Manufacture of Welded Tubing 1

[SnTMan]

OK, I am over my head here, but I know I'm in the right place, if not necessarily the right forum

We have to buy small quantities of the various product forms of duplex stainless steel, plate, pipe forgings, tubing and fittings. There is a rat's nest of specification requirements covering the purchase of the duplex materials. We attempeted to boil down the rat's nest for our vendors by writing purchase specs for each product form.

My specific issue at this time is welded lo-fin tubing. The strip manufacturer (as opposed to the tube manufacturer) has taken some exceptions / clarifications which I don't really understand.

To the requirement that the tubing be solution annealed at 1950-2010 F, exception is taken: Vendor only performs bright solution anneal with forced gas quenching.

My question: What is bright solution anneal?

To the requirement that each tube be eddy current tested in finished condition over its entire length a clarification is offered discussing no EC end effect if product is mill annealed, annealing above about 1925 F requires off-line annealing, cutting of each end to remove end effect.

My questions: What is mill anneal, end effect and offline anneal?

I just do not have enough knowledge of the tube manufacturing process to make sense of this, but I know people here do. Any explanation welcome, and thanks in advance.

Mike

 

[metengr]

SnTMan;

Vendor only performs bright solution anneal with forced gas quenching.

Bright anneal is typically in a furnace with hydrogen atmosphere which is reducing, meaning oxides on surface are reduced and you have bright metal surfaces. Upon leaving the bright anneal operation, the tubing is quenched with gas instead of liquid, as part of a solution treatment.

My questions: What is mill anneal, end effect and offline anneal?

Mill anneal means the tubing is subjected to an elevated temperature hold to ensure a homogenous single phase (austenite) for ferrous-based or nickel-based alloys before quenching.

The mill anneal can be done with either in-line processing (furnace chamber and subsequent quenching medium) or after tube manufacture can be done as a separate operation (off-line) using a stand alone annealing furnace with quench tank.

End effect is the beginning and end of a tube manufacture run where the skelp is formed, seam welded, solution annealed and quenched, followed by running through a shaper and ECT on-line. The ends of the tube are usually trimmed to length.
The above is based on my tube manufacturing audits over the years when we purchased heat exchanger or condenser tubing. One thing is this always ask the mill for an explanation if you are unsure or do not understand a process. Most reputable mills will fully explain any clarifications to your specification.

 

[SnTMan]

metengr, thank you, I intend to talk to the mill and our purchasing people as well, this forum is an additional (and valuable) resource.

I mis-stated in my OP, the vendor is the tube manufacturer not just the strip manufacturer. Another vendor will do the finning.

Can I assume that for this purpose that bright anneal is the same as mill (in-line) anneal?

By "beginning and end of a tube manufacture run" I interpret that the tube is manufactured in some convenient long length and afterwards trimmed to the order length.

The requirement that each tube be eddy current tested in finished condition over its entire length would appear to preclude (or be in addition to) the on-line ECT, do you agree?

Thanks again,

Mike

 

[metengr]

Mike;

Can I assume that for this purpose that bright anneal is the same as mill (in-line) anneal?

Yes, but this needs to be verified.

The requirement that each tube be eddy current tested in finished condition over its entire length would appear to preclude (or be in addition to) the on-line ECT, do you agree?

If this were my specification, I would require an ECT of the tubing prior to forming fins because this material needs to be confirmed as having no inherent defects either in the skelp (strip) or from welding.

The second part of this is the lo-fin process. I have dealt with heat exchanger tubing where the fins are rolled into the tube OD surface. You need to be made aware of two issues with this integral fin operation - the original solution annealed tubing will have local cold working imparted to the tube that will result in significant residual tensile stresses. This may or may not effect tube performance in service because this level of residual stress can induce stress corrosion cracking in certain alloys and in certain environments. This means the fin tubing may need an in-situ stress relief operation to remove the effects of cold work.

The second problem is having a final ECT performed on lo-fin tubing. This needs to be evaluated by the NDT organization that will examine the finished tube product. Integral fins can present a problem with ECT.

 

[Duwe6]

metengr, you are correct. Most finned tubing cannot be Eddy-Current tested and get any meaningful results. The welds/brazes of the fin attachment show up as small defects. Thus the ET of finned tubing can only detect large defects.

You can hydro and find the large defects, so ET after finning is a waste of money.

My guess on the Bright Anneal requirement is to make it easier to geet good quality fin attachment welds/brazes. Even a thin, thin oxide coat gives no-filler-added processes problems. Bright-to-bright metal welding/brazing is always best, and worth the extra money.

 

[SnTMan]

metengr, on reading the finning vendors quote I see that they will ECT each tube after finning. If the mill performs its normal ECT as well this would seem to meet the needs.

As to the cold-work, one part of the rat's nest is a specification dealing exclusively with duplex materials, with specific requirements for each product form. Spec seems most concerned with confirmation that intermetallic phases are not present. Testing per A-923. (I should say that I don't have a good understanding from a metallurgic standpoint of the composition, formation, etc of these phases, however I do understand that they have detrimental effects.) Lo-fin tubing is specifically exempted from soln anneal after forming.

Service would not appear to be particularily corrosive, air and water, but probably only the owner (who is not our customer) knows.

At any rate, cold-work of the tubing does not seem to be a concern.

Thanks again, this is going to be interesting

Mike

 

[metengr]

Duwe6;
I just went through a failure investigation of lofin tubing for an oil cooler heat exchanger we own that was retubed under an EPC contract (I was not involved but should have been). The fins are not brazed, they are rolled into the tube similar to the appearance of threaded rod to increase heat transfer.

The problem with my application was that the heat exchnager tubes were admiraltiy brass and locally cold worked from the lofin operation. Guess what, the lofin tubes failed in service from SCC because of dissolved ammonia levels in the canal water.

 

[rmw]

Does your spec mention cold working of the weld bead in any way?

rmw

 

[SnTMan]

rmw, no, it does not