Not sure what the PO did, but cracking at the welds is usually caused
by either insufficent penetration, poor toe blending or excessive
heat. My money is on insuffficent penetration. Cut one of the failed
welds an you will likely find it fails to penertate all the way
through the base metal. This is very common when welding tubing.
There is another possibility, using the wrong filler rod. Not usually
a problem with mild steel tubing, but I see it a lot when guys use
plain steel filler to weld on stainless headers. Works ok for a
while, but eventually the weld cracks and fails.
The heat cycle will defintly reduce and stresses that may have formad
in the material while you were welding, and id a good idea for mild
steel tubing. just heat the entire part up to cherry red, then let it
cool slowly. Do not try this on a hardened or heat treated items,
like frame rails, or case hardend parts, as you will destroy the heat
temper of the metal.
WRT the metal thickness, the best thing to do is use the thinest
material that is capable of handling the pressure that you are
devloping. I suspect you aren't going for a lot of boost, so
relatively thin material is ok. But I doube the heat cycle is what
got you anyway. It is most likely the welding that caused the cracks.
On 3/7/06, Glen Byrns <grbyrns@ucdavis.edu> wrote:
> I'm going to be welding up a manifold from mild steel tubing, three short
> stubs of 1.25" going into a 'log' 2" in diameter. This will mate a turbo to a
> 1275.
>
> My question is, would the finished manifold be less likely to crack if the
> manifold is heated up to red heat and cooled slowly before use? The original
> was made by the PO and has cracked at the welds several times over the years.
> This one will be a copy made with thinner gauge steel. I presume the cracks
> are from the stress of the 'log' expanding as it gets hot, while the faceplate
> bolted to the head is much cooler and doesn't expand nearly as much. Would
> thick or thinner steel be less lkely to crack in this situation?
>
> Once I finish this one for the bugeye, I plan to design one to mate a modern
> small turbo (VW?) to a 948 (Austin Powerless).
>
> Thanks,
> Glen Byrns
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