Cutting Tool Materials

The cutting tool materials must possess a number of important properties to avoid excessive wear, fracture failure and high temperatures in cutting, The following characteristics are essencial for cutting materials to withstand the heavy conditions of the cutting process and to produce high quality and economical parts:

• Hardness :At elevated temperatures (so-called hot hardness) so that hardness and strength of the tool edge are maintained in high cutting temperatures. 
• Toughness: Ability of the material to absorb energy without failing. Cutting if often accompanied by impact forces especially if cutting is interrupted, and cutting tool may fail very soon if it is not strong enough.
• Wear resistance: Although there is a strong correlation between hot hardness and wear resistance, later depends on more than just hot hardness. Other important characteristics include surface finish on the tool, chemical inertness of the tool material with respect to the work material, and thermal conductivity of the tool material, which affects the maximum value of the cutting temperature at tool-chip interface.

Common cutting tool material used:

• Carbon steel: Carbon steels having carbon percentage as high as 1.5% are used as tool materials however they are not able to with stand very high temperature and hence are operational at low cutting speed.
• High speed steel (HSS): These are special alloy steel which are obtained by alloying tungsten, Chromium, Vanadium, Cobalt and molybdenum with steel. HSS has high hot hardness, wear resistance and 3 to 4 times higher cutting speed as compare to carbon steel. Most commonly used HSS have following compositions.

1. 18-4-1 HSS i.e. 18% tungsten, 4% chromium, 1% vanadium with a carbon content of 0.6 - 0.7%. If vanadium is 2% it becomes 18-4-2 HSS.
2. Cobalt high speed steel: This is also referred to as super high speed steel. Cobalt is added 2 – 15%. The most common composition is tungsten 20%, 4% chromium, 2% vanadium and 12% cobalt.
3. Molybdenum high speed steel: It contains 6% tungsten, 6% molybdenum, 4% chromium and 2% vanadium.

• Cemented carbide:  These are basically carbon cemented together by a binder. It is a powder metallurgy product and the binder mostly used is cobalt. The basic ingredient is tungsten carbide-82%, titanium carbide-10% and cobalt-8%. These materials possess high hardness and wear resistance and it has cutting speed 6 times higher than high speed steel (HSS).
• Ceramics: It mainly consists of aluminum oxide (Al2O3) and silicon nitride (Si3N4). Ceramic cutting tools are hard with high hot hardness and do not react with the workpiece. They can be used at elevated temperature and cutting speed 4 times that of cemented carbide. These have low heat conductivity.