High speed steel

High-speed steel (HSS or HS) is a subset of tool steels, commonly used in tool bits and cutting tools.

It is often used in power-saw blades and drill bits. It is superior to the older high-carbon steel tools used extensively through the 1940s in that it can withstand higher temperatures without losing its temper (hardness). This property allows HSS to cut faster than high carbon steel, hence the name high-speed steel. At room temperature, in their generally recommended heat treatment, HSS grades generally display high hardness (above Rockwell hardness 60) and abrasion resistance (generally linked to tungsten and vanadium content often used in HSS) compared with common carbon and tool steels.

Although development of modern high speed steel began in the second half of the 19th century, there is documented evidence of steels produced earlier with similar content. These include hardened steels in China in 13th century BC, wootz steel manufactured in India around 350 BC and production of Damascus and Japanese layered steel blades in years 540 AD and 900 AD. High-speed properties of those steels would be mostly coincidental (as no machining technology that involved quantification of speeds and feeds existed at the time) and would be the result of local iron ores containing natural traces of tungsten or other favorable alloying components.

In 1868 the English metallurgist Robert Forester Mushet developed Mushet steel, considered to be the forerunner of modern high speed steels. It consisted of 2% carbon (C), 2.5% manganese (Mn), and 7% tungsten (W). The major advantage of this steel was that it hardened when air cooled from a temperature at which most steels had to be quenched for hardening. Over the next 30 years the most significant change was the replacement of manganese (Mn) with chromium (Cr).

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