[croshaul]

I'm building an exhaust for my project and have been working on making pie cuts and general fitting issues. I watched a you tube video where they were purging the pipe. The pipe had numerous pie cuts and just tacked in place sections. What is the advantage of the purge in this scenario?

[cj737]

Purging for stainless exhausts is mandatory if you want to prevent backside "sugaring". Even though the pie cuts are tacked, when welding out, you should cover the seams with tape to force argon to fill the interior. Run your purge line from as close to the lower point, and allow it to vent at the highest (small pinholes for evacuation). Argon is heavier than the atmosphere and will fill bottom to top.

Now when you weld, you can get full penetration welds with stainless filler or autogenous welds with no oxidation on the backside.

Is it truly required? No. But if you don't purge, the welds will corrode and crack prematurely. If you do purge, the exhaust will last a LOT longer. Plenty of good videos on stainless exhaust purging on YT.

[croshaul]

I really didn't even notice if they were welding stainless. My exhaust is mild steel and I just assumed he was welding mild steel also. I know better. Is there a benefit to purging mild steel?

[cj737]

Not really for an exhaust. I did one recently using Silicon Bronze because it was aluminized metal, no purge.

[croshaul]

Thanks CJ, pipe welders at the mill that I used to work at did alot of purging on SS pipe. They also used a backing tape but I don't remember if they were using both at the same time. I thought it was cool to see the weld from the inside. Looked like they were welding the inside.