High-speed steel is a term given to a tool steel material that can be used to machining at high speeds.
The origin of high-speed steels dates back to 1861, when an Englishman developed the first high-speed steel. The high-speed steel developed was carbon manganese tungsten type (C-Mn-W) with a carbon content of 1.5%~2%. Then in 1898, in the USA, engineers called Taylor and White succeeded in developing a high-speed steel that maintained it’s hardness under high temperature conditions. This was achieved by heating a high carbon tungsten chromium (C-W-Cr) steel with a carbon content of 1% near to its’ melting point, this was followed by quenching then a tempering process.
This step forward in heat treatment of steels was revolutionary and for about 80 years thereafter, high-speed steels played a major role as cutting tool materials. Presently, approximately 30% of cutting tools are made of high-speed steels and new types of high-speed steels with superior cutting performance are still being developed.
Components & use
There are tungsten (W)-based and molybdenum (Mo)-based high-speed steels. While W-based high-speed steels offer high temperature hardness properties, the Mo-based high-speed steels offer high toughness and are easily heat-treatable due to a low quenching temperature. Therefore, Mo-based high-speed steels are more generally used.
The table below shows the effects of the alloying elements in high-speed steels.
The ffigure below shows a graph that depicts the heat-treatment process of high-speed steel. The tempering process is carried out in 2~3 stages. The most commonly used method is to heat the workpiece in a vacuum condition and then cooled using nitrogen gas.
The figure to the right shows a simplified graph that shows that when high-speed steel is tempered at temperatures of around 550°C the material itself becomes harder than when it was initial quenched.