Casting defect
[1] The terms "defect" and "discontinuity" refer to two specific and separate things in castings.
Isolated pools of liquid form inside solidified metal, which are called hot spots.
They require a nucleation point, so impurities and dissolved gas can induce closed shrinkage defects.
Nitrogen, oxygen and hydrogen are the most encountered gases in cases of gas porosity.
Hydrogen is produced by the reaction of the metal with humidity or residual moisture in the mould.
[12] Gas porosity can sometimes be difficult to distinguish from micro shrinkage because microshrinkage cavities can contain gases as well.
Proper foundry practices, including melt preparation and mould design, can reduce the occurrence of these defects.
Because they are often surrounded by a skin of sound metal, blowholes may be difficult to detect, requiring harmonic, ultrasonic, magnetic, or X-ray (e.g., industrial CT scanning) analysis.
A misrun occurs when the liquid metal does not completely fill the mould cavity, leaving an unfilled portion.
Cold shuts occur when two fronts of liquid metal do not fuse properly in the mould cavity, leaving a weak spot.
[15] Misruns and cold shuts are closely related and both involve the material freezing before it completely fills the mould cavity.
The point is difficult to predict in mould design because it is dependent on the solid fraction, the structure of the solidified particles, and the local shear strain rate of the fluid.
These usually are impurities in the pour metal (generally oxides, less frequently nitrides, carbides, or sulfides), material that is eroded from furnace or ladle linings, or contaminates from the mould.
In order to reduce oxide formation the metal can be melted with a flux, in a vacuum, or in an inert atmosphere.
Other ingredients can be added to the mixture to cause the dross to float to the top where it can be skimmed off before the metal is poured into the mould.
This happens because the metal is weak when it is hot and the residual stresses in the material can cause the casting to fail as it cools.
This type of defect can be avoided by proper cooling practices or by changing the chemical composition of the metal.
[3] Additional methods of minimising hot tears are not overheating the casting material and increasing the temperature of the mould.
[23] A longitudinal facial crack is a specialized type of defect that only occurs in continuous casting processes.
This is caused by sand particles which are too coarse, lack of mould wash, or pouring temperatures that are too high.
[26] An alternative form of metal penetration into the mould known as veining is caused by cracking of the sand.
[27] These defects are caused by overly high pouring temperatures or deficiencies of carbonaceous material.
This defect can be avoided by reducing the temperature of the liquid metal, by using a mould wash, and by using various additives in the sand mixture.
