CAST STEEL

The cracking indicates that your weld area is getting high carbon or high alloy admixture from the base material. Proper welding will require specific weld materials and at least some measure of pre and post heat treatment. Net: non trivial.

But I wouldn't even th> HELP

You will need to heat the parts first - both parts. Pre-heat your cast iron parts slowly to just before it turns red hot. Do your weld, then slowly allow the cast iron to cool.

How do you get a big piece of cast iron that hot then slowly cool it? Expensive equipment! That's why a lot of people will just go buy another cast iron flywheel.

Besides, like RoyJ said, it will be a dangerous piece after you are done with it.

how condesending