Factors Affecting Microstructure of Cast Iron
The structure of Cast iron is affected by the following factors:
The higher the iron’s carbon content, the greater will be the tendency for it to solidify grey. To ensure that the structure is completely graphitic, the carbon content is kept less than 2 per cent when casting.
Presence of Alloying Elements
Alloying elements like silicon and nickel, have a tendency to promote graphite in the structure of iron. Presence of silicon also reduces the rate of oxide formation at high temperature.
Metal chips are sometimes put into the sand moulds, in areas where a high surface hardness is required.
Rate of Solidification
Casting made in sand moulds has a tendency to become grey on solidification. Slow rates of solidification result in graphite formation, while rapid solidification gives white iron structure. When iron solidifies from the molten state, the graphite crystals form irregularly shaped flakes. In this condition, the graphite makes the material brittle.
A small amount of magnesium or cerium is inoculated immediately, before casting. This makes the graphite to solidify as spherical nodules. In this form, the iron is stronger and tougher than the iron-containing flake graphite.
Heating of white Cast iron for more extended period results into graphitization. This phenomenon is used as the basis for the manufacture of malleable iron. Irons for high-temperature service must be in a fully graphitized state before being put into service.
Nickel increases cast iron’s strength by changing the coarse pearlite to fine pearlite and finally to martensite. The addition of chromium counteracts the graphitizing effect of nickel.
The combined effect of Nickel and chromium results in better wear and abrasion-resistant material is known as Ni-Hard. It contains about 3 to 5% nickel, 1 to 3% chromium. It has a hardness of 550 to 650 BHN. Ni-Hard liners are used in grinding mills because of better wear resistance.