Iron casting is a ferrous alloy made from pig iron, which is melted in a furnace along with alloying elements like carbon and silicon. The liquid molten iron is poured into molds to be casted for manufacturing an array of products. These cast iron products are used in a variety of industries ranging from agriculture to automobiles and hydraulics.
During the process of casting, some defects may be found in the products. Defects are usually undesirable in any casting product as they affect the quality of the product. The type of defect decides whether the fault can be tolerated, corrected or eliminated. It is very important to identify the type of defect so that you can decide what you need to do with them. More often, recognizing the defect correctly can help the quality control department to identify the root cause of the defect and find ways to fix them. Here are the common types of defects that may be seen in casting products.
As the metal solidifies after being poured into the molds or casts, it is bound to shrink. When there is not enough metal available, shrinkage of cast iron products will lead to holes in the casting product. There are many types of shrinkage depending upon its cause.
When the casting iron in the center (axis) takes a longer time to solidify or freeze in comparison to the metal on the sides, it leads to a cavity in the middle known as axial shrinkage. This can be caused due to various factors like the temperature at which the molten iron is poured, the speed at which it poured the type and quality of alloy present and any defect in the thickness of the casting mold.
A variation in the alloy elements can lead to this type of shrinkage where cavities are formed perpendicular to the casting surface. High nitrogen content or low carbon content can lead to this type of casting defect.
Sometimes, all the casting products may have the same type of defect variation in the dimensions. When the various alloy elements that were added to the furnace solidify at different rates, it leads to improper shrinkage. When this type of defect is found in the casting product, the product has to be remade.
It is a metallurgical defect, which is characterized by the presence of depressions on the surface of the casted product. When the graphite moves into shrinkage cavities, it leads to scars or seams.
They are shallow spots or drops found on the surface of the casting products caused due to inclusion of contaminations like metal carbides, calcites, oxides and sulfides from ladles, furnaces and casting molds.
If you find tiny projections on the surface of the casted products, it could be the rat tail defect. It usually occurs in sand casting ferrous products. Altering the sand mixture will eliminate this type of defect.
When you find cavities on the surface of the casting products, they are called blowholes or pinholes depending upon the size of the cavities. They are caused when gas gets trapped in the casting products during solidification. Solidifying the casting products in vacuum can eliminate this defect.
When there are unfilled portions leading to cavities of varying depths on the product surface, they are called coldshuts. This defect is often accompanied with another defect called misruns, where the edges of the casting are ill-formed. Both these defects are commonly caused due to the lack of fluidity of metal alloys.