## 4 Discussion  
The effects of the different heat-treatment procedures on the hardness and microstructural characteristics of the EN 100 CrMo7 bearing steel high-thickness rings were analyzed through a dedicated industrial-scale investigation. At first, the effects of martempering and austempering were compared (Trial n.1). As expected, the microstructure of the steel after martempering carried out at 180 \({}^{\circ}\)C is composed of a mixture of untempered tetragonal martensite, carbides, and retained austenite (see Figure 7a). These microstructural features led to the highest HRC values (Figure 6). The HRC evolution along the ring's height is almost constant and the role of the detected incipient formation of upper bainite (see arrows in Figure 7b), as suggested in [34], appeared to be negligible in the measured bulk hardness evolution. By contrast, austempering at 240 \({}^{\circ}\)C and 220 \({}^{\circ}\)C led to a reduced HRC along the ring's height (Figure 6) and the corresponding microstructures (Figure 7c-f) revealed appreciable microstructural variations between the surface and the center of the rings where the incipient formation of upper bainite was detected too. By the comparison between the untempered (Trial n.1) and tempered (Trial n.2) conditions in the martempered rings, the tempering treatment promoted the microstructural transformation and the consequent softening of the martensite (see Figures 7a,b and 9a,b), as confirmed by the decrease in hardness (see Figures 6 and 8), from about 67 HRC down to about 62 HRC. According to [21], after quenching, the microstructure of high-carbon bearing steels is mainly martensitic and composed of carbon oversaturated martensite with a significant amount of retained austenite--because the martensite start (M\({}_{\text{S}}\)) and martensite finish (M\({}_{\text{F}}\)) temperatures are low enough (M\({}_{\text{F}}\) is below room temperature) to retain austenite--and alloyed carbides that were not fully dissolved during austenitizing (named undissolved carbides). For the 100 Cr6 bearing steel, it was reported that quenching results in a predominantly martensite microstructure with approximately 6 vol.% of retained austenite and 3-4 vol.% of cementie particles that remain undissolved after the austenitizing process [3]. In the present case, the martempering produced up to 13.1 vol.% of RA (see Table 2), which is considerably higher than the one obtained with a direct quenching. Upon tempering, the RA amount was notably reduced to about 2 vol.% in the center and less than 1.0 vol.% in the surface. Concerning the austempering, the amount of RA was less than that of the martempering both at the center and at the surface (about 3.0-4.9 vol.%), which was considerably reduced--less than 1.0 vol.%--after tempering \begin{table}
\begin{tabular}{c} \textless{}1.0 \\ \textless{}1.0 \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Measured RA contents (vol.%) for all the investigated heat-treatment routes. within all the volume (see Table 2). In addition, for asttempered and tempered rings, the different slight hardness reduction in the center of the ring for both temperatures was put in relation to the observed bainite microstructures (see Figure 9d,f). Regarding the role of austenitizing temperature and austempering holding times (Trial n.3) on HRC evolution (Figure 10) and microstructural outcomes (Figure 12), several issues should be considered. The detected HRC evolutions can be ascribed either to the austenitic grain size (see Figure 11) or the hardenability of the investigated steel. The comprehensive review of bearing steels by [PERSON] [3] explained that, while small bearings are usually through-hardened, since the hardenability is sufficient to attain the martensitic microstructure throughout the section, large bearings can be through-hardened by increasing the hardenability of the steel using higher concentrations of alloying elements.