Rolling AL6XN Tubes into SS316 tubesheet

[Afox88]

Hi Guys,

I have a customer asking us to roll AL6XN tubes into a SS316 tubesheet. I am obviously suggesting that they go with the AL6XN tubesheets as well, but I am not familiar enough with AL6XN to know if it is even possible to roll AL6XN tubes into a SS316 tubesheet. I know that some materials cannot be rolled into others (ex. SS316 tubes into brass tubesheet) because of differences in strength and elasticity but what is the criteria in making this decision?

Please advise,
Allan

 

[metengr]

The AL6XN tubes can be roll expanded into a 316 stainless steel tubesheet. By the way, 316 stainless steel tubes can be easily rolled into a Muntz metal or even brass tube sheet because the tube wall is relatively thin in comparison to the much thicker or stiffer tubesheet. The real concern in tube rolling is ensuring proper roll reduction and to minimize galvanic corrosion concerns between dissimilar materials.

You will need to develop a tube rolling procedure (measurements before and after rolling to ensure proper tube wall reduction) between the AL6XN and 316 stainless steel and provide a mock-up to validate the tube rolling procedure.

 

[SnTMan]

Afox88, some guidance can be found in Appendix A, see definition of fy. Per this, your expanded tube-ts joint would not require qualification by test (fy = 0.67).

The problem of course is that the tubesheet potentially yields more than the tubes and you don't get a good joint. Rolling a given tube can loosen previously rolled tubes.

This could be an adequate joint or not, depending on the service. I'd kind of consider it a marginal choice for a rolled-only joint.

A mockup would perhaps be in order.

Regards,

Mike

 

[metengr]

If this is for a condenser application, I see no concerns. If this is for a feedwater heater or other higher pressure heat exchanger component as mentioned by SnTMan , the requirements of ASME Section VIII, Div 1 would be required.

 

[Afox88]

Thank you for the feedback. We consider the requirement to do a mockup procedure to qualify a rolling procedure a no-go. Therefore, if a mock up procedure is required, we consider the materials as they are incompatible for rolling. (We obviously still do mockup for qualifying weld procedures)

Sntman, thanks for the info about Appendix A. At design temperature of 422F, the fy value is ~0.65 which is of course higher than 0.60. Therefore, we will advise our clients that it is acceptable to roll al6xn into ss316 but it is recommended to go al6xn also.

Thank You,

 

[Afox88]

Metengr, this is a low pressure exchanger. 300 psig 422F.

Thanks again.

 

[EdStainless]

Will these be seal welded?
Just asking because crevice corrosion of the 316 at the roll joint will be astronomical.

You really need a mock up and careful control of rolling torque, expansion alone is not a suitable control. With strong tubes and soft tubesheet there is no room for error (or re-rolling).

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P.E. Metallurgy, Plymouth Tube

 

[Afox88]

Ed, the tube-ts joint will not be seal welded. The tubesheet is a solid SS316 tubesheet, no overlay or cladding. We previously offered them al6xn tubesheet and tubeside as well, but I guess it became to pricey for their budget so they want just the tubes. My stance is that they should at least go with AL6XN tubesheet as well but this is a customer that hasn't heeded my advice before.

 

[metengr]

Afox88
Considering the design conditions (if this were a condenser application it would not matter), I would use a mock-up to demonstrate the effectivness of a rolled tube joint using the stated materials. If anything you can consider grooves in the tubesheet holes accepting the tubes to enhance leak tightness.

 

[Afox88]

Metengr, we will definetly double groove our tubesheet holes. That is a certainty as it is our standard.

 

[EdStainless]

Don't double groove, instead use more, and smaller grooves. You won't be able to force the 6XN into the larger grooves because the 316 is too soft.
There is data reported on dissimilar metal crevice corrosion.
At the joint the 316L tubesheet will see faster attack than if there were 316L tubes in it.


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P.E. Metallurgy, Plymouth Tube

 

[ASMEQC]

All great answers above. Just my thoughts to add to the conversation would be check the difference in thermal expansion on the tubes and the shell of the exchanger.