Mumbai University > Mechanical Engineering > Sem 4 > Production Process II

Marks: 10M

Year: Dec 2014, Dec 2015

Ceramics: Ceramic tool material, introduced in the early 1950’s, consist a primarily or fine grained, high purity aluminium oxide. They are pressed into insert shapes under high pressure and at room temperature, sintered at high temperature and are called white, or cold-pressed ceramics. Titanium carbide and zirconium oxide can be added to improve properties such as toughness and resistance to thermal shock.

Alumina based ceramic tools have very high abrasion resistance and hot hardness. Chemically they are more stable than high speed steels and carbides, thus they have a less tendency to adhere to metals during machining and hence lower tendency to form a built up edge. Consequently, good surface finish is obtained with ceramic tools, particularly in machining cast irons and steels. However ceramics lacks toughness, which can result in premature tool failure by chipping or fracture. The shape and setup of ceramic tools is also important.

Carbides:

The tool materials described so far possesses sufficient toughness, impact strength and thermal shock resistance for many applications, but they have significant limitations on characteristics such as strength and hardness, particularly hot hardness. Consequently, they cannot be used as effectively where high speeds, and carbides also known as cemented or sintered carbides, were introduced in the 1930’s to meet the challenge of higher machining speeds for higher production rates.

Because of the high hardness over a wide range of temperatures, high elastic modulus and thermal conductivity and low thermal expansion, carbides are among the most important, versatile, cost effective tool and die materials for a wide range of applications. However, stiffness of the machine tool is important and light feeds, low cutting speeds, and chatter can be detrimental. The two basic groups of carbides used for machining are tungsten carbide and titanium carbide.

1) Tungsten Carbide:

Tungsten carbide (WC) is a composite material consisting of tungsten carbide particles bonded together in a cobalt matrix hence it is also called cemented carbide. The amount of cobalt significantly affects the properties of carbide tools. As the cobalt increases strength hardness and wear resistance decreases, while toughness increases. This material is generally used to machine steels, cast iron and abrasive nonferrous materials. WC tools are manufactured by powder metallurgy techniques.

2) Titanium Carbide:

Titanium Carbide (TiC) has higher wear resistance than tungsten carbide but it’s not tough. With a little nickel molybdenum as the matrix, Titanium Carbide is suitable for machining hard materials mainly steels and cast iron and for machining higher speeds than those for tungsten carbide.