## 2 Materials and Methods  
### The Material Used in This Study and the Dataset Generation Process  
The SCM 440 supplied by the China Steel company was used as the experimental material studied in this paper. SCM 440 is a Japanese grade of structural steel for machinery manufacturing. According to JIS (Japanese industrial standard), the alphabet SCM indicates that the major alloying elements of the steel are Cr-Mo. The following numeric codes represent the main alloying element content and average carbon content. The material is widely used in a variety of devices, such as bolts, shafts, spindle barrels, gears, motorcycle components, etc. Metallographic microscopy is mainly used to identify and analyze the internal structure and organization of metals such as SCM 440. In study, the optical image samples were etched with 3% Nital solution. Vickers micro-hardness (500 g, 15 s hold) of 15 individual measurements in each optical image were performed on the metallographic specimens using an Akashi MVK-H100 (Osaka, Japan) machine. Figure 2 shows example results of triangular pyramidal indentations obtained by the Vickers hardness test [8]. Figure 3 shows the data generation process applied in this paper, in which the Vickers hardness tester (MVK-H100) is used for the Vickers hardness testing manually to generate the training and test datasets consisting of figures as shown in Figure 2, which were used by the computer-assisted method for calculating the HVs. In general, the harder the specimen is, the smaller the size of the indentation is and vice versa. The entire process is elaborated as follows. The sample was generated by tensile testing for the material under test. After etching with a 3% Nital solution, the Vickers hardness tester was used to produce the specimen images of the sample shown in Figure 2 by pressing the indenter into the surface of the sample with a certain load and holding the load for a certain period to form an indentation, as illustrated in Figure 1a, where L1 and L2 are the diagonal lengths of the indentation. Figure 1b shows the side view of the diamond indenter, which has a relative angle of 136\({}^{\circ}\). The image of the indentation displayed on the screen of Figure 3c was photographed by an optical microscope. Finally, the micro-hardness (the HV) was obtained by measuring the diagonal lengths of the square indentations (L1 and L2) on the surface of the captured  
Figure 3: The data generation process applied by this study. (**a**) The sample used for tensile testing: (**b**) the Vickers hardness tester used in this study; (**c**) the usage of the Vickers hardness tester software. Figure 2: Example results obtained by the Vickers hardness assessment method for SCM 440.  
image, as shown in Figures 1a and 3c, calculating the average value and applying it to Equation (1):  
\[\mathrm{HV}=\frac{F}{S}=\frac{2F*\sin\left(\frac{136^{\circ}}{2}\right)}{g*d^{2} }\cong 0.1891*\frac{F}{d^{2}}\left[\mathrm{N/mm^{2}}\right], \tag{1}\]  
where \(F\) represents the load (in Newton force) and \(S\) represents the calculated area. The indenter angle is \(136^{\circ}\), \(d\) is the average diagonal length of the indentation, and \(g\) is the acceleration of gravity. Because the samples of the SCM 440 material are limited and the aforementioned manual generation process of the indentation image for the specimen and its corresponding HV is extremely tedious, only 105 indentations were obtained in this study. Figure 4 illustrates an example of the original specimen; the microstructure that consists of ferrite, banite, pearlite, and carbide was shown in different contrast in Figure 4a and the same specimen after the Vickers hardness test in Figure 4b. Dark and grey contrast areas present pearlite and banite microstructure, while lighter regions include a mixture of microstructures, mainly ferrite and spherical cementie, as shown in Figure 4a. Each microstructure has different hardness; ferrite is soft and banite is hard.