holding time of solution annealing for tube (Stainless Steel)

[Kurniatullah]

Dear all,
i have U-tube material with solution annealing due to extreme fiber elongation greater than 5%,

material : SA312-TP304
Thickness : 2.11mm
In mill certificate mentioned the holding time is 15 second. it is correct?

please advise or give me reference whether 15 second is correct or no?

Regard
KRN

 

[metengr]

Not enough information. What was the method of heating? Induction or convection?

 

[EdStainless]

I would guess either induction or resistance.
Resistance heating is common for ubend annealing. They clamp use beyond the tangent points and then use the tube itself as the heating element. You do need to be on the look out for arc burns at the clamp locations and the risk of overheating the inside radius of the bend on tighter bends.
If they were using optical temperature controls and aiming for 1950F as an anneal temp, then 15 sec is more than long enough.

Most people are under the mistaken impression that this anneal reduces residual stresses. It does remove the localized residual tensile stress on the inside of the ubend, but depending on the cooling method and rate it may leave the ubends with very high (but random) residual stress patterns.

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

There is no reference or standard for correctness. The organization that performs the solution anneal for u-bends should have a procedure qualified as such from actual u-bends because time is based on thickness and temperature. This is one of the items I check for when doing vendor audits on material we purchased in the past.

 

[EdStainless]

If these were purchased to a ubend specification (A688) then there is a requirement that you follow the minimum temperature in the tube spec for the ubend anneal.
That is the only specification guidance that there is.
Metengr is correct, if you are buying ubends this procedure needs to be on the pre-purchase document check list.
The other issue is do you want/need the ubends anneled?
In the US they are annealed, especially for power plant feedwater heaters. Even though there is no service related reason. It does result in more uniform ubends and better control of the radius and leg spacing which makes fabrication easier.
In most ubend heat exchangers it is more important to have low residual stress in the straight legs of the tubing since the conditions at the inlet are usually much more sever than at the bends.
And without special precautions the residual stress in the annealed ubends will be high anyway.

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P.E. Metallurgy, Plymouth Tube