Cast Cobalt Tools and Carbides
Cast Cobalt Tools
Also called white bits, these contain costly materials like cobalt (50%), chromium (28%), tungsten (20%) and carbon (2%). Although very hard and convenient for cutting forgings and castings 'stellite' can be shaped only by casting and is very difficult to machine and grind. This limitation of workability has rendered cast cobalt tools uneconomical and obsolete.
Carbides
Extremely hard and brittle tungsten carbide powder is mixed with soft and tough cobalt (bond) powder. The mixture is pressed to form blanks. These are pre-sintered at 900oC to impart strength and machinability to the blanks. The blanks can be suitably shaped by turning, drilling, grinding, etc. before a second sintering is carried out between 1400oC and 1600oC. The second sintering melts the cobalt bond and facilitates absorption of carbides. Solidification at 1350oC during cooling imparts excellent toughness and wear resistance to cemented carbides. Finer carbide grains enhance hardness while coarse grain and more cobalt increase toughness. Addition of Tantalum and Niobium carbides improves wear resistance making the tool more suitable for machining of steel and other materials producing long continuous chips.