Effect of Impurities on Cast Iron

The effects of impurities on the cast iron are as follows:

1. Silicon: It may be present in cast iron up to 4%. It provides the formation of free graphite which makes the iron soft and easily machinable. It also produces sound castings free from blow-holes, because of its high affinity for oxygen. It reduces the melting point but enhances the percentage content of uncombined carbon.
2. Sulphur: It makes the cast iron hard and brittle. It causes red-hardness of metals (Brittleness at high temperatures). Since too much sulphur gives unsound casting, therefore, it should be kept well below 0.1% for most foundry purposes.
3. Manganese: It makes the cast iron white and hard. It is often kept below 0.75%. It helps to exert a controlling influence over the harmful effect of sulphur by forming MnS (which is not injurious in small quantities).
4. Phosphorus: It aids fusibility and fluidity in cast iron, but induces brittleness. It is rarely allowed to exceed 1%. Phosphoric irons are useful for casting of intricate design and for many light engineering castings when cheapness is essential. It gives rise to cold shortness (Brittleness at ordinary temperatures).