What is Casting Defect and Types of Casting Defect
Casting Defect is an undesired irregularity in the casting formed during metal casting processes. Casting defects are generally classified in to five major categories (like cracks, gas porosity, shrinkage defects, mould material defects, pouring metal defects and metallurgical defects).
Types of Casting Defect
Shrinkage:
Shrinkage can occur in casting when standard liquid metal is not available to compensate the shrinkage of metal during solidification from liquid state to solid state. Shrinkage defect may be either open shrinkage or closed shrinkage.
Gas Porosity:
Gas Porosity is formed due to air bubbles forming during pouring of liquid metal from ladle to mould; which is entrapped in casting itself during solidification. Insufficient vents or excess gas in liquid metal causes gas porosities in castings. Following a proper de-oxidation process and techniques like AOD and VOD can reduce the gas porosity formation in castings.
Pouring Metal Defects:
Pouring Metal Defects may be like misruns, cold shuts and inclusions. Misrun and cold shuts are closely related to each other because in both cases, liquid metal solidifies before the mould completely fills. The severity of defect depends on castability and viscosity of metal. High pouring temperature can avoid such defects. Inclusion in casting may be due to various reasons like improper slag removal during tapping, in-sufficient time for oxides formed during de-oxidation to reach slag, furnace lining erosion due to high melting temperature of alloy, etc. Depending on the source, various remedial measures are taken to avoid inclusions in castings.
Metallurgical defects:
Metallurgical defects are mainly due to hot tear and hot spots. Hot tear type of defect occurs when the material casted is weak at high temperature or residual stresses in material itself cause failure during solidification of metal. Hot spot in casting is formed in region which cools very slowly. Due to this, abnormal shrinkage of material occurs during solidification resulting in formation of porosities and cracks. This can be avoided by using proper cooling techniques and methods to promote solidification.
These defects may have various reasons to occur, but identifying exact root cause helps in making proper corrective actions to arrest these kind of defects in future castings. Thus, defect analysis is important in a casting industry to produce a sound casting. Optical microscope is one tool used to identify the defects and classify them to map the root cause.
Optical Microscope:
Optical Microscope also called as light microscope works on principle of reflection of light. Optical microscope has a series of lenses placed to generate magnified images of object to be observed. They are simple and easy to operate and handle. Optical microscope consists of light source, condenser lens, objective lens, sample stage, and eye piece lenses. Object can be viewed at different magnifications by using objective lenses with different magnifications like 2.5X.
So, the lenses used in optical microscope help to analyze and inspect tiny cracks and bubbles in metal casting both in surface and internal structure. It detects different defects, such as porosity and cracks, and with its distinct features finds very small holes or thin lines in casting. Then after identifying the type of defect, it also helps to determine whether the defects are caused by shrinkage, gas entrapment or other issues.
This analysis helps to reveal causes such as contamination, mold problems or improper pouring techniques. Thus, optical microscope plays a vital role in improving manufacturing methods by pinpointing root causes of defects.
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