# Research on Roughness and Microhardness of C45 Material Using High-Speed Machining
## 2 Materials and Methods
A Mitutoyo SJ-400 digital surface roughness tester (Kawasaki, Japan) (Figure 6) was used for roughness analysis, a Neophot 21 light electron microscope (Oberkochen, Germany) (Figure 7) to evaluate the microstructure, and a CV-400 DAT hardness tester (Bjerringbro, Denmark) (Figure 8) to evaluate the Vickers microhardness in accordance with STN EN ISO 6507-1, STN EN ISO 6507-2, and STN EN ISO 6507-3. For the evaluation of microhardness, the samples were prepared in a special way in accordance with existing standards, cold-covered in acrylic resin, and wet-sanded with SiC-based sanding papers with 500, 800, and 1200 grit. Prior to the observation, the samples were polished with a disc with artificial diamond fiber and a polishing suspension Al\({}_{2}\)O\({}_{3}\) 5 \(\upmu\)m followed by etching in 2% HNO\({}_{3}\) solution. Figure 4: Illustration of conducted experiment. Figure 5: Illustration of the testing sample after the experiment—\(v_{c}\) = 1400 m/mm, \(a_{p}\) = 1.5 (**left**); \(v_{c}\) = 2000 m/mm, \(a_{p}\) = 1.5 (**middle**); \(v_{c}\) = 700 m/mm, \(a_{p}\) = 1.5 (**right**). In order to determine the surface quality, a graph shown in Figure 10 was also created. In the given graph, a mathematical prediction of determining the quality of the machined surface was also created with the following notation:  
\[f(x)=p_{1}\cdot x^{2}+p_{2}\cdot x+p_{3}, \tag{7}\]  
with 95% reliability and individual coefficients:  
\(p_{1}=-0.004176\)  
\(p_{2}=0.1873\)  
\(p_{3}=-0.4834\)  
Microhardness was observed on the prepared samples at three basic points, always before the machining process and subsequently after the end of the machining process. A comprehensive overview of the achieved results is presented in Table 3.  
Figure 10: Graph of dependence of roughness Ra on cutting speed. Figure 9: Graph of dependence of roughness Ra on cutting speed and depth of cut. For individual alternatives of cutting parameters, the basic machining parameters in the primary zone for high-speed machining processes were calculated, the values of which are given in Table 4.  
Chips were assessed from a metallographic cut and under an electron microscope in each alternative of the research. During the first alternative, a smooth material removal process was observed in the cutting zone, with no significant changes observed in either the surface roughness or the microhardness (Table 5). During the first analysis, only slight wear of the tool was recorded, assessing the condition before the start of the machining process and at the end of machining. \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{1}{c}{**Parameters**} & \multicolumn{2}{c}{**Values—Experiment Alternatives**} \\ \cline{2-3} \multicolumn{1}{c}{} & \(\
u_{c}\) = 700 [m/min] & \(\
u_{c}\) = 1400 [m/min] & \(\
u_{c}\) = 2000 [m/min] \\ \hline Chip compression & & 1.16 \\ plastic deformation limit angle & & 47\({}^{\circ}\) \\ chip removal rate & 681.3 [m/min] & 1362.6 [m/min] & 1946.6 [m/min] \\ chip deformation & & 1.32\({}^{\circ}\) \\ shear speed & 924 [m/min] & 1848 [m/min] & 2640 [m/min] \\ deformation rate in the shear plane & 9240 [s\({}^{-1}\)] & 18,480 [s\({}^{-1}\)] & 26,400 [s\({}^{-1}\)] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Machining values for high-speed machining.