Hey I have been given a cast iron sump to weld the engine blew and put a rod through the sump and they cannot be got. I have welded the cracks with a cast rod and worked well but there was a 4 inch hole and the bits couldn't be found so I cut a piece of mild steel and set it in and welded it with the correct rods from a weld shop. Once welded it cracks along the side of the weld on the cast side I have tried it both hot and cold it has also caused the cracks that were there to get bigger. Can anyone shed some light on this or where I am going wrong. Thanks.