Post Weld Heat Treatment (PWHT) 2

[VPN16]

Currently I am designing on ASME section-VIII Div-1 (Code)drum with 'U' stamp. Drum is SA 516 Gr 70N and the shell and head thickness approximately 80 mm. Stainless steel SS 316L internals are placed inside drum. The fabricated drum shall be PWHT as per the Code. The SS 316L internals are welded inside the SA 516 Gr 70N Shell. How to perform post weld heat treatment (PWHT) for the vesel. Is there any pre-caution or notes, I have to include in the data sheets for the fabricatot to observe?

 

[metengr]

Yes, one comment regarding PWHT. Is the welding for attaching the stainless steel internals to the drum scheduled after the vessel PWHT? Since the 316L series stainless does not require PWHT, I would consider the attachment welds to be completed after PWHT of the vessel to avoid any possible carbide/sigma phase precipitation of the 316L series stainless internals. It is not good practice to have 3XX series SS exposed to the PWHT time at temperature for the P1 material.

I would think, and you have to confirm this with your design, that the attachment welds joining the stainless (P8) internals to the carbon steel vessel wall (P1) would be exempt from PWHT, based on the nominal thickness of the weld deposit.

Second comment concerns the selection of filler metal for attaching the stainless internals to the carbon steel drum. I would give serious consideration to using a nickel-base filler metal like Inconel for these attachment welds. Two reasons; the linear coefficient of expansion for the nickel-base filler metal is between the linear coefficient of expansion for 316L SS and carbon steel reducing thermal stresses across the weld fusion zones. Second reason is the potential for carbon migration resulting in premature failure of the weld fusion zone on the carbon steel side of the attachment weld should the operating temperature of the vessel exceed 650 deg F. A 309SS filler metal is common for dissimilar metal welds. However, over time the carbon will diffuse from the carbon steel side of the fusion zone to the higher chromium weld deposit. To reduce carbon diffusion, a nickel-base filler metal is used.

 

[unclesyd]

We try to never weld SS attachments to a CS vessel as yours. We would weld a properly designed clip from the same material as the drum material and after PWHT weld/bolt the SS to said clip. We have been quite successful in this approach both with the AI and the operational point of view.

SS to CS with temperature doesn't give me that warm fuzzy feeling. Mainly due to all the reasons stated above and some.

 

[VPN16]

Thanks for both the inputs. However the internals shall be placed through the dished end opening and after the internals are welded, dished end also welded. The whole vessel shall be PWHT. The internals are welded inside of the shell and not bolted. Based on the size of the internals, it not possible to install or remove through the man-hole opening.

 

[unclesyd]

I still would use clips attached to the pressure boundary in case there is problems as mentioned by metengr in the transition weld area. The clips would again allow you later replacement of the SS without any problems with PWHT, in other words remove the possibility of having to PWHT the any part of the shell.

 

[arto]

Use the clips like they say above -
we usta call them "Poison Pads" - they keep the metallurgically/thermally bad stuff out of the pressure retaining wall of the vessel.