Re: Re Welding Heads

Posted by Stuart on February 14, 2010 at 16:16:36 from (64.198.86.189):

In Reply to: Re Welding Heads posted by LarryM on February 14, 2010 at 13:32:20:

You can 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, first preheat 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). You can use 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 your building up a valve seat, make a carbon graphite plug to fill the hole, then weld up around it. 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. Therefore, the casting must be cooled very slowly to prevent these undesirable metallurgical changes.

Wrap the head 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. 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

All aluminum heads are tig welded.

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