[Stubbe]

I’m wondering how you would weld small pipe together so it bends nicely. At work we weld 1/2 and 3/4 pipe sch 80. All end to end. It’s beveled from our supplier, not super nice steel though. Then it’s pulled into an about 8” radius. The guys doing it are having trouble with the welds ripping either in the weld or right next to it after its rolled. They run 25 volts and 500 ipm and no gap.

So what’s better to make the weld “soft” or ductile?

I tend to think hotter to spread the heat zone out further, and gap it some so the weld lays flatter.

What do you guys think?

[Coldman]

Your pipe joint prep and mig process are both inappropriate.
You need a proper vee butt joint prep with gap and a full penetration tig weld. External reinforcement can be ground flush and you're good for bending.

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[Stubbe]

Unfortunately I think I’m stuck with mig ( not my company). I could get different wire or gas if I asked. I agree tig makes the most sense as the welds are soft. So do you think a smaller gap and welding closer to spray and faster, or colder and slower with a gap.

I’m do auto Resto on the side and the longer or hotter you get the metal seems to help it be softer. Oxy being the nicest to planing out. That’s why I’m thinking whatever heats the pipe up the most.

[Coldman]

External reinforcement has to be ground flush with the pipe so it fits into the die and bends neat. That means you need a full penetration weld with internal reinforcement to ensure the weld is stronger than the parent metal according to good welding practice. You also don't want cold lap in the weld.

If you can achieve that with mig on 1/2" pipe go for it. I have doubts after reading your description of what goes on in your shop.

Your understanding of heat is flawed. A good weld has enough heat to ensure complete fusion while keeping the heat affected zone to a minimum. Comparison with oxy is not accurate either. Oxy flame is what something over 1000 degree C. Electric arc is the heat of the sun.

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[Coldman]

If a die bender is not being used the pipe should be heated to cherry red while bending.

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[Poland308]

If a die bender is not being used the pipe should be heated to cherry red while bending.

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Probably your best bet! Or try welding the tubes on in lengths where the welds don’t end up in a bend.

[Stubbe]

Thank for the replies. The welded pipe gets pulled onto a mandrel like a spiral. So all of it gets “bent”. It always seems to hump up some where the weld is and not pull tight to the mandrel (with the weld ground flat too).

Played with it some today after work. On mig one hotter pass with a full bevel worked good. Only a slight hump.
A colder pass with more gap left more hump. I hit some with a rosebud after welding an heated 2 inches or so and those bent nearly perfect.

Did one tig. Of course that worked nearly perfect. I agree about this shop not being the best btw, so my feeling aren’t hurt. Were just a factory that welds some.

Overall I’m thinking increasing the heat affect helps it roll more evenly and that reduced the stress in the weld. So even if the weld is a little weaker it works better for this part.

I’m going to have the guy try it for a week or so and see how it turns out.

[Poland308]

Heating with the torch after it has cooled once is called normalizing or annealing. It is a common way to soften the area that was hardened from the welding process.