit a clear peak and subsequent steady-state flow stress caused by DRX [36; 41]. Therefore, for most deformation conditions, the flow curves had increasing flow stress until the start of the saturation stress (\(\sigma_{\text{sat}}\)) region. The results show that DRV involved the softening mechanism. Similar flow stress-strain curve behaviour results are available in the literature for creep-resistant steels [42; 43].  
Figure 5: Flow stress–strain curve of steel A.