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Old 31-03-2014, 12:49   #1
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seamless versus welded tubing

I want to build a custom pole to support both a wind generator and to be used as part of an outboard hoist. Without shopping around I find the following prices for the main tube (2.5" OD x .065 wall x 96" long):

Seamless $309
Welded $123

I'm would like your help in determining whether there is any reason not to use the welded tube.

The welded tube is smooth on the outside but has a bead on the inside. That's not a problem.

The strength of the welded is fine.

The only thing I can see as problematic is whether there are impurities (more iron) in the weld material that would cause it to rust. Does anyone have any idea about this?

Thanks in advance for your help.
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Old 31-03-2014, 12:53   #2
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Re: seamless versus welded tubing

After thinking about it, they surely use 316 weld rod. The question then becomes whether the heat of welding does something to enhance rusting. Hmmm. This is far beyond my expertise.
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Old 31-03-2014, 12:56   #3
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Re: seamless versus welded tubing

Welded tube should be more than adequate. Seamless tube is generally used to convey liquids or gases at high pressure. Tube welding generally does not require filler metal so the weld seam is identical to the parent metal provided the process is done correctly.
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Old 31-03-2014, 12:59   #4
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Re: seamless versus welded tubing

Thank you so very much. This will save me a bunch of money.
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Old 31-03-2014, 13:19   #5
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Re: seamless versus welded tubing

Welded tubing is intended as a structural member, e.g.: a pole to hold something up. Seamless tubing is intended to be used to hold and convey liquids or gasses, perhaps under high pressure (think pipe). High pressure inside a welded tube may split the seam.
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Old 31-03-2014, 15:34   #6
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Re: seamless versus welded tubing

I concur if the weld is professional. Welding can cause premature rusting if the piece got too hot or wrong filler rod,just be careful on who welds it in.
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Old 31-03-2014, 16:24   #7
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Re: seamless versus welded tubing

I just noticed that the 1-inch stainless tubing sold for railings on boats is 304 not 316. I always thought that 316 was better in terms of corrosion resistance. Is there any reason to use 304?

Maybe they use 304 because it is slightly less expensive.
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Old 31-03-2014, 16:28   #8
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Re: seamless versus welded tubing

Quote:
Originally Posted by mlibkind View Post
I just noticed that the 1-inch stainless tubing sold for railings on boats is 304 not 316. I always thought that 316 was better in terms of corrosion resistance. Is there any reason to use 304?

Maybe they use 304 because it is slightly less expensive.
304 is less expensive, easier to weld, stronger, and for railings wore than sutable since it won't be subject to tensile loads (which exacerbate stress crack corrosion). 316 is more corrosion resistant, but that isn't always the controlling factor.
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Old 31-03-2014, 16:46   #9
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Re: seamless versus welded tubing

We use 304 on aircraft due to it's greater strength, but it is nowhere near as corrosion resistant. If you want it pretty use 316 slightly oversized to make up for the strength loss.
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Old 31-03-2014, 17:07   #10
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Re: seamless versus welded tubing

Thanks for the info on 304 vs. 316. Now all I need to do is make a decision. Ugh.
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Old 31-03-2014, 17:17   #11
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Re: seamless versus welded tubing

It depends on alot of things, but you are on the right track. Much seamless tubing is simply drawn down to cold work the whole tube after welding. This pretty much makes the weld disappear. The cold worked weld ends up with properties closer to wrought material rather than cast properties of a weld. If plain welded tubing was annealed after welding, then the weld will not rust worse than the other part. If you only need a little bit, why skimp? the labor etc is a lot more problem than the cost of tube.
regarding 304 vs 316... there isnt a huge amount of difference really, either one will fail in poor circumstances. Every sailor is hot on 316, but it's not a huge difference. What's more important is the surface finish, passivation, electro polishing or polishing.
Regarding welding electrodes, I cant remember now for sure, but in the aircraft industry, but I think 309 is used alot for most the 300 series or 321 which has titanium in it.
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