## 3 Results and Discussion
### Assessment of Cutting Temperature  
The cutting temperature in hard turning is a significant factor that influences both the machining performance and the longevity of the cutting tool. The regulation of cutting temperature is of utmost importance as an excessive amount of heat can result in an expedited deterioration of the cutting tool, thermal harm to the workpiece, and a decrease in dimensional precision. By effectively controlling the cutting temperature, the hard turning process can yield favorable results, including an enhanced surface finish, a prolonged tool lifespan, minimized tool deterioration, and an increased productivity. Several strategies are utilized in order to control the cutting temperature and optimize the hard turning process. These strategies encompass the selection of cutting parameters, tool geometry, cutting fluid application, and cooling techniques. The utilization of optimal cutting temperatures in hard turning operations serves to guarantee the achievement of appropriate chip formation, the minimization of tool wear, and the enhancement of surface integrity in the machined components. These outcomes subsequently contribute to improved productivity, reduced costs, and enhanced product quality. In this recent research, to reduce the unexpected temperature rise in the hard turning of an AISI 52100 steel rod, ZnO nano-cutting lubricant through a dual jet was used. From the experimental results (Table 1), it can be observed that the cutting zone temperature varied from 43.8 \({}^{\circ}\)C to 269.8 \({}^{\circ}\)C throughout the experiment. The maximum temperature generated during the hard turning of AISI 52100 steel for each experiment is shown in Figure 14. The highest cutting temperature was generated at run no. 7 at a higher cutting speed level of 210 m/min, with the lowest MQL flow rate of 20 mL/hr, a higher feed (0.15 mm/rev), and a moderate depth of cut (0.2 mm). Cutting temperature was boosted accordingly with an increase in cutting speed, due to the rise in friction between the workpiece and cutting tool during the turning process. At higher cutting speed levels (210 and 280 m/min), the formation of the chip is faster; and thus, the chip was accumulated near the cutting zone and enabled a higher temperature range. At the lowest cutting speed of 70 m/min, the maximum temperature was recorded as 86.9 \({}^{\circ}\)C, which was very low for hard turning. Moreover, at highest cutting speed (280 m/min), the maximum temperature was recorded as 269.8 \({}^{\circ}\)C (Figure 14), which was more than 3 times higher than the maximum temperature obtained at 70 m/min. At the highest cutting speed (280 m/min) with an improving flow rate (20 to 50 mL/hr), the temperature reduced due to the increasing amount of nano-cutting lubricant supplied in hard turning. A higher amount of lubricant helped toward a rapid reduction in cutting zone temperature. \begin{table}
\begin{tabular}{l l l l l l l l} \hline
**Source** & **DF** & **Seq-SS** & **Adj-MS** & **F** & **P** & **\% Impact** & **Significant** \\ \hline ap & 3 & 0.023368 & 0.007789 & 9.95 & 0.046 & 3.78 & Yes \\ s & 3 & 0.037242 & 0.012414 & 15.86 & 0.024 & 6.02 & Yes \\ v & 3 & 0.469742 & 0.156581 & 200.04 & 0.001 & 75.94 & Yes \\ q & 3 & 0.085909 & 0.028636 & 36.59 & 0.007 & 13.89 & Yes \\ Error & 3 & 0.002348 & 0.000783 & & & & \\ Total & 15 & 0.618609 & & & & & \\ \hline \end{tabular} Summary: S = 0.0279773; R\({}^{2}\) = 99.62\%; R\({}^{2}\) (adjacent)= 98.10\%.  
\end{table} 
Table 4: ANOVA findings for P.  
From the main effect plot (Figure 15), it can be observed that the cutting temperature was boosting with rising cutting speed. The variations in temperature by changing the feed and depth of cut were uneven and varied near about the mean-value straight line, thus signifying a lower impact of both terms toward the cutting temperature. Furthermore, MQL flow rate has a significant effect on mitigating cutting temperature gradually when the flow rate is enhanced to 50 mL/hr.