When the head preheat temperature has stabilized (it takes about an hour), a "neutral flame" oxyacetylene torch with slightly more oxygen than acetylene is used to melt the cast iron (which melts at approximately 2700 degrees F). Geertsema says he uses a several varieties of cast iron filler rod and borax flux. The trick here is to keep the weld clean by adding a little flux so the impurities will rise to the top. The impurities can then be floated out of the repair area with the torch.
"If we're building up a valve seat, we'll make a carbon graphite plug to fill the hole, then weld up around it," said Geertsema, "The puddle will be about half-an-inch deep and maybe two-inches in diameter. It takes a lot of heat to do this, about 5000 degrees F.
After the crack has been filled, a long, slow cool-down follows. This step is also important to prevent the head from recracking. If cast iron cools too quickly, one of two things can happen. The surrounding metal can shrink away from the weld causing new cracks to open, and/or the carbon in the iron can turn into carbide making the metal too hard and brittle to machine. *Geertsema said, therefore, the casting must be cooled very slowly to prevent these undesirable metallurgical changes.
Geertsema says he wraps the heads in an insulating blanket and keeps the head in a hot box so the head will cool at a rate of no more than 200 degrees F per hour.
At these rates, it can take quite awhile for the head to cool down to ambient temperature: 8 hours to overnight. So one can't be in a hurry when *fusion welding cast iron heads. Geertsema says once the head has cooled, it is cleaned to remove the scale, then rough machined and submersion pressure tested in hot water at 100 psi for leaks. . ."