ANSWER
Cast iron is very brittle and this makes it difficult to weld unlike steel. Heat treatment is therefore required.
(i) Heat treatment of cast iron before welding prevents cracking due to expansion and contraction.
(ii) Heat treatment enables a sound weld to be made as the welded cast iron cools down slowly.
(iii) It enables a weld that can be good surface finish to be produced.
(iv) It causes grey cast iron to be formed instead of the hard white unmachinable weld that would occur if it cooled off rapidly.
