Characteristics of Tool Material
Following are the tool materials in growing up order of their hardness:
High Carbon Steel
Usually these are plain carbon steel containing 0.6 to 1.5 % C. Method of fabrication for High Carbon Steel (HCS) is forging. HCS has hot hardness temperature of around 250°C. Also, its maximum cutting velocity is around 5 m/min.
Hence, generally HCS is used for machining soft materials like aluminium, copper, magnesium etc. HCS is harder and the cheapest tool material.
High Speed Steel
Usually High Speed Steel (HSS) is carbon steel having 1.5 to 2% carbon, 18 % tungsten, 4% chromium, 1% vanadium and rest is iron. Tungsten is added to improve hardness. Chromium is added to enhance hot hardness. Vanadium is added to enhance wear resistance. Technique of fabrication for HSS is forging. Cutting velocity of HSS is 40-60 m/min. It provides higher speed than HCS. Hot hardness temperature of HSS is around 600°C.
Sometimes 18% molybdenum is added rather than tungsten to enhance the wear resistance of tool. Then this HSS is known as molybdenum based HSS. However tungsten based HSS is commonly utilized. HSS contain only disadvantage that during machining of pure carbon work material, diffusion of carbon atoms into iron is much more since iron has stronger affinity to attract carbon.
These are non-ferrous cast alloys. Stellite have 40-45% cobalt, 14-25% tungsten, 30% chromium, and 2% carbon. Method of fabrication for stellite is casting. Its hot hardness temperature is around 800°C. Its hardness is same as that of HSS. It is a substitute for HSS during machining of pure carbon work material.
Carbides are either cemented carbide or tungsten carbide. It has 85-95% tungsten carbide and 5-15% cobalt. Hot hardness temperature of carbide materials is around 1000°C, and its cutting speed is 10 times greater than HSS. Since the cost is high, therefore only tip is made by carbide, and regrinding is also not possible. Fabrication for carbide is performed by employing powder metallurgical techniques.
These are also called cemented oxide. Its major constituent is aluminium oxide and hot hardness temperature is around 1200°C. It also has higher cutting speed of around 300 to 400 m/min. It has better resistance to abrasion than cemented carbide.
It also exhibits low coefficient of friction with most of work materials. These are utilized for higher production rate and for continuous operation only. The disadvantages of ceramic are very low toughness and very brittle in nature. Fabrication for ceramic is done through powder metallurgical techniques.
It is combination of ceramic and metal. It is substitute for ceramics. It has 90% ceramics and 10% nickel. The method employed for fabrication of cermets is also powder metallurgical technique. It's hot hardness temperature is around 1000°C and cutting speed is around 250 m/min. Cermets gives better toughness than ceramic. It gives very good wear resistance.
Diamond possesses all the desirable characteristics but has very high cost. Diamond is made by graphitization technique. It's hot hardness temperature is around 2000°C. Its hardness is higher than any other material. It is chemically inert and contains high thermal conductivity. Its cutting speed is around 500 m/min. Diamond is widely used for machining of non-ferrous alloys as aluminium, copper, magnesium, brass etc. Diamond is not utilized for machining of ferrous material because diamond is basically pure carbon and diffusion of carbon into iron takes place due to affinity of iron atoms toward carbon.
Cubic Boron Nitride
Cubic Boron Nitride contains atoms of boron and nitrogen. It is hardest material next to diamond. It is substitute for diamond during machining of ferrous alloys. It has high hardness and thermal conductivity.
It is combination of titanium, columbium and tungsten. It has 30% titanium, 50% columbium and 20% tungsten. Method of fabrication is rolling. Its hardness is 2500 to 3000 Vickers. The speed for UCON is in range of 250 to 500 m/min of steel of 200 BHN. Expense of UCON is higher than diamond and is utilized only for highly hard material. It has outstanding shock resistance, resistance to diffusion and adhesion wear.
It contains silicon, oxygen, aluminium and nitrogen. It is even costlier than ucon. Sialon is utilized during machining of interrupted cuts. Sialon tips are utilized for machining of aerospace alloys.