Post Weld Heat Treatment of a stainless steel to carbon steel weldment 1

[RobsVette]

Hi Guys, Looking for some help here.

We have a situation involving a deaerator that I would like some assistance with.

We are instructed to run a stainless steel vent through a carbon steel shell that is 1" thick. Given that this is a deaerator, we will have to stress relieve the weld to prevent thermal shocking cracks and damage.

Aside from the need to qualify a procedure to do this, is tehre any reason not to perform the weld explained above? I had always thought you should try to avoid post weld heat treatments of stainless steel due to the possiblity of additional sensitization past what already happens during the weld.

Can you guys provide any insight as to what would be the best course of action here? I am planning to install a carbon steel line to penetrate the vessel wall and then do the dis-similar metal weld away from the deaerator.

Also, if you have any good links to resources concerning sensitization of stainless steel, please share them.

Thanks in advance for any help.
Rob

 

[EdStainless]

You should have welded a short extension or pad of CS with a low CE (low enough to not require PWHT) to the DA and then done the regular PWHT.
Then you could weld the SS to the extension and not worry about PWHT.

If you PWHT the SS you run the risk of sensitization.
How low is your C content?
The sensitized SS will be much more susceptible to both pitting corrosion and Cl stress corrosion cracking.

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P.E. Metallurgy, Plymouth Tube

 

[weldstan]

You can butter the carbon steel, PWHT the butter layer, and make the strength weld of the stainless steel to the buttered carbon steel without PWHT. You must still use a low carbon grade of stainless steel.
Or you can use a low carbon grade or extra low carbon grade of stainless steel to mitigate sensitization and weld directly to the carbon steel vessel followed by PWHT.

 

[metengr]

First, you do not need to perform local PWHT of the penetration because the thickness of the CS shell does not require PWHT based on the original code of construction. If you must PWHT because of other concerns, I would use a stub-in CS nozzle for the thickness of the shell, weld and PWHT. After this operation, butt weld the stainless steel vent line directly to the CS stub-in nozzle.

 

[weldstan]

I've seen more than a few deaerators built to ASME I.

 

[metengr]

Agreed. However, Section I has relaxed the nominal thickness to be in line with Section VII, Div 1.

 

[metengr]

Section VIII, Div 1, sorry for the typo.

 

[weldstan]

Thanks, metengr.

 

[RobsVette]

Hi Everyone,

As usual this site is a great aid in the engineering profession. You guys have basically confirmed what i had thought and i do appreciate the quick responses.

Thanks again to everyone.

Rob