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Requirements for stress relieving of heat exchanger tube u-bends? 3

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thermmech

Mechanical
Dec 13, 2004
103
Client's specifications say that u-bends of tubing in HEx need to be heat treated / solution annealed. I cannot figure out where does this requirement come from.

*TEMA RCB-2.33 Heat Treatment* says that: "...heat treatment may be performed by agreement..." but does not give any specifics.

API 660 says nothing about it.

I was not able to find any requirements in the Code in either SA-179 or SA-450 spec's.

Any help would be appreciated.

 
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s27289

There is not a Code or Standard clause asking you to heat treat the tube U bends;- it is the PV Code however, asking for the heat treatment of any component cold formed with residual stresses larger than 5% (please check the figure, it could be 3% or similar).

Since the plastic deformation of the tubes will be balanced by an amount of residual stress, say again 5%, that amount has to be removed by heat treatment, other ways the bend will crack in time, shorter for cycling stresses.
 
s27289, this requirement may be based on experience with a particular service, to improve corrosion resistance, for example.

Typically, only a couple of rows would be required to be stress-relieved per Code requirements on strain. See, for example, UCS-79 or UG-44. Even then the design temperature may have to be very high before before stress-relieving is required.

Regards,

Mike
 
gr2vessels, SnTMan;

Thank you for your posts. I have checked the above-mentioned paragraphs and found out that UG-79(a)/ UCS-79(d)(4) provide basis for heat treatment of U-bends for P-1 materials like SA-179.

Also, my question "How will heat treatment be carried out?" has been answered in UCS-79(d) as it refers to UCS-56 where temperature range and holding times are defined.

UHA-44(a) answered my question re solution annealing of U-bends of austenitic tubes. I guess that U-bends of SA-268-TP410 are not required to be heat treated as this is ferritic-martensitic steel.

Thank you again!
 
s27289, all well and good, but if the customer spec requires stress-relief I would not fail to do it without getting his OK first.

Good luck,

Mike
 
Good point, Mike.

S&T vendor also raised concern re TP410 tubes and stress relieving of U-bends. I will further investigate this requirement as customer's spec's are pretty generic, and post my findings for comments.

Thank you,
Sean
 
Most likely, the U-bends for TP410 SS will be formed under temperature (induction heating). If this is the case, SA-268 provides maximum hardness requirements for TP410. These will require post bend tempering to assure max hardness requirements. The TP410 max hardness criteria is important to assure minimal exposure to stress corrosion cracking in service.
 
Thank you, guys.

The last piece of info that I got from the tubing supplier said that SA-268-TP410 is a ferritic steel that is not succeptible to stress corrosion cracking - like austenitic grades. Therefore, solution annealing would be counterproductive and should not be performed.

I will try to get more information re forming and will post it up.

Regards,
Sean
 
The last piece of info that I got from the tubing supplier said that SA-268-TP410 is a ferritic steel that is not succeptible to stress corrosion cracking - like austenitic grades.

The TP 410 stainless steel is a martensitic stainless steel and beyond a certain strength level can develop stress corrosion cracking in certain environments. This material does not go thru a solution anneal like austenitic grades, it is supplied tempered at a minimum of 1200 deg F as a final heat treatment. So, you may indeed have a final heat treatment of the U-bends to meet the maximum hardness requirement of 95 HRb or 207 BHN.
 
metengr, thank again...

I guess I really need to brush up on my materials knowldege. Upon reviewing the chemistry of SA-268-TP410 UNS S41000, I found out that the Carbon content is 0.15% which makes it martensitic. That kinda makes me think that I need to confirm with the tube supplier the exact UNS of the alloy they are looking at.

Maybe we are talking about apples and oranges.

Sean
 
The 410 that will be used for tubing is probably less than 0.08% C and it is a ferritic. If so you will not be able to CSCC this alloy.

In my experience I have seen dozens of failures in U-bends related to the way that the stress relief was done. The biggest problem is getting the cooling rate correct for the alloy. It is easy to screw up microstructures.

In feedwater heaters you will not get CSCC or stainless steels in the ubends. The temperature is lower than at the inlet and since the steam is sturated there is not possability of wet/dry cycling to concentrate the impurities.
The bigger problem is in the desuperheat zone. You have wet/dry concentration of impurities, vibration and high temp. Everything is working against you.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
 
s27289,

I believe you already got the answers for your original questions (where is the HT requirement come from). Now you may have to consider below;

- What heat treatment procedure shall be applied? [See ASME Section VIII, Div.1, UCS 56 & UHA 32 per material]
- What is the required length to be stress relieved? i.e. whole U-tubes, one foot from T.L., or 3 feet from T.L. [Depends on the client’s requirement]
- How do you meet the tolerance of reduction of tube section area? [Depends on the client’s requirement and/or threshold delta pressure]
 
UCS-79 of Sect.VIII, Div. 1 does not necessarily apply. There is a list of 5 conditions (one of which must be met) for heat treatment to be required on the u-bend. (Lethal service, impact testing required, t > 5/8", wall thickness reduction of more than 10%, and cold forming between 250F and 900 F). For many (if not most) tube bundles, none of these conditions apply. (Note that bends are usually formed at room temperature)
 
After cold bend Exchangers U tubes is good practice stress relive U bends because cold forming induces tensions on the bends that if not relived are points of preferential accelerated corrosion.
 
Thanks to everyone for their comments!

I would like to address the issue whether stress relief is required or not:

UG-79 directs us to UCS-79(d)(4) which says that "other pressure boundary parts of carbon and low alloy steel plates fabricated by cold forming shall be heat treated subsequently (see UCS-56) when the resulting extreme fiber elongation is more than 5% from the as-rolled condition and any of the following conditions exist.

(4) The reduction by cold forming from the as-rolled
thickness is more than 10% at any location where the
extreme fiber elongation exceeds 5%."

TEMA limits the minimum bending radius, which sets the reduction of tube wall of tube bends in inner tuberows to up to 17%. That is more than 10% in the above paragraph. Also, if extreme fiber elongation for the same minimum radius bend is calculated, the value is 16.6% which is > 5%. So, the stress relieving due to cold forming is required, at least for inner tube rows and for P-1 materials.

I still haven't gotten the answer to what exactly grade of SS tubing are we getting from vendor. I will follow up on that.

Thank you once again.
Sean
 
I failed to mention in my last post that this paragraph only applies to plate products fabricated by cold forming.

Still, u-bend stress relief is very good practice. I would strongly recommend that u-bends be stress relieved, but I am not aware of any code/standard requirements regarding u-bend stress relief.
 
Stress relief of u-bends in stainless steel tuibing for use in steam service has no advantages and may introduce unintentional weaknesses.
The residual stresses from bending will result in high tensile stress on the inside of the bend. If you have enough Cl to drive CSCC, and you have an alloy that could crack, you will get failure in the desuperheat zone first. Temperature is a much stronger factor in CSCC than either Cl content or stress level.

It is easier to build a unit that has stress relieved tubes. The bends are more uniform and will stuff easier.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
 
I am not agree with you EdStainless.

As you said, residual stress from bending would result in high tensile stress on inside of the bend, I would complement saying, especially on short radius bends.

For this reason after cold bending, I keep on thinking that would be more beneficial to stress relieve the tubes than not to stress relieve.

Austenitic stainless steels may be susceptible to chloride stress corrosion cracking (CSCC). The standard 304/304L and 316/316L grades are most susceptible. High residual or applied stresses, temperatures above 65-71C (150-160F) and chlorides increase the likelihood of CSCC. Crevices and wet/dry locations such as liquid vapor interfaces are particularly likely to initiate CSCC in susceptible alloys.
 
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