## 3 Results
### Trial n.3--Effect of Austenitizing Temperature and Austempering Holding Times  
Lastly, to assess the possibility of reducing the austenitizing temperature up to the minimum expected within the range of the datasheet for the EN 100 CrMo7 steel [27], 850 \({}^{\circ}\)C and 880 \({}^{\circ}\)C austenitizing temperatures were compared. Furthermore, the contribution of three different holding times, i.e., 60 min, 180 min, and 300 min, was investigated. The experimental outcomes are reported in Figure 10. For the lower austenitizing temperature, it was observed that, regardless of the holding times, the center of the rings exhibited a drop in HRC values; the results reported in Figure 10a show that the drop is up to 16% with respect to the surface for the shorter holding times. Note that, for the 300 min holding time, this decrease is less than 10%. After austenitizing at 880 \({}^{\circ}\)C, no remarkable changes in HRC values were detected along the ring's height, regardless of the investigated holding times. In comparing the austenitizing temperatures (Figure 10a,b), it is worth noting that the hardness values at the surfaces are similar, about 61 HRC for the same austempering temperatures and irrespective of the holding times. In addition to the HRC evolution, the average grain size G was estimated, as depicted in the representative LOM micrographs of Figure 11. Specifically, for the ring quenched from 850 \({}^{\circ}\)C, the grain size was equal to 10, while for the ring quenched from 880 \({}^{\circ}\)C, the grain size was equal to 8. Figure 8: Hardness evolution (HRC) along the ring’s height (d) for the heat-treated EN 100 CrMo7 rings after tempering—Trial n.2. The representative SEM micrographs of the EN 100 CrMo7 rings austenitized at 850 \({}^{\circ}\)C and 880 \({}^{\circ}\)C are displayed in Figure 12. Concerning the austempering routes that followed the austenitizing performed at 850 \({}^{\circ}\)C, a comparison between the microstructure at the surface (Figure 12a,c) and at the center (Figure 12b,d) for 60 min and 300 min of holding time, respectively, can be carried out. The lower bainite matrix is predominant at the surface, whereas, at the center, an increased amount of upper bainite is detectable (see red arrows in Figure 12b,d). Such an amount is appreciable for the 60 min holding time (Figure 12b). Figure 9: SEI-SEM micrographs of the heat-treated EN 100 CrMo7 ring—Trial n.2: (**a,b**) austenitizing at 880 \({}^{\circ}\)C for 150 min, matterpering at 180 \({}^{\circ}\)C for 30 min, cooling to room temperature in calm air and tempering at 230 \({}^{\circ}\)C for 120 min, and cooling to room temperature in calm air for the upper surface and central region, respectively; (**c,d**) austenitizing at 880 \({}^{\circ}\)C for 150 min, austempering at 240 \({}^{\circ}\)C for 300 min and cooling to room temperature in calm air, and tempering at 230 \({}^{\circ}\)C for 120 min and cooling to room temperature in calm air for the upper surface and central region, respectively; (**e,f**) austenitizing at 880 \({}^{\circ}\)C for 150 min, austempering at 220 \({}^{\circ}\)C for 300 min and cooling to room temperature in calm air, ad tempering at 230 \({}^{\circ}\)C for 120 min and cooling to room temperature in calm air for the upper surface and central region, respectively. By contrast, on increasing the austenitizing temperature to 880 \({}^{\circ}\)C, the 60 min holding time (Figure 12e) still revealed regions of upper bainite (see red arrows in Figure 12f) but these were finer than the ones observed for the 850 \({}^{\circ}\)C austenitizing temperature. Finally, the amount of residual austenite (RA) was measured using the diffractometric method. This aims to provide insight into RA presence after quenching and after quenching and tempering for the different process routes, having practical effects related to the dimensional stability of the investigated large-size rings. Table 2 summarizes the RA contents (vol.%) for all the investigated heat-treatment routes.