I've done some.
Preheat for an hour to 500 degrees in the oven.
Weld quickly allowing it to cool as little as possible.
Put back in 500 degree oven for another hour.
Turn oven off, but leave the part in it and the door closed.
Come back the next day to a cold and welded part.
Also - I have had some success welding one bead down on half of the crack, another parallel bead down the other half of the crack, and then a third bead to connect the first two. The first two get to get a good bite into the base metal without being subjected to the cooling stress, and then the third bead, you're mostly just welding to filler, not to cast iron any more. Finally - a convex weld puts less surface tension into the part than a concave weld, so pile the filler on.
