For centrifugal casting, molten metal is introduced
into a mould that is rotated during solidification. The speed of the
rotation and metal pouring rate vary with the alloy and size and shape
being cast. The centrifugal force improves the feed and filling
consistency achieving surface detail. This method has been specifically
adapted to the production of cylindrical parts and eliminates the need
for gates, risers and cores. The process is typically unsuitable for
geometries that do not allow a linear flow-through of metal.
The idea of employing centrifugal force to make castings had been known
for a long time, it was A. G. Eckhardt's original patent of 1809 which
revealed understanding the basic principles involved. Centrifugal
casting has greater reliability than static castings. They are
relatively free from gas and shrinkage porosity. Many times, surface
treatments such as case carburizing, flame hardening and nitriding have
to be used when a wear resistant surface must be combined with a hard
tough exterior surface.
One such application is bimetallic pipe consisting of two separate
concentric layers of different alloys/metals bonded together. Such pipes
can be economically used in many applications and can be produced by
centrifugal casting process. Typically, in centrifugal casting, the
following structure or zones may occur, Chill Zone - This layer is of
fine equiaxed structure which forms almost instantaneously at the mould
wall, Columnar Zone - This is next to chill zone. It consists of
directionally oriented crystals approx. perpendicular to the mould
surface, Equiaxed zone - this region may occur next to columnar zone
characterized by large number of uniformly grown crystals. Centrifugal
casting is suitable for the production of hollow parts, such as pipes.
The process is suited for producing structures with large diameters -
pipes for oil, chemical industry installations and water supply, etc.
The
casting is relatively free from defects.
Non
metallic impurities which segregate toward the bore can be machined off.
Less
loss of metal in tundish compared to that in gating and risering in
conventional sand casting.
Better
mechanical properties.
Production
rate is high.
Can
be employed to manufacture bimetallic pipes.
Centrifugal
casting process can be used for fabricating functionally gradient
Metal
matrix composite material.
Mould
preparation time is relatively short in comparison to many other
processes.
The
relative simplicity of the process makes it ideally suited to
mechanization.
High
levels of sand reuse are achievable.
It
produces less waste than other techniques.
Defects in Centrifugal
Casting: Conventional static casting defects like internal
shrinkage, gas porosity and nonmetallic inclusions are less likely to
occur in centrifugal casting.
Hot Tears : Hot tears are developed in
centrifugal castings for which the highest rotation speeds are used.
Longitudinal tears occur when contraction of casting combined with the
expansion of the mould, generates hoop stresses exceeding the cohesive
strength of the metal at temperatures in the solidus region.
Segregation: Centrifugal castings are
under various forms of segregation thus pushing less dense constituents
at centre.
Banding: Sometimes castings produce zones
of segregated low melting point constituents such as eutectic phases and
sulphide and oxide inclusions. Various theories explain this, one states
vibration is the main cause of banding.
