SR Of U-Bends

[SnTMan]

Looking for some advice here

Supplying two U-tube exchangers exposed to a common process, blowdown at a high temp and pressure. One set of tubes is welded 304, specified to be supplied as solution annealed and meeting ASTM A262, Practice A or E. The other set is seamless alloy 600, N06600.

Customer has reported CISC of the 304 tubes as operational experience, while the Inconel tubes were reported to have cracks or pitting at the tangent.

Question is: should either or both sets of U-tubes be stress relieved after forming? The customer has not specified such, but I'll do it if it makes sense.

Thanks in advance,

Mike

 

[metengr]

Yes and Yes for post forming thermal treatment to ensure maximum corrosion resistance.

 

[SnTMan]

metengr, I suspected so, but thanks.

I would normally electric resistance 304 per SA-688. Could you provide a reference for the Inconel? Not overly familiar with it.

Thanks again,

Mike

 

[metengr]

For Inconel 600, the nuke folks did a lot of work regarding susceptibility to SCC in PWR enviornments

http://www.ijaea.com/admin/archive/IJAEA-5-6-131.01.20141391163744.pdf

 

[SnTMan]

metengr, thanks for the link. My (brief) research suggests elec resistance SR at maybe 1100F and quench.

Comments?

 

[metengr]

SnTMan
No. Follow the recommended minimum heat treatment temperature based on Table UNF-79 for post fabrication strain limits. 1900 deg F and forced air cool or water quench.

 

[SnTMan]

Table UNF-79. Duhhh. Thanks, metengr

 

[SnTMan]

metengr, a little more discussion, please.

Design temperature is not so high as the post-fab requirements of UNF-79, it is 600F. Would you still stress relieve, given the reported experience? Strain of the innermost row would be around 30%.

Would an 0.095 wall tube really be subject to a 10 minute SR? I thought tubing was treated for more like seconds rather than minutes.

Thanks again,

Mike

 

[SnTMan]

Sorry, should have said .049 wall.

 

[metengr]

SnTMan
Understood. I cited the table for temperature exposure for this material. From my 30+ years of experience with heat exchanger tube materials, I always found that thermal treatment performed on u-bends rendered maximum corrosion resistance and elevated temperature strength/ductility (not applicable in your situation). Residual forming stresses in u-bends to me pose a risk of stress corrosion cracking. It is more expensive, no doubt.

If you use induction heating for this tube wall thickness, you are talking about heating in seconds. If you use conventional heating resistance heating, it is minutes. By the way, I would recommend you qualify your u-bend thermal treatment procedure on a few spare bends to ensure proper microstructure and hardness. Send the samples to a metallurgical lab.

 

[SnTMan]

metengr, given the experience I am going to recommend that it be done. The forming and any SR will be done by an approved vendor. I think I now know how to specify. i.e "electric resistance or induction SR at 1900 F with rapid cool".

Be nice if there was a spec to cite, such as SA-688 for SS.

Thanks again,

Mike

 

[Haiker]

If you want the best corrosion resistance you should consider pickling after solution annealing as well.

 

[SnTMan]

Haiker, yeah well, whole issue is up to the "bean counters" now

We'll see if it gets done at all, or not.

Thanks to you both for the input though.

Regards,

Mike