## 4 Discussion
In this respect, the Mo-modified EN 100 CrMo7 bearing steel, with its greater hardenability than the typical EN 100 Cr6 steel, is a widely adopted material for large-size components, e.g., with wall thicknesses above 30 mm [34]. Nevertheless, regardless of the austempering holding time, the 850 \({}^{\circ}\)C austenitizing routes did not yield uniform hardness between the surface and center of the rings (Figure 10a). Even though, according to the supplier datasheet [27], the austenitizing temperature can be lowered down to 850 \({}^{\circ}\)C, such a temperature has been proven to be detrimental for the hardness in the center of the ring. The detected HRC values at the surface are above 60 HRC for all the investigated holding times (Figure 10a), as was the case for the 880 \({}^{\circ}\)C austenitizing routes (Figure 10b). Such an outcome should be analyzed from a twofold point of view: the increased grain size for the 880 \({}^{\circ}\)C austenitizing routes (Figure 11) promoted the martensitic transformation at the surface (Figure 12a,c) but, at the center, the microstructures were inhomogeneous, with large amounts of bath-like ferrite with cementie on the lath boundaries, likely to be referred to as lower bainite [35]. Thus, in agreement with [3], the properties of the core are adversely affected by the abnormal growth of austenite grains during heat treatment. As the austenitizing temperature is lowered, the pearlitac nose of the cooling curves is shifted to the left [12], i.e., the hardenability is lowered, thus affecting the microstructural transformation. Indeed, dealing with large components of uniform shape that must be rapidly cooled, this could result in a heterogeneous temperature distribution [3], as in the present case. In the center, it is highly likely that, before reaching the austempering temperature, the cooling curve had come across the upper pearlific/bainitic transformation regions. Conversely, at the surface, an entirely upper bainitic microstructure has been observed (Figure 12a,c). In addition, the most important contribution to the bainite volume fraction and hardness is related to austempering temperature rather than austempering time, as reported in the literature for bearing steels [15; 36]. This issue has been stressed in a previous study by the authors [37] and, in the present investigation, it has been further assessed. SEM observations highlighted the more significant differences between the surface and center of the ring (Figure 11a vs. Figure 11b and Figure 11c vs. Figure 11d) rather than between 60 min (Figure 11a) and 300 min (Figure 11c) holding times. This evidence is supported by the HRC values that are about 62 HRC for the surface and about 55 HRC for the center for the 300 min holding time, and about 62 HRC for the surface and about 51 HRC for the center for the 60 min holding time. Lastly, even if it is beyond the scope of the present investigation, the contribution of primary and secondary carbides against the holding times, and their effect on the bainitic transformation, cannot be neglected [38]. Furthermore, when comparing austempering routes at austenitizing temperatures of 850 \({}^{\circ}\)C and 880 \({}^{\circ}\)C, it was observed that an increase in the austenitizing temperature led to a corresponding increase in the RA. It is established that the amount of RA in hypereutectoid steels increases with austenitizing temperature [39] and is affected by the dissolution rate of the spheroidized carbides, which depends, in turn, on their size and distribution prior to the heating phase [4; 34]. With the increase in the austenitizing temperature, more carbides are dissolved, and thus more carbon and alloying elements enter the solid solution of the austenite; hence, Ms lowers and more austenite is retained after quenching [21]. Consistent with these findings, the amount of RA achieved through austempering with an austenitizing temperature of 850 \({}^{\circ}\)C was lower than that attained through austenitizing at 880 \({}^{\circ}\)C (1.9-2.8 vol.% compared to 3.0-4.9 vol.%).