## Materials and Methods  
### Steel Samples and Image Data Setting  
Low-carbon steels subjected to different heat treatments were considered, and the chemical compositions are indicated in Table 1.  
Metallographic samples were investigated by the authors for this research, according to well-established procedures. In this way, no adversarial attacks are expected regarding model creation [20]. These steels can present different microstructures depending on the type of heat treatment that has been undergone. In the case of annealing, the observable microconstituents are ferrite and pearlite. Martensite is the dominating phase that appears after quenching. Depending on the steel composition, some minor quantities of retained austenite can appear. Tempering after quenching provides different microstructures depending on the temperature; however, for tempering at intermediate temperature, the typical microstructure is composed by tempered martensite that is contoured with a thick coating of precipitated cementite. For annealing, quenching and quenching-tempering, other constituents than those mentioned can appear depending on the steel composition and the way in which the heat treatment is performed. In Figures 1-3, representative microstructures corresponding to some of the experimented steels are shown. All the steel samples were prepared for the optical microanalysis at 400\(\times\), which was performed using an inverse microscope Nikon equipped with a Nikon FX-35 WA camera. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline
**Steel** & **Samples** & \multicolumn{8}{c}{**Chemical Composition Weight \%**} \\ \hline  & & **Mo** & **Si** & **Mn** & **P** & **S** & **Cr** & **Mo** & **Ni** \\ \hline C22E & 0A 1Q 0T & 0.17 & & 0.6 & & & & & \\ C45E & 4A 1Q 2T & 0.45 & & 0.65 & & & & & \\
37 Cr4 & 1A 1Q 2T & 0.37 & & 0.75 & & 1.05 & & & \\
34 CrMo4 & 1A 0Q 0T & 0.35 & & 0.75 & & & 1.05 & & \\
40 NiCrMo6 & 0A 1Q 1T & 0.4 & & 0.7 & & 0.8 & 0.25 & 1.8 \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition (weight %) of low-carbon steel samples according to ISO 683-1:2019 and ISO 683-2:2019 standards. Number of samples for each heat treatment: annealing (A), quenching (Q) and tempering (T). Figure 1: Typical annealing microstructures of a C45E steel (**a**), a C22E steel (**b**) and a 34 CrMo4 steel (**c**); globular annealing of C45E steel (**d**). Figure 2: Typical quenching microstructures of C22E steel (**a**), C45E steel (**b**), 37 Cr4 steel (**c**) and 40 niCrMo6 steel (**d**). This procedure permitted digital images to be taken at a resolution of 2080 \(\times\) 1542 pixels. Optical microscopy under those conditions is a typical microstructure characterization technique. Steel microstructures were selected taking into consideration several criteria. Although the expected microstructures should be similar in all steels, some heterogeneities can be considered. In Figure 1a, we observe perlite (dark areas) and ferrite (white areas) as the typical constituents of an annealing structure of a low-carbon steel. Perlite is a lamellar structure of alternative bands of ferrite and cementie; however, for many islands of perlite, this structure cannot be resolved due to the microscope augmentation, and these islands appear as dark areas. This lack of uniformity of perlite is due to the fact that annealing is performed with a progressive cooling, and the temperatures at which perlite is formed are not uniform in all areas of the piece. In addition, the different plane orientation of the perlite respect to the observation one, led to a different appearance of this constituent. The microstructure of Figure 1b presents a similar aspect to Figure 1a but with a different perlite-ferrite proportion. In Figure 1c, a very fine troostific perlite can be observed. Finally, Figure 1c corresponds to an annealing treatment for which only ferrite and cementie appear, since perlite was transformed into these two phases.