Titanium tube to tubesheet joints 2

[PressuresOn]

Dear All,

I know this topic has been briefly touched upon in other forums but I would like to ask a question and develop a better understanding of the problem.

We are currently reviewing a project where we would be asked to produce an expanded and seal welded tube end joint in Titanium material. We have expanded and welded literally millions of tube end joints of all different configurations for different codes - ASME, PD5500, EN13445, China Regulations, Stoomwezen etc.

The conventional knowledge and good practice is that the seal welding for most material grades is performed prior to expanding for two principal reasons as follows;

1) To enable the welding gases that may be trapped behind the weld to escape down the gap between the tube and tubehole - thereby avoiding blowoout through the weld
2) To avoid the situation where the presence of oil and fluids used during expansion do not have the opportunity to contaminate the weld pool.

Whilst we have some experience in Titanium there seems to be an opion based on experience whereby the expansion of Titanium tubes should take place prior to seal welding. How does this methodology overcome the two problems noted above?

In addition, we are lead to believe that the extrusion of the tube during conventional rolling would lead to problems with a clad type tubesheet - namely extrustion forces potentially causing the clad layer and interstitial layers to separate. In this instance we would commonly be directed towards tube hydroswaging as the extrusion forces are deemed to be lower with this application.

I would really appreciate it if anyone could shed any light or offer any direction towards guides, papers on the subject etc.

Thank you

Brian

 

[stanweld]

Rolling first mitigates air contamination at the root of the Ti seal weld.

 

[chicopee]

Are the tube ends beaded after they were expanded? if so why would you seal weld?

 

[EdStainless]

1.You must seal weld for two reasons:
a. Ti is soft and has a low modulus resulting in very low pullout strengths
b. the tubesheets are clad so you only have 1/4" of Ti on the face, you can reliably roll into this.
2. You don't want to hard roll before welding. You will have blow hole issues in the welds. Sometimes a light roll is used to control the weld gap.
3. When you do hard roll you have to stay at least 1/8" back from the weld, and the same (or more) in from the back face to the tubesheet.

The tube and hole had better be very clean when they go together. If there is any contamination you won't get good welds. Oxygen in backside isn't an issue as there is so little the Ti weld getters it right away.

Do not hydroexpand Ti, unless you are doing very small axial lengths. Otherwise you will pin the tube in the hole and end up thinning the wall. This often results in tears or splits.

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Plymouth Tube

 

[rmw]

Ed,

It has been a few years, but unless the rules have changed since I was deeply involved in that world, Ti to ti, clad or othewise was the only, and I mean only exception to the rules you outline above. It was hard (not contact) rolled first then welded, autogenous, of course. But that doesn't mean that is how it is now. It has been a while as you know.

If it has reversed, I'd be glad to learn that.

rmw

 

[EdStainless]

Depends a lot on tube size/wall/hole fit.
I have seen it go both ways.
Of course I have seen it screwed up both ways too.

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Plymouth Tube

 

[PressuresOn]

Hi folks,

Thank you so much for providing your insight and experience.

I think I may have worded the question badly and I would like to ask again in a slightly different manner.

The advances in tube end joining are such that 'experts' within the industry would advise us to proceed with a Ti to Ti tube end joint as follows;

1) Hydroswage lightly to pin the tube in position
2) Seal weld the tube end to the tubeplate
3) Hydroswage the tube to the required closeness of tube to tubehole desired

The advice is that hydroswaging is a better option than roller expanding when using clad tubesheets for the following reason - hard roller expanding basically extrudes the tubes and sets up stresses between the expanded region and the clad face where the end is welded - potentially leading to failure of the bond between the cladding and the base material when in operation.

Any thoughts?

Many thanks

Brian