Seemles vs. Welded Tubing
Seemles vs. Welded Tubing
(OP)
I am trying to figure out the strength difference between seamless tubing, drawn-over-mandrel tubing, and welded tubing. I know seamless is strongest and welded is weakest, but I don't know if this is a 10% difference, 2x, or 10x.
Just to give context, I'm building bicycle parts. Right now, I've been buying seamless 4130 chromoly, and copying wall thicknesses from existing parts (with a bit extra for safety margin). This has been getting kind of expensive, and I'm considering switching to other types of tubing, especially for early prototypes and non-critical components (racks, baskets, and the like). I haven't been able to find even ballpark numbers on how much weaker such parts would be, or alternatively, how much thicker I'd need to make walls. Right now, most of my tubes are 7/8" diameter, and 1-3mm thick. I expect to use some very narrow tubing later as well (1/4" or so).
Just to give context, I'm building bicycle parts. Right now, I've been buying seamless 4130 chromoly, and copying wall thicknesses from existing parts (with a bit extra for safety margin). This has been getting kind of expensive, and I'm considering switching to other types of tubing, especially for early prototypes and non-critical components (racks, baskets, and the like). I haven't been able to find even ballpark numbers on how much weaker such parts would be, or alternatively, how much thicker I'd need to make walls. Right now, most of my tubes are 7/8" diameter, and 1-3mm thick. I expect to use some very narrow tubing later as well (1/4" or so).





RE: Seemles vs. Welded Tubing
RE: Seemles vs. Welded Tubing
The key is how much NDT is performed as part of seam weld fabrication. If no NDT is used, you would need to use a knock down factor of 0.80 to compensate for inherent weld defects because a seam weld is considered cast material. So, the strength level would be reduced by 80% for no NDT.
RE: Seemles vs. Welded Tubing
If I use this for something like a rack or a basket, where there is no risk to the user, I can overspec width 2x relative to existing chromoly products, and be okay. For things like handlebars, frames, etc. where there is a risk, I would want to overspec 5x, since I don't know if the cheap steel tubing undergoes NDT. This makes seamed tubing not feasible for those applications.
In practice, the difference is likely to be lower, since the 4130 has a heat effected zone near my weld joints, whereas the seamed tubing does not.
Does this sound correct?
RE: Seemles vs. Welded Tubing
Why, because the wall thickness and microstructure are very uniform.
W&D will have better fatigue performance because the inside surface does not have the micro-tears and fractures from piercing.
In most Code applications generic welded tube is assumed to have 0.85 joint efficiency.
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RE: Seemles vs. Welded Tubing
But bicycle components have an extreme "wow" factor - where many geeks and uber-specialists (er, buyers) "want" the more expensive, fancier-sounding components and are willing to pay more for to get it first.
So, advertising "all seamless tubing" might be a sales factor, a price factor you can use in some models or some locations of your product. Not because it is any stronger with 100% NDE , but because it "sounds" stronger. 8<)
RE: Seemles vs. Welded Tubing
For thin wall tubing, welded + drawn tubing will perform as good or better than seamless, for the reasons mentioned by EdStainless (who has expert level knowledge on this proecss). It is very difficult to obtain small diameter, thin wall tubing from the seamless process, with aircraft-grade 4130 being the only real exception.
RE: Seemles vs. Welded Tubing
The carbon and alloy steels are available as seamless because those materials have very low work hardening factors and can be reduced to finished size from large starting hollows economically. Me make such tubing marketed as ProMoly.
Thanks TVP.
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