Machinability of difficult materials - Application to hardened steels

Machining heat-treated steels in their hardened state is nothing new, and has been around since hard tool materials were first developed. This has only been possible thanks to the development of new tool materials. Such is the case with ceramics. Ceramic products (known as pottery) have been around since the Neolithic period. These were articles for everyday use, such as hand-made vessels. Ceramic production is therefore one of the oldest human technologies. The first patents and publications on aluminum-based ceramic cutting materials (Al ₂ O ₃ ) date back to the early 20th century in Germany. Systematic research began in the 1930s. But it was not until after the Second World War that research and development were intensified. Results enabling the exploitation of ceramics were developed in the USA, the USSR and Germany in particular. The practical application of cutting ceramics was presented for the first time at machine tool exhibitions in Chicago in 1956 and Hanover in 1957. The first cutting metals were mainly very pure ceramics, based on aluminum oxide. This material offers very high mechanical strength. It is the most stable oxide, with excellent chemical stability up to temperatures close to its melting point (2,050°C).

This oxidized ceramic made it possible to machine cast irons and steels at speeds that, for the first time, exceeded the 1,000 m/min limit. The term "high-speed machining" was born. The performance of cutting ceramics was far superior to the capabilities of existing machine tools. Machine tools had to be improved in terms of stability, power, speed and control. In the early 1960s, a major reorientation began, resulting in a reciprocal evolution of tools and machines.

At the beginning of the 20th century, no one would have thought that a ceramic cutting material would play such an important role in machining.

Thanks to their remarkable properties (non-metallic, inorganic, refractory), these ceramic products offer a wide range of possible applications, and their field of application is growing by the day, despite the particular image of brittleness, fracture, and so on.

The same applies to cubic boron nitride (c-NB), which was first synthesized in 1957 at the General Electric Company in the USA. However, it was not used industrially for metal cutting until the mid-1970s.

Industrial awareness of the hard turning technique only really took off in the early 1990s. Some thirty years therefore elapsed between the possibility of using the technique and the birth of the need for it. This technology transfer time may seem long, but in retrospect we could well ask ourselves what point there...