## 5 Conclusions  
This research seeks a deeper insight into the knowledge and control of heat treatment parameters from an industrial perspective. Based on the experimental evidence, the study draws the following conclusions:  
* Martempering led to the highest hardness values, about 66 HRC, and the evolution of hardness along the ring's height was quite uniform without detectable differences between the center and the surface regions. The obtained microstructure was composed of a mixture of untempered tetragonal martensite, carbides, and RA, whose content was up to 13.1 vol.% in the center before tempering;
* Regardless of the austempering temperature (220 \({}^{\circ}\)C vs. 240 \({}^{\circ}\)C), a slight decrease in hardness was measured at the center of the rings with respect to the martempering heat-treated condition. Nevertheless, a lower amount of RA (in the range 3.0-4.9 vol.%) was detected in comparison with the martempering;
* After tempering, the rings revealed overall HRC values lower than those of the untempered ones; for the martempering, the evolution along the ring's height was almost constant, whereas, for the austempering routes, a hardness reduction up to about 3 HRC points in the center of the ring was detected;
* For the martempering ring, after tempering, the amount of RA was notably reduced to about 2.0 vol.% in the center and less than 1.0 vol.% at the surface. The austempering routes showed a deeper reduction in RA, to less than 1.0 vol.%, both in the center and at the surface;
* Regarding the effect of the austenitizing temperature, the HRC evolutions along the ring's height detected for the 850 \({}^{\circ}\)C austenitizing temperature revealed that such a temperature was detrimental to the hardness in the center of the ring, regardless of the holding times. The measured hardness values were lowered down to 16% with respect to the surface for the shorter holding times (60 min). Hence, for the considered large-size rings, the choice of such a low austenitizing temperature seemed ineffective, as also confirmed by the high amount of upper bainite detected in the center of the ring. Conversely, the 880 \({}^{\circ}\)C austenitizing temperature was proven to be effective as the high and quite constant values of HRC along the ring's height;
* Overall, the experimental findings revealed that the role of the cooling rate (surface vs. center of the ring) was more detrimental to the resulting hardness and microstructure rather than the adopted austempering holding time.  
Conceptualization, E.B. and M.M.; methodology, M.C.; investigation, A.F. and M.C.; data curation, A.F.; writing--original draft preparation, A.F. and M.M.; writing--review and editing, E.B.; visualization, A.F.; supervision, M.M. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The data presented in this study are available in the article.  
**Acknowledgments:** The authors wish to gratefully acknowledge [PERSON] for his valuable support during the experimental activities. Thanks are also due to [PERSON] for his assistance.  
**Conflicts of Interest:** Author [PERSON] and [PERSON] were employed by the company Timar Snc. They declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.