Casting defects and their inspection
Casting is a very precise process and any lacking at any stage of manufacturing process may lead to casting defects which may also be non fixable. Casting defects are not limited to only sand casting but can be seen in all type of castings.
Few major and common type of casting defects are listed below:
- Shrinkage casting defects
- Gas porosity casting defect
- Pouring casting defects
Shrinkage casting defects:
It is understood that when hot metal solidifies its overall size is reduced due to shrinkage and as the name indicates this type of defect occurs due to shrinkage of molten metal into its mold during solidification.
Now anyone can think if shrinkage during solidification will happen anyhow then how is it a defect.
To understand this we have to first know that shape of mold cavity is as per pattern design and in pattern design shrinkage allowance is provided which means OK part will be produced even if metal shrinks during solidification.
So to control shrinkage of metal more than its allowance runners and risers are attached to mold cavity function of which is to compensate the reduced size due to shrinkage during solidification of molten metal. This type of defect arises when reserve metal is not available to overcome the shrinkage.
Shrinkage defect is of two types i.e. Open and closed type.
Open shrinkage defects occur at surface of castings.These defects looks like web of pipelines over surface and shallow cavities spread across the surface. Closed type shrinkage occur within the casting and looks like porosity.
Gas porosity casting defect:
Reason of gas porosity is the basic understanding that liquid metal can carry more gases within than the metal in solid state. Gas porosity can be seen on surface as well as inside castings.
Gas porosity inside casting are more critical than the surface gas porosity because amount of porosity is not know and it directly affects the strength of casting.
When metal solidifies, gases present in molten metal becomes bubbles and tries to come at surface. Bubble which make their way to surface may escape the casting leaving behind nothing or may form surface porosity of bubble shape.
When bubbles gets trapped with in castings, these bubbles form internal gas porosity. To prevent gas porosity, presence of trapped gasses is minimised from melting metal into furnace to pouring into mold.
To melt metal with minimal gases vacuum chambers are used so that gases does not mixed or melting is performed in the presence of gases having low solubility like argon or carbon dioxide.
At extreme high temperatures metal absorbs maximum gases and to prevent this, temperature is kept lowest possible. Gas mixing is also possible at pouring stage as if path of pouring is not steady and smooth or shaky. This has to be considered to prevent gas mixing during pouring of metal.
Pouring casting defects:
Defects occurred during pouring of metal into mold is called Pouring casting defect. There are few type of pouring defects explained below:
Misruns: Suppose metal is poured into mold and before it reaches its far most end it freezes leaving behind unoccupied mold cavity. This state of defective pouring is called Misruns.
Misruns occurs if mold cavity is vary narrow for passage of molten metal or if temperature of molten metal is low (In low temperature condition ,molten metal will have high viscosity and low fluidity).
These may also occur if pouring is done too slowly. Due to slow pouring temperature of metal drops and fluidity decreases. Fluidity is also effected by composition of metal.
Cold shut occurs when molten metal in mold starts freezing from two different ends and created a weak casting zone at a point where they meet. This happens because these two zones does not fuses properly. These type of defects are very critical as these zones have very low strengths and tends to have high failure chance.
These are generally contamination due to oxidation of carbides formation. These are in pure molten metal in the form of slags or dross. These make casting irregular and poorly aesthetic.
Tests to detect these defects:
- Visual inspection is primary inspection criteria which involves inspection of exposed surfaces for defects
- Dimensional inspection to check for any excess shrinkage, shifting.
- Metallurgical inspection to check composition and hardness.
- Mechanical test to check strengths, either tensile or compressive.
- Magnetic particle testing or radio graphic tests to check internal cracks and flows.
- Pressure test to check leakages in castings.