Cast iron is a ferrous alloy that is made by re-melting pig iron in a capola furnace until it liquefies. The molten iron is poured into molds or casts to produce casting iron products of the required dimensions. Based on the application of cast iron, the alloying elements added to the furnace differ. The commonly added alloy elements are carbon followed by silicon. The other alloying elements added are chromium, molybdenum, copper, titanium, vanadium, etc.
Based on the alloying elements added, the variation in the solidification of the cast iron and heat treatment used, the microstructure of the cast iron can vary. Depending upon the application and the preferred mechanical properties, iron castings can be classified into the following.
White cast iron
When the white cast iron is fractured, white coloured cracks are seen throughout because of the presence of carbide impurities. White cast iron is hard but brittle. It has lower silicon content and low melting point. The carbon present in the white cast iron precipitates and forms large particles that increase the hardness of the cast iron. It is abrasive resistant as well as cost-effective making them useful in various applications like lifter bars and shell liners in grinding mills, wear surfaces of pumps, balls and rings of coal pulverisers, etc.
Grey is the most versatile and widely used cast iron. The presence of carbon leads to formation of graphite flakes that does not allow cracks to pass through, when the material breaks. Instead, as the material breaks the graphite initiates numerous new cracks. The fractured cast iron is greyish in colour, which also gives it the name. The graphite flakes make the grey cast iron exhibit low shock resistance. They also lack elasticity and have low tensile strength.
However, the graphite fakes gives the cast iron excellent machinability, damping features as well as good lubricating properties making them useful in many industrial applications. The graphite microstructure of the cast iron has a matrix that consists of ferrite, pearlite or a combination of two. The molten grey iron has greater fluidity and they expand well during the solidification or freezing of cast iron. This has made them useful in industries like agriculture, automobile, textile mills, etc.
Malleable cast iron is basically white iron that undergoes heat treatment to convert the carbide into graphite. The resultant cast iron has properties that vary from both grey and white cast iron. In case of malleable cast iron, the graphite structure is formed into irregularly shaped spheroidal particles rather than flakes that are usually present in gray cast iron. This make the malleable cast iron behave like low-carbon steel. There is considerable shrinkage that results in reduced production of cast iron as well increased costs. Malleable cast iron can be identified easily by the blunt boundaries.
Ductile cast iron is yet another type of ferrous alloy that is used as an engineering material in many applications. To produce ductile iron, small amount of magnesium is added to the molten iron, which alters the graphite structure that is formed. The magnesium reacts with oxygen and sulphur in the molten iron leading to nodule shaped graphite that has earned them the name-nodular cast iron. Like malleable iron, ductile iron is flexible and exhibits a linear stress strain relation. It can be casted in varied sizes and into varying thickness.