Seemles vs. Welded Tubing 3

[RobertHM]

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).

 

[blacksmith37]

For low alloy steels (like 4130) ,the strength level depends primarily on the heat-treatment. Generally such steels will not be used in a mill condition (hot rolled, cold rolled, etc)since you a paying a premium for a heat-treatable alloy (more than carbon steel).

 

[metengr]

Well, from my experience with codes/standards, for material that is seam welded (with filler metal or autogenously) and subsequently nondestructively examined with RT ot UT, the Codes and Standards do not typically differentiate strength differences for application where time dependent properties (creep, plastic deformation, etc) do not apply.

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.

 

[RobertHM]

Thank you both. From your comments, my conclusion is that seam-welded steel is of a material that doesn't improve from heat treatment, so I can assume that, if properly welded (as guaranteed by NDT), it is the same strength as seamless of the same type. Weaker, non-heat-treatable steels are about 2x weaker than 4130, so I can assume a factor of 2 worst-case difference.

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?

 

[EdStainless]

With proper heat treatment and NDT the welded and drawn tubing will always out perform the seamless in destructive testing.
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.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[racookpe1978]

Yes - ALL of the above is true.

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<)

 

[TVP]

RobertHM,

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.

 

[EdStainless]

The highest performance bicycle frame tubes made are all welded tubing. They have been cold worked enough after welding that you will never find the welds. These are exotic PH stainless grades.

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.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube