## 2 Materials and Methods  
This study analyzes large rings (132 mm in outer diameter, 32 mm in thickness, and 64 mm in height, see Figure 1a) manufactured with the EN 100 CrMo7 bearings steel provided by Ovako (Ovako AB, Stockholm, Sweden). The simplified geometry of the rings fully represents real components with high-wall thickness, even if they exceed the upper limit for hardenability, as indicated in the supplier's technical information datasheet [27]. The chemical composition of the steel measured by a G.N.R. S7 Metal Lab Plus (G.N.R., Agrate Conturbia, Italy) quantimeter is reported in Table 1. The chemical composition was measured at the ring's core, at half thickness, and near the surface; n. 3 samples for each location, on a longitudinal cross-section, were analyzed. As can be seen, the composition meets the standard for EN 100 CrMo7 steel according to UNI EN ISO 683-17:2023 [28]. For the microstructural analysis, standard metallographic procedures were followed, and the final chemical etching was performed using a 4% Nital solution (4% HNO\({}_{3}\) in ethanol). The analyses were conducted on a radial section of each ring (see Figure 1b). The investigated sections were the upper surface of the ring (i.e., the last that faced the quenching medium and, in turn, the worst condition considering its cooling rate) and the central region (i.e., half the height of the ring, at the center). Micrographs were captured either by a Leica DMi8 A (Leica, Wetzlar, Germany) light optical microscope (LOM) or by a Zeiss EVO MA15 (Carl Zeiss, Jena, Germany) scanning electron microscope (SEM) in secondary (SEI-SEM) and backscattered electron imaging (BSE-SEM) modes. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline  & & & & & & & & & & & & \\ \cline{2-13}  & **C** & **Si** & **Mn** & **P** & **S** & **Cr** & **Mo** & **Ni** & **Al** & **Cu** & **Fe** \\ \hline Average & 0.959 & 0.300 & 0.295 & 0.011 & 0.010 & 1.679 & 0.181 & 0.192 & 0.024 & 0.178 & Bal. \\ St. Dev. & 0.018 & 0.017 & 0.010 & 0.002 & 0.003 & 0.023 & 0.008 & 0.006 & 0.006 & 0.008 & \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition (wt.%)—Fe Balance  
Figure 1: Digital images of the EN 100 CrMo7 rings: (**a**) top view; (**b**) radial section. Brinell hardness measurements were carried out under a 187.5 kgf load and using a 2.5 mm diameter spherical indenter (referred to as HBW 2.5/187.5) with the AT130D Ernst (Cisam-Ernst, Induno Olona, Italy) hardness tester. Hardness tests were carried out according to the UNI EN ISO 6506-1:2014 standard [29]. The mean Brinell hardness value was calculated as an average of five random indentations obtained from various locations along the thickness of the ring. The Rockwell C hardness (HRC) was measured by the AT130D Ernst hardness tester in agreement with the UNI EN ISO 6508-1:2016 standard [30]. HRC tests were carried out on a radial section and in the center of the ring's thickness, analyzing the hardness evolution from the upper to the lower surface of the ring (i.e., along the ring's height), with 19 indentations at 3 mm intervals. Different heat treatment routes were settled to study the effect of heat treatment parameters on the microstructural characteristics of the steel and hardness according to the dimension of the component and comprising both austempering and martempering. In all cases, austenitizing was performed in an Albaplant FP 120.200 C-A (Albaplant SrI, Pessano con Bornago, Italy) pit furnace. Based on the supplier datasheet, the austenitization temperatures were chosen according to the indicated temperature range. The dwelling time and the cooling parameters were defined to obtain the thermal equilibrium of the large-size investigated rings and follow practical experience from the industrial perspective. According to the supplier datasheet, the reported critical temperatures were Ms 186 \({}^{\circ}\)C, Ac1 730 \({}^{\circ}\)C, and Ac3 750 \({}^{\circ}\)C.  
Trial n.1 comprised a comparison between the martempering and austempering process routes.