Alloy 600 U-bend tube stress relieving

[TommasoDG]

Dear Readers
I heve several SB 163 UNS N06600 (Ø31.75 x 3.759) U-tubes. The cold U-bended area, plus approximately 6 in. [150 mm] of each leg beyond the tangent point of the U-bend, shall be heat treated at 1150°C.

I want know what is the minimum soaking time, fixed by Code or others, in case of stress relieving by resistance-heating method.

 

[EdStainless]

There is no min time. We used to use 5 sec as an internal control. For stress relief there is no kinetic limit involved (no phase transformation or diffusion) so time is almost irrelevant. In every case that I have seen by the time that they reach temp they are fine.
Now since this is an austenitic alloy you will be reheating to the solution anneal temp.
The ID does need to be purged with an inert gas to minimize oxidation, and the contact locations need to be inspected for arc burns.


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P.E. Metallurgy, consulting work welcomed

 

[isoca]

If you want to perform this HT under ASME code you can check it in the ASME Code SECTION VIII — DIVISION 1 UNF-79 REQUIREMENTS FOR POSTFABRICATION HEAT TREATMENT DUE TO STRAINING

 

[EdStainless]

But UNF-79 does not strictly apply, tubing is material not fabrications.
I'll also add that the residual stresses in the bend is controlled by the cooling process. Cooled in a controlled manner (moderate rate and uniform) the residual stress will be low, rapidly cooled (especially unevenly) and the stresses will be high (random but high).

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P.E. Metallurgy, consulting work welcomed

 

[isoca]

Yes I know, but if what you want is a reference, it is the only thing that I have found that mentions some parameter of heat treatment for these cases.
In fact, if one is based on what is indicated by the code, this type of treatment is generally not necessary.
As Ed mentions, cooling and radius calibration after heat treatment can reintroduce stresses into the material.