Metal working processes involve operations on metals like cast iron, steel, brass, bronze, aluminium, etc,
with the help of machine tools like lathe, shaper, miller, grinder, planner, etc.
The following are the metal working processes:
• Finishing, plating, honing, galvanizing, anodising.
• Forming, casting, forging, hot-rolling, extruding.
• Heat treatment, hardening, tempering, annealing, normalising.
• Joining, welding, soldering, brazing, riveting.
• Machining, milling, plaining, shaping, grinding.
Metal working processes can be described as under:
Casting: A casting may be defined as a molten material that has been poured into a prepared cavity and
allowed to solidify. Casting may be classified as:
Sand Casting: It is used mainly for steel and iron and it can also be used for brass, aluminium, bronze,
copper etc. and relatively large amount of metal is to be removed.
Features of Sand Casting:
• Clay and water act as a bond.
• Compressed moist sand is used.
• Less costly.
• More machining of finished goods is required.
• More weight.
There are two methods of sand casting:
Green Sand Moulding and Dry Sand Moulding.
Green sand Moulding utilises a mould made of compressed moist sand. This method is not suitable for
large and heavy castings. In case of Dry sand Casting, the mould surfaces are given refractory coating and
are dried before the mould is closed for pouring. This method is useful for large and heavy castings.
Centrifugal Casting: The molten metal is poured into a hollow cylindrical mould, which is spinning and
the centrifugal force causes the liquid metal to flow to the outside of the mould and to remain there. Then
it is allowed to cool.
Die Casting: The dies are expensive but the advantage is that no finish machining is required in many
cases. This is limited to low melting point alloys.
Investment Casting: Machining the part made out of an alloy is very difficult and in such cases, the
investment casting is used.
Permanent Mould Casting: This is developed to avoid the above disadvantage. The advantages are smoother
finish, higher mechanical properties, good dimensional uniformity and ease of adaptability to automatic
high production. But this is having high initial cost of tooling and the size of casting is also limited by the
mould making equipment.
Plaster Mould Casting: Only one casting is made and then mould is destroyed. The advantages of plaster
are the superior surface finish, improved metal characteristics and good dimensional accuracy. But the
disadvantage is that the mould is destroyed each time.
Forming: Forming processes are those which accomplish the rough sizing or shaping of manufactured
articles. Any cutting tool which produces a desired contour on the work piece comes under forming process.
Heat Treatment: It is a process of heating and cooling metals in order to obtain certain desired properties.
Then the right combination of hardness and toughness is achieved for enabling the cutting tools to be able
to successfully machine other metals. In addition to hardness and toughness, this process will improve
heat and corrosion resistance and relieve stresses. The various types of heat treatment processes are briefly
discussed below:
1) Annealing: It refers to the heating and cooling operations which are usually applied to induce softening.
Annealing softens overly hard, overly brittle part, through heating, short of critical point, followed by
gradual cooling. Thus it relieves further the brittleness introduced by “hardening” and reduces metal
electricity slightly.
2) Case Hardening: This process involves two operations. The first is a carburizing process where carbon
is added to the outer surface by heating low carbon steel. The second operation is heat treatment of
the carburized parts so that the outer surface becomes hard.
3) Hardening Steel: Hardening is the process of heating a metal to the decalescence point, soaking it for
a considerable time to allow thorough penetration of the heat in the metal structure and then suddenly
quenching it in cold liquid.
4) Normalising: Normalising is the process, in which parts are allowed to cool in still air at room
temperature.
5) Quenching: Quenching is the process of rapidly cooling the metal from a high temperature. Rapid
cooling is required for obtaining high strength and hardness and the purpose is to harden steel so that
it can withstand wear and tear.
6) Tempering: Heat treatment should relieve stress as stated above. But after quenching, the metal is
hard and brittle. It requires reheating. Tempering is the process of reheating which will reduce the
brittleness and soften the steel.
No comments:
Post a Comment