Bend a tube with heat 7

[legacy]

I want to find someone who can demonstrate that he or she can cause a semi-rigid tube to repeatedly bend in all directions beyond 4 (or so) degrees of its axis due to the tube being heated along one edge. The tube can be made of metal, a combination of metals, or any other heat-resistant, resilient, fatigue-resistant material(s), with the length of the tube not to exceed a multiple of 30 times its diameter.
Can you do this, or do you know of someone who can? Has it already been done?
I'm working on an invention.

 

[EdStainless]

Find an old time muffler guy. They used to torch bend everything. By using alternate heating and cooling they could put a dog-leg into an exhaust pipe. It doesn't do great things to the metals microstructure, but it works.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[deanc]

Go to any boiler/vessel Job Shop. This is still common practice. Used to do it alot myself(couple years back) Books? Try Amazon and look for The Welding Encyclopedia by LB MacKenzie/T Jefferson. "Flame Straightening"

Also check with Lincoln,AWS,and old blacksmith books.

 

[unclesyd]

I have taken SS hypodermic and using glass blower torches and made reproducible bends with reasonable tolerances, though they exceed your requirement of 30 x D.

Get a piece of say 1/2" or 1" diameter SS pipe and small rosebud/heating torch and go to work and after a little practice add a water hose to you toolbox. If you haven’t witnessed any of this type work you will be in awe what can be accomplished.


Aside from the above mention areas of expertise you should also check any established steel erection, ironworker, shop. There are many ironworkers that can take a rosebud torch and water hose and do marvelous things with steel sections. There are/were companies that specialize in reclaiming steel distorted by fire.

We use flame straightening routinely in our fab shop to realign bearing journals on large SS shaft,16'long, assemblies. It also routine to flame straighten shafts of all types in the machine shop between centers.

Older pipefitters/boiler makers were especially adept in using heat to bend and align pipe either on slab or during erection to ensure fit. One fitter I had the pleasure of working with didn't know what a pipe fitting other that a weldneck flange was. His pipe work from the 50's and 60's still impresses people and makes them ask how did they do this.

I have personally straightening SS components, agitator hubs, pump impellers, and assemblies using either a heating torch or fusion with a tig torch. There is one big caveat on SS assemblies that are welded, heat will move it but who knows where.


There is another older book "Flame Straightening" by Stewart, Canada.

http://www.steelstraightening.com/index.htm

 

[btrueblood]

Um, not to take anything away from the experts, but I read his post and interpreted "repeatedly bend" as having the tube bend and straighten some number of times. The flame bending/straightening that Ed, unclesyd, and deanc mention are techniques to create a more "permanent" or plastic deformation, are they not? I mean, you might be able to bend & flame straighten a tube a few times, but may soon end up with low-cycle fatigue failures? Or am I off my nut (again)?

 

[legacy]

Thanks, btrueblood, for picking up on my wanting the pipe to bend "repeatedly." I want to flex the pipe repeatedly and I want to avoid any extreme heat that would fatigue the metal.
You know how a thermostat can flexfor years with not breaking? That's the kind of thing I'm looking for, except that I need the pipe to start out straight and return to straight after the heat is removed. I know I'm not being perfectly clear, but I don't want to disclose the "secret" of what I'm attempting to do. However, I will share the details with the one who can demonstrate what I need.
I appreciate all of you responding to my question.

 

[Rich2001]

Take a look at Titamium Nickel Memory metal Nitinol

http://mrsec.wisc.edu/edetc/memmetal/

http://www.dynalloy.com/index.html

http://www.memry.com/

 

[unclesyd]

Sorry about that, it seems that I was told a long time ago to read the question but have seem to have forgot or have just got too lazy.

Rich2001 has the answer.

The problem with most MOC for pipe and tubing is that repeated heating and cooling will cause the line to axially grow especially if there is a bend.

As mentioned by btureblood there is a fatigue problem especially with SS called differential thermal fatigue. I have never seen any problem with mechanical fatigue upon heating and cooling.

 

[btrueblood]

Unclesyd, I wasn't thinking of differential thermal fatigue (aka ratcheting, although I'm not sure if thats a well defined term). I was just thinking of the repeated bending (by any means) of a tube, where a plastic deformation is induced. Essentially, you are saying that with each thermal straightening "cycle" the tube will be thinning - because one side is getting stretched or squashed - won't this repeated stretch/squash in the plastic region cause a low cycle fatigue crack to open up? Or...thinking about it some more, maybe not, because the plastic work is always done on the compression side...but then how does the tube end up stretching? Now I'm really confused.

 

[metengr]

btrueblood;
Flame straightening from my experience is a technique that is done using one or two cycles (applications) for purposes of either straightening an object or to produce a unique bend or to remove previous distortion. The concept is to apply heat locally to introduce a residual stress that locally exceeds the yield strength of the material. Spot heating provides the maximum benefit to move material.

The initial application of heat to the surface of the component will produce a tensile stress that upon rapid cooling will revert to a compressive stress at the location where heat was applied. The local compressive stress is what pulls the material in and results in local permanent deformation.

The bottom line is that the application of local repetitive heating and cooling cycles can result in low cycle fatigue crack propagation at any stress risers on the surface of the component.

I personally have never seen a tube or pipe bend elongate as a result of heating the bend with a torch because we had never needed to use more than two applications of heat. However, if the tube does indeed thin as mentioned above from multiple applications of heat, the tube material has to move in other directions (circumferential and axial) because of conservation of volume.

One example of the effects of repeated heating and cooling cycles is a steam line that suddenly cools as a result of thermal stratification (unit trip that results in steam collapsing to condensate as the line cools). A horizontal pipe that suffers from this phenomenon takes on a "banana" shape that is referred to as steam line humping. When a steam line develops this condition, low cycle fatigue damage can occur at girth welds that can result in cracks or failure.

 

[unclesyd]

Low cycle differential thermal fatigue (DTF) especially in SS is real and working all the time. It also happens with CS and other alloys. A good place to look for DTF where you would use a tee for mixer. .
We have found DTF in service with as little as 55° F temperature differential. This was the in mixing of two streams using the afore mentioned tee and it occurred just by injecting a hot stream (125°F) into a larger stream at 70° F. Even though the velocity and turbulence was very high at this point we had DTF for about 2' from the injection point. They were no stresses other than hoop stress operating at 150 psig (8" Sch 40 S/S pipe). I’ve seen both single and multiple cracks. The single cracking was normally found in the lower temperature ranges or changes in section of a SS vessel. I’ve seen this DTF several times a low temperatures and lower temperature differentials, quite a bit lower than most literature mentions. Another good place is where you have a cold liquid dripping on a hot line

On pipe that is affected, growing, you aren't always working on the compression side. We see this in jacketed pipe, mainly SS. It is most pronounced in pipe heated with vaporized oil operating at 250°C with several bends. After about 10 cycles the inside bends start touching the jacket at each bend. We compensate for this pulling the core pipe to where it almost touching on the inside radius of the jacket. We also see this with steam heated lines but it takes few more cycles.

We also see growth in straight runs of piping due to the cleaning process. The uneven heating comes from the pipe being partially full of polymer. The pipe is flame straightened by a fab shop mechanic after each cycle and after several cycles the pipe is replaced or has to be shortened.

We have demonstrated this on the bench using SS pipe by making a bend and reversing it. Normally there is a measurable difference in length after 3 to 4 cycles.


The banana boat effect mentioned by meteng is very common on our jacketed SS equipment heated with vaporized Therminol at 283°C. This affect is seen every time the equipment is stated up. Introduction of the Therminol vapor results in a tea kettle effect, causing extreme temperature differentials. This causes the condensate to quickly accumulate in lower part of the vessel and both ends rise about 5/8" on a 16' to 20' long; 5' to 6' dia. vessel. We have suffered DTF on the older vessels judging from the surface appearance at the crack site. Also the older vessels are 1 1/4" longer than the original dimensions.
We have other smaller diameter though longer vessels that have the same problems from the puddling of Therminol condensate in lower part of the vessels.