Tool steels

(ii)Tool steels:

          The requirements in a tool steel are that it should be capable of becoming very hard and further, that it should be able to retain its hardness at high temperatures commonly developed during cutting of steel and other materials. This property is called "red hardness". Further tool steel should not be brittle and should have good strength. High speed steel (HSS) is the name given to a most common tool steel. Its name implies that it can cut steel at high cutting speeds. At high cutting speed, the temperature rise is higher but high speed steel tools can retain their hardness up to 600-625. The property of red hardness comes from addition of tungsten. A typical composition of H.S.S is tungsten 18%, chromium 4%, vanadium 1%, carbon 0.75 - 1%, rest iron. Tungsten is a costly metal. It has been found that molybdenum can also impart "red hardness" to steel and actually half per cent of molybdenum can replace one percent of tungsten. Molybdenum is far cheaper than tungsten. H.S.S with tungsten are known as T-series and H.S.S with molybdenum are known as M- series steels. A very useful H.S.S has a composition of tungsten 6%, molybdenum 6%, chromium 4% and vanadium 2%, besides iron and carbon. Another version of H.S.S is called super high speed steel. It is meant for heavy duty tools and has about 10-12% cobalt, 20-22% tungsten, 4% chromium, 2% vanadium, 0.8% carbon, rest iron. These days, tools are made of tungsten carbide and other materials, besides H.S.S.