Duplex u-bended tubes

[EuroWeld]

Hello all,

My question relates to the different methods used for bending and heat treating duplex stainless steel (heat exchanger) u-tubes.
To avoid metallurgical problems with the duplex grade material it is prefered to bend the tubes cold. In some cases specifications prescribe solution annealing and quenching after bending even if the bending is done cold.

Can somebody inform me about the exact reason(s) for this? Does someone have some expierence with this subject before?

What methods are commenly used? What are do's and what are the dont's with bending and heat treating duplex tube material?
Is it sufficient to quench the tubes with air after solution annealing? What is common practice?
We try to get more insight information on this issue because we recently intercepted a bundle of tube that was full with intermetallic phases located just before and in the bended area. We want to avoid this to happen again and we are working on preventive measures at this moment.
For your information we are already is discussion with our supplier.

Many questions but all little information is more than welcome.

Thank you, Weldfan

 

[EdStainless]

It greatly depends on the service that they are going into.
The post-bend anneal can be used in austenitic alloys to achieve low residual stress levels in the bends.
There is no reason to anneal the ubends in duplex tubes.
With a duplex there is more chance to screw them up than possible benefit to be gained.

That said, non-annealed ubends will not have as good of dimensional uniformity (of the bends).
This can make stuffing a heat exchanger bundle a real challenge.

Ubends are usually annealed by heating using electrical resistance.
They clamp the tube on either side of the bend and pass a current through the tube.
optical systems are commonly used for temperature control.
cooling is the big issue. To achieve low residual stress the cooling has to be gentle and very uniform.
With a duplex alloy you have to cool faster, and any slight non-uniformity will result in significant residual stresses.
It is best to not bother.

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Plymouth Tube

 

[OGMetEngr]

The only issue with cold-bent duplex stainless steels is possible hydrogen embrittlement. Once you get over about 32 HRC for super duplex this can be an issue. I have seen super duplex piping cold bent to a small radius with hardness at the extrados around 35 HRC. Many bends cracked after exposure to cathodic protection in seawater. Ambient and below-ambient temperatures are the worst temperatures for this. Depends entirely on operating conditions if this is an issue for you (and if it's duplex or super duplex).

As for intermetallic phases, this is due to spending too much time in the intermediate temperature range. This could be as wide an interval as 1300-1950F depending on grade. Lowered fracture toughness and corrosion resistance will result.

 

[XL83NL]

(Cl-)SCC may be or become a problem. You might want to get a copy of API TR 938 C, as it discusses these phenomena, and the use of DSS in refinery applications.

 

[EdStainless]

OGM; If you had HE cracking in a duplex your cathodic protection system was way out of control.

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Plymouth Tube

 

[OGMetEngr]

CP was a passive system, AlZnIn alloy. The protection potential is around -1050mV. When super duplex is at the potential hydrogen is produced at the surface. I would imagine duplex would react the same, just not sure at what hardness it would be an issue for the lower alloyed duplex grades.

Tubes closest to the anodes cracked, further away none cracked. I explain this in that tubes closest to anodes were at or very near protection potential, while further away it was more positive and less hydrogen was produced.

 

[metengr]

Duplex stainless steel tubing that is cold bent should be locally re-solution annealed to avoid susceptibility to stress corrosion cracking in service.

 

[EdStainless]

Unless the environment in highly likely to drive SCC you will likely cause more damage doing local re-anneal.

OMG: Over 1000mV! sounds like a design problem. There is no reason for over 750mV for any CP system that I have ever seen.

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Plymouth Tube

 

[EuroWeld]

Thank you all for given information.

@Metengr: If you perform a locally re-solution annealing of the bend area how can you secure that the transfer zone is free of intermetallic phases due to uncontrolled/slower cooling? I assume that this zone is also very sensitive for SCC during service? Is it better to heat treat the complete bended tube to avoid this transfer zone to occur? What is the correct method to do this?

Do you believe that duplex bends should always be re-sa&q or only if SSC is expected?

 

[metengr]

I will answer your last question first, despite what suppliers, tube manufacturer’s say cold formed bends in austenitic, Ni-base and duplex stainless steel in either elevated (above ambient) temperature service and/or corrosive service should be thermally treated to remove harmful residual tensile stresses after forming (this gives the material the best chance at optimum corrosion performance). Being on the Code committee, I can't tell you how many times I have seen code cases submitted with forming limits and reports provided of failures caused by harmful residual forming strains.

Second, local re-solution treatment is commonly done over the entire u-bend and tangent points. If done correctly, the so-called thermal transition region is so narrow and with proper quenching will not result in harmful intermetallic phases. You need to develop a u-bend solution treatment procedure to assure proper controls and consistency. Practice on several pieces of tube material and qualify the procedure using metallographic examination and hardness testing.

I would not re-solution treat the entire tube because you may get significant distortion and trying to install the tube in a heat exchanger could be tricky.

Do you believe that duplex bends should always be re-sa&q or only if SSC is expected?

Only if the application for the formed tube or pipe is for critical plant or process equipment, and the environment contains the following -cyclic service duty (fatigue) and corrosive conditions.

 

[EdStainless]

The issue that I have is the myth that by re-annealing the ubends you have addressed residual stresses.
This is not the case.
If ubends are rapidly cooled, as some alloys require, then the residual stress will be equal to the yield strength of the material.
The only difference is that instead to the inside of the bend being in tension and the outside in compression and annealed bend will have a random stress pattern.
It is possible to make low residual stress bends by carefully controlling the cooling rates, but this could be very detrimental to a duplex alloy.
The transition regions have never been a problem that I have ever seen.
But if that worries you what about the oxide that forms on the ubend, this significantly lowers corrosion resistance on the outside of the tube.
In practice I have never seen a ubend corrosion or CSCC failure that didn't have much worse attack on the straight legs near the inlet.

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Plymouth Tube

 

[EuroWeld]

@XL83NL: Thank you for informing me about the API TR938-C, this is exactly the information were I was looking for.

@Edstainless: Tubes are filled with Argon during the heat treatment after bending and the outside of the tubes are pickled and passivated followed by a neutralizing and rinsing treatment.

In our case we found the intermetallics (around 2-3%σ)in the transition zone of the bend/straight legs at both sides at appr. 80% of the total quantity of all tubes. So something went wrong during bending or heat treatment and we are not sure yet what.