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Welding vs Threading in Fatigue

09/23/2015 2:27 AM

In fatigue loading conditions, which type of connections work better, welded or threaded? Why?

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#1

Re: Welding vs Threading in Fatigue

09/23/2015 3:30 AM

Search CR4 for some answers!

Here is a good one from a very respected member of the forum!

Then there is this and this.

Last but not least you need to know the use of the pipe.

One thing to check for example: vibration!

The rest is up to you to sort out in your homework assignment.

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#2

Re: Welding vs Threading in Fatigue

09/23/2015 3:34 AM

I've never seen threaded sheet, bar, angle or strip. I've never seen brass or galvanised parts welded. You work it out.

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#3
In reply to #2

Re: Welding vs Threading in Fatigue

09/23/2015 4:44 AM

So we are not talking pipe? We are talking fasteners against welding?
We are in for a treat from OP.
Some minimum information!

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#7
In reply to #2

Re: Welding vs Threading in Fatigue

09/23/2015 10:56 AM

I can weld galvanized all day long!

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Al B. Coffin

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#16
In reply to #7

Re: Welding vs Threading in Fatigue

09/24/2015 2:16 AM

LOL!!!

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#14
In reply to #2

Re: Welding vs Threading in Fatigue

09/24/2015 12:49 AM

And you are funny that way.

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#4

Re: Welding vs Threading in Fatigue

09/23/2015 4:54 AM

Welded obviously, because everyone always starts with a threaded connection, then it breaks off, then they weld it. Usually with an overlapped coupling welded until there is no more room for weld and you end up a connection twice as thick as the original threaded one.

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#5

Re: Welding vs Threading in Fatigue

09/23/2015 6:31 AM

Welded, a threaded connection has many stress risers which are affected by cyclic loading.

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#15
In reply to #5

Re: Welding vs Threading in Fatigue

09/24/2015 1:13 AM

Fatigue points, you mean?

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#18
In reply to #15

Re: Welding vs Threading in Fatigue

09/24/2015 7:13 AM

No, please look up stress risers.

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#6

Re: Welding vs Threading in Fatigue

09/23/2015 10:08 AM

That could go either way. Depends on the situation. Bolts could be used and sacrificial in the loading as they are easily replaced.

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#8

Re: Welding vs Threading in Fatigue

09/23/2015 4:28 PM

Its hard to find threaded pipe, if that is what we are talking, any larger then about 6 inches. With flange fittings (weld neck or slip-on) welded by a class A welder, I have seen the pipe fail before the weld joint did.

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#9

Re: Welding vs Threading in Fatigue

09/23/2015 9:36 PM

Just glue it. That'll give you some stress relief.

Oh wait, what are we joining?

Must be a sewer because you're putting in garbage. You know the old saying? Garbage in, garbage out.

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#10

Re: Welding vs Threading in Fatigue

09/23/2015 11:41 PM

First, welcome to CR4!

Some of those who frequently post here are MUCH too harsh on posters who don't plan far enough ahead, to give appropriate details.

You are assuming too much! If you go back and re-read your original post, assuming NOTHING, you should be able to see that there is no information on what kind of connections you are asking about. A person whose principal training/experience is in the electrical/electronic field could interpret your question as: "Is it better to use screws or solder (commonly translated as 'weld' from some other languages) to attach wires that will flex frequently?".

You are probably talking about pipe or tubing, but we have no way of knowing.

Be as specific as possible, and you are much more likely to get a useful answer. If you are talking about pipe, then texasron has provided your answer.

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#11
In reply to #10

Re: Welding vs Threading in Fatigue

09/24/2015 12:26 AM

This was my first time posting a question on a forum ever...so I apologize for the obviously missing important details! You correctly deduced that I was talking about pipes.

Thank you so much for the advice! I'll keep this in mind going forward.

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#13
In reply to #11

Re: Welding vs Threading in Fatigue

09/24/2015 12:33 AM

Perhaps some time spent reading the CR4 Rules of Conduct and the CR4 FAQs would be well spent.

Then, some time spent simply observing the workings of the forum would also have helped you.

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#25
In reply to #11

Re: Welding vs Threading in Fatigue

09/24/2015 4:37 PM

What size pipe?

What material?

What is it transporting?

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#12

Re: Welding vs Threading in Fatigue

09/24/2015 12:30 AM

Welding and proper annealing would be best. But say maintenance wise, if you consider convenience and replacement of parts is necessary, threaded would be a good choice.

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#17

Re: Welding vs Threading in Fatigue

09/24/2015 2:29 AM

Firstly, I have to admit to only being a rank amateur welder.

But the RN, always threads and welds high pressure steam flanges.

In WW2, they just welded for speed of production.

One such weld broke on a WW2 built aircraft carrier sometime in the 1950's and killed everyone in that particular engine/boiler room.

So I would guess its decided by the the application and the stresses involved, plus the possible effects of a failure?....you gave no details of that to help the professional welders here (NOT ME!!) to answer you!!!

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#19

Re: Welding vs Threading in Fatigue

09/24/2015 7:51 AM

Pro welders are often quoted saying that a good weld should be as strong as the surounding substrate.

Compare three scenarios.

1. One single solid part

2. Two sub-parts welded together to give something that looks and feels like 1.

3. Two sub-parts screwed together to give something that looks and feels like 1.

Suppose we can assume that good welds with correct bead diameter and penetration with no embrittlement or hardening ( ideal world stuff ) are as strong as the surounding substrate then it seems reasonable to assume cases 1. and 2. deliver about the same level of strength - lets just assume an ideal case so there really is not a whole lot of difference between single solid part and welded part.

With all these caveats in place the question seems to boil down to ....

1. Can I cut something made of a single homogeneous material and join it back together again in such a way that it is stronger than before I cut it?

Answer is most likely NO ?

I am sure there are some esoteric exceptions but unless the welds are particularly vulnerable to stress or fracture I think the above reasoning might be ok.

Also as another poster observed machining involves removing material and thinning - ok an ideal threaded pipe joint may have same OD and ID everywhere including the joint but the contact surfaces of the thread are not the same as a single fused part - not all threaded areas will be taking the strain or resist strains in the same way as a homegeous single solid part.

Add real world things like couplings that have considerably more material in them than the surounding pipe and you probably change things but I started out assuming similar geometries for the above.

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#20
In reply to #19

Re: Welding vs Threading in Fatigue

09/24/2015 8:45 AM

Even welding engineers will tell you that the actual weld is not the weakest point of a joint. A properly designed and executed weld joint should outlast the parent material and not be a point of failure - the parent material should fail prior to the weld.

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#21

Re: Welding vs Threading in Fatigue

09/24/2015 10:43 AM

Welding if executed correctly, is mechanically stronger than threaded.

However, even though it is stronger it may not be feasible or cost effective in some applications and therefore will not work better in those instances.

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#22

Re: Welding vs Threading in Fatigue

09/24/2015 11:47 AM

If we are comparing welded pipe against threaded pipe, I would go for Welded pipe based on expereince. If you are using a steel pipe, you would be threading a SCH 80 pipe but welding a SCH 40 pipe.

Although you have a thicker pipe to thread, the threads have high stress concentrated areas and can become a shear point under load. The exposured threads after you connect and fastened can also become a weak point due to corrosion if you do not remove the remaining exposed thread tape, wire brush and apply anti-corrosive paint for protection.

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#23

Re: Welding vs Threading in Fatigue

09/24/2015 11:59 AM

For corrosion reasons, it is best in an exposed environment (and under insulation and lagging to weld the joint, as threads always produce weakness in the metal (if standard thread tapers are used or not). If you want to go on the cheap, and there is no possibility of harm to any personnel by having a failure, then go for cheap threaded stuff - at least for a prototype. If you go into production of numbers of this (whatever), then you should weld it and anneal the metal according to codes that apply. If this is for hazardous material transfer piping, then forget threads.

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#24

Re: Welding vs Threading in Fatigue

09/24/2015 3:50 PM

In the high reliability semiconductor business, parts are frequently welded because they often must pass helium leak testing. Organic materials such as epoxy can't be trusted to pass stringent leak testing.

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