**5. Conclusions**

This study investigates the microstructure and mechanical properties of taps made of M35 high speed steel and vacuum heat treated at increasing pressures of quenching gas, in order to improve their fracture toughness and durability. Based on the results, the following conclusions can be drawn:


**Author Contributions:** In the frame of the present investigation, C.S. conceived the experiments and wrote the manuscript. A.F. contributed to data analyses and to the manuscript. R.S. and E.F. supported the experimental investigations. M.M. and G.L.G. supervised the experiments and aided the results discussion. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Department of Engineering, University of Ferrara (Ferrara, Italy), grant number 2238/2010.

**Acknowledgments:** The authors would like to dedicate this study to the memory of Vincenzo Gabrielli, whose invaluable contribution to the experimental activity and whose guidance have been essential to achieve the present results. The authors also thank Milvia Chicca for help in revising the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
