Grey cast iron is used in casing of a gear box for the following reasons-
1) Grey cast iron has high compressive strength and as such it can absorb the varying load of high magnitude.
2) High tensile - strength - Due to presence of high Carbon and other alloying elements like Manganese, Nickel and Chromium the tensile - strength of the Grey cast - iron is of high order - having to the tune of average 7 ton per square inch. This property offers the same high impact to varying load and as such crack would not develop easily.
3) High Corrosion - resistance material - Vanadium is used in the Grey cast iron so that it can fight with corrosion giving the material an excellent life and as such the same is used for casing purpose in the Gear - box.
4) Low- cost availability - The price of Grey cast iron is cheaper than steel but suitable to withstand the load of varying degree. So, this material has gained popularity for its specific application in casing.