Tube to Tubesheet Strength Weld - Material selection

[Metlgst007]

Hello Material Experts,

My questions are regarding material selection for the tube to tubesheet joints that require strength weld as detailed in UW20.4.

(1) Is it possible to obtain strength weld between 410S tubes and 410S tubesheet ? I heard that it's difficult to get 410S plates with 0.08% max carbon. In that case, can we allow 410 plates with 0.12% max carbon and obtain a strength weld with 410S or similar consumable and preheat to avoid PWHT or it's advisable to use 309 or Nickel alloy consumables ?

(2) I understand that strength weld means, the fillet weld size is atleast equal to tube wall thickness. Tube wall thickness are generally less than 3mm thick, If we have cladded tubesheet, do we need more than 3mm clad thickness ? e.g. 316 tubes with 316 cladded carbon steel tubesheet require more than 3mm clad thickness ?

(3) How does axial tube strength is related to material of tube and tubesheet. Can I have a T to TS strength weld between 304 tube to carbon steel using 309 filler ? 304 is softer and I can achieve the matching strength with 309 filler.

(4) If I am using cladded tubesheet, do i have to clad tubesheet on the tubeside as well as shell side both to avoid galvanic corrosion ? E.g. 316 cladded carbon steel tubesheet and 304/316 tubes. Let's assume no strength weld is require.

Thank you very much to eng-tips and you all experts for your time and advise.

 

[MJCronin]

Thank you for your complete and well worded questions ....

Based on the specifics of your HX design questions, you may want to post in "Heat Transfer and Thermodynamics engineering" forum

MJCronin
Sr. Process Engineer

 

[EdStainless]

I'll skip #1 for now, that is an entire separate set of issues.

2) There are different styles of strength welds, this will dictate how thick your clad needs to be.
Do the tube ends project? Is there a chamfer in the tubesheet? If so, how deep is it?
But in general, if you are trying to develop full strength welds then the clad needs to be thicker than the tube walls.
You need to remember that full strength tube welds are usually found in thin wall tubing. Thicker tubing usually will have no trouble developing adequate strength as expanded, and if you are super concerned about sealing then seal welds are added.
3) Strength welds need to develop enough strength to suit the design, this should be far less than the strength of either the tube or the tubesheet. And since the welds are loaded in shear it is usually easy to have welds that are plenty strong even when they are softer than either of tube or tubesheet materials.
4) If there is that much risk of corrosion the shell side then the tubesheet should be solid and the shell, baffles, and tie rods should all be alloy construction. Shell side clad is very rare, as it is a real pain to build. You have to actually seal the tubes to the back clad in order to prevent crevice corrosion in the tube holes. This either involves thick clad and very careful tube expansion (expanding too near the back of the tubesheet can cause may problems) or a back-welded design.

If you want to discuss any of these further perhaps it would be best to start a new thread for each topic.
Regards

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P.E. Metallurgy, consulting work welcomed