CASTING
DISCONTINEUTIES IN RADIOGRAPHY.
Casting is a manufacturing
process in which liquid material is pored into mold the mold contain hollow
cavity of a desire shape and then allow to solidify, the solidify part is
called as Casting.
In radiography the casting
product have different types of discontinuities found as shown below,
1) Micro-Porosity Defect :
The micro-porosity is the defect found in Casting due to evolution of gas
very fine gas pores form around grain boundaries . The micro-porosity occurs in
those places of the casting products which solidify last. Miro-Porosity is not
always immediately visible to the human eye. The least micro-porosities occur
in Die-Casting Products and their causes originate in a shrinkage porosity.
Micro-Porosity mainly effects
Non-Ferrous metals such as Magnesium & Aluminum.
Pic.01 : Micro-Porosity.
For
Prevention
i) Avoidance of sharp-cross-sections and
joints.
ii) Keep binder contents in mold materials.
iii) Adjust optimum castings temperatures.
iv) Reduce
gas contents in the melt.
v) Reduce solidification interval.
2) GAS HOLE
DEFECT:
We know
that Rounded cavities generally greater than 1 mm Diameter, quite large and
after more randomly dispred through the casting due to gas involve from the
metal during solidification. Appears as dark areas with a smooth outline which
is either circular or in elonged shape.
Generally Gas Hole porosity occurs when
the metal traps gas (Nitrogen, Oxygen or Hydrogen) during casting. When casting
solidify there is formation of bubbles This bubbles appears on casting as
rounded, circular or hole type.
Pic.02 : Gas Hole
3) PIN-HOLE POROSITY:
Small
rounded in shape cavities in casting generally less than 1 mm Diameter cause by
same evolution of gases during solidification of molten metal of casting
products is called Pin-Hole Porosity. Pin-Hole Porosity sometimes occurs below
the surface of casting products and such type of discontinuities called
Sub-Cutaneous pinhole porosity. Which type of porosity appears on radiograph in
dark rounded images.
Basically, Pinholes
are very tiny holes found in upper part of mold. This type of porosity appears
in large numbers together.
Pic.03 :
Pin-Hole Porosity.
4) WORMHOLES:
Wormholes is Tube
like cavities similar to the gas hole discontinuities generally located just
below the surface of casting products. The Wormhole discontinuity is caused by
progressive expansion of entrapped superheated steam from moisture in a mold.
In order to prevent The Wormhole ,
eliminating the gas and cavities to prevent wormholes in casting. Avoid joint
geometry which creates the cavity.
5) SHRINKAGE
CAVITY :
Shrinkage
Cavity is the defect or depression in casting which formed during
solidification process. As compared to the Rounded Gas Porosity the Shrinkage
Cavity appears with angular edges.
In other words, the Shrinkage Cavity
is caused by centration of metal during solidification.
6) AIRLOCK
DEFECT :
Airlock is basically a large cavity formed by air is entrapped in the mold during pouring of the metal. Airlock is appears as a generally smooth and often irregular shaped.
7) FILAMENTARY
STRINKAGE :
The continuous cracks of various dimensions and different densities observed usually under thick part of the material is called filamentary shrinkage . The filamentary shrinkage is difficult to detect. As compared to Filamentary Cracks the filamentary Shrinkage is less dense. Appears as a network of branched irregular shapes.
8) HOT TEAR :
Hot Tears is also called as Hot Cracking. The Hot Tear defect occurs when casting cools. It is one type ofd fracture which cause in metal during contraction as it cools during the early edges after solidification. Hot Tear is appears as dark lines. It is plane type discontinuity.
Pic.08 : Hot Tear.
9) COLD SHUT :
Cold Shut is a
discontinuity formed when a stream of liquid metal flows through a mold fails
to fused with other metal in the mold. A smooth dark line appears as cold shut
on radiograph. Cold Shut is planner type discontinuity . It is detect by
visually even visible by naked eye.
Pic.09 : Cold Shut
10) INCLUSION :
Sand from mold can be washed into the
stream of metal as enters as flows through a mold and become trapped in the
metal as it solidifies. It may appear as a light or dark irregular shaped image
in a radiograph and may difficult to distinguish from a void. Such type of
discontinuity is generally not acceptable.
Inclusions is usually impurities in a pour metal. Inclusions are mostly a problem in aluminum casting but their are issues in zinc and magnesium also.
Pic.10 Inclusion
:
11) STRESS CRACK
:
A sharper more well define fracture of a meterial during the later stages of cooling from solidification is called Stress Crack. May appears like dark line . They also can formed when casting is cold or during heat treatment. Stress Crack is most common source of failure at molds made of maragging steels.
Pic.11 :
Stress Crack
12) SEGREGATION
:
This continuity comprises particular component of a metal composition that have different solidification temperatures and tend formed by solidification of metal and segregation at particular areas particularly at central zones of casting products. May appear as a dark or light area on radiograph. Generally when casting an alloy Segregation occurs.
Pic.12 :
Segregation.
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