**Influence of Vacuum Heat Treatments on Microstructure and Mechanical Properties of M35 High Speed Steel**

**Chiara So**ff**ritti <sup>1</sup> , Annalisa Fortini 1,\* , Ramona Sola <sup>2</sup> , Elettra Fabbri <sup>1</sup> , Mattia Merlin <sup>1</sup> and Gian Luca Garagnani <sup>1</sup>**


Received: 6 April 2020; Accepted: 11 May 2020; Published: 15 May 2020

**Abstract:** Towards the end of the last century, vacuum heat treatment of high speed steels was increasingly used in the fabrication of precision cutting tools. This study investigates the influence of vacuum heat treatments at different pressures of quenching gas on the microstructure and mechanical properties of taps made of M35 high speed steel. Taps were characterized by optical microscopy, scanning electron microscopy with energy dispersive spectroscopy, X-ray diffraction, apparent grain size and Vickers hardness measurements, and scratch tests. Failure analysis after tapping tests was also performed to determine the main fracture mechanisms. For all taps, the results showed that microstructures and the values of characteristics of secondary carbides, retained austenite, apparent grain size and Vickers hardness were comparable to previously reported ones for vacuum heat treated high speed steels. For taps vacuum heat treated at six bar, the highest plane strain fracture toughness was due to a higher content of finer small secondary carbides. In contrast, the lowest plane strain fracture toughness of taps vacuum heat treated at eight bar may be due to an excessive amount of finer small secondary carbides, which may provide a preferential path for crack propagation. Finally, the predominant fracture mechanism of taps was quasi-cleavage.

**Keywords:** high speed steel; vacuum heat treatment; microstructure; plane strain fracture toughness
