Schedule 10 1-1/4" and 2" 300 series stainless pipe warping

by Chad
(Austin, TX)

Hello, I weld a LOT of 300 series stainless for exhaust systems, everything from schedule 10 to 16 gauge tube. I am trying to implement as many ways as I can to prevent warpage of the manifolds and flanges, since the vehicle the components are for has VERY tight tolerances. I have been trying to use an airgun to keep the part cool, an infrared thermometer to keep on eye on the exact interpass temp and try to keep it below 350 F...but this is very time consuming. I also put a tack every inch or less (on a 2" pipe there are 8). The manifolds are bolted through every hole onto a jig. I use a Dynasty 300 with my dc high-speed pulse settings at:
PPS: 175-500
Percentage: 60-80
Low Amperage: 25-60
High Amperage: 40-110

The low numbers are for the 16-gauge 3" tubing, the high numbers apply to the schedule 10 manifold material.

I also keep the post-flow up and of course, purge.

At my work it is considered correct to use a crap-ton of heat and feed like an inch of filler into each dime, using the filler rod to cool the puddle enough to not put a hole in the pipe. This penetrates well, but actually leaves a raised bead inside the pipe as well.
My understanding is that I should use as little heat as possible, just enough to penetrate through, while adding the smallest amount of filler I can. I want a flat weld with the heat colors on the sides as close as I can get them to the weld.

What are the best ways to avoid warpage and what are the primary industry accepted methods for reducing it?
What errors or techniques may actually exacerbate warpage (too many tacks, low speed pulse, etc.)?
Should I try to move fast or slow?
Will too much heat in the overall part cause a "spring effect" in the metal, or should a strong jig prevent this as well as the local distortion from the welding itself?

Sorry for the book. It seems like the more I learn, the more questions I have. I look forward to tapping in to the wealth of information that you all have. Thanks!




try lowering the pps to around 50 with % peak time at 30 and background at 30.

take no more than 2 or 3 seconds to establish a puddle and get moving and make a dime every second or faster and move the torch about 1/8" each dime. I think if you are adding that much filler rod, you could drop the amperage and still get in and get moving without all that heat.

sometimes that many tacks dont help and might even hurt. try making a few less tacks if you think the part wont misalign without them.



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