How Fusion Welding Cast Iron Works

". . .Fusion welding cast iron is called the "black art"' of crack repair because it involves a lot of heat and requires a highly skilled welder. Learning how to "recast" damaged heads is not something an inexperienced welder can pick up overnight. . .

"If furnace welding was easy, everybody would be doing it." said Rick Geertsema of *A&C Casting Rebuilders, formerly of Excelsweld USA.. "It's a hot job that doesn't exactly appeal to a lot of people."

Today, most of the work he does is specialty restoration work on older heads from classic cars and antiques. Examples of Rick's fusion welding experience.

Geertsema says he may use either *fusion or powder welding to repair a crack depending on the application. Either way, the first step is to fully identify the cracks, then grind them out with a hand-grinder.

If a crack is being repaired by *fusion welding, Geertsema first preheats the head to 1300 degrees F (cherry red) in an oven. . .Preheating is absolutely essential to minimize thermal shock, and to relax the metal so it won't distort when the torch is applied to the casting.

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. . ."

*Some words have been changed from the original document.