Furnace pwht or locally pwht for stainless nozzles in clad vessel

[honourable]

Dear sirs,
There is vessel with hydrogen service with material of sa 516 70 normalized with thickness more than 50 mm. We have 3mm clad of SS 304L (carbon content less than 0.02%).
It seems that heat treatment is necessary due to thickness and service. Vendor wants to use furnace for pwht. Our small nozzles(2 inches) are solid stainless. The question is that these nozzles must be welded before furnace or vessel must go to furnace without these nozzles and shall be locally pwht after furnace?in this case doesn't we have two times stress releiving which can make more probability for senthisizing?

 

[metengr]

I would suggest for the small nozzles to have the weld prep ends on the vessel where the nozzles will be attached by welding buttered with Inconel 82 or 182, PWHT the vessel and then install the solid nozzles on the butter layer weld preps using Inconel filler with no PWHT required.

 

[r6155]

PWHT without nozzle 2 NB 304L
Then Install the solid nozzles 304L,local preheat 200ºF and fillet weld each side of shell thickness.

Regards
r6155

 

[pradipgoswami]

Hydrogen service is an unforgiven environment. Clients usually insist on complete penetration type welds for integrity. Typical practice followed by fabricators for welding a 2" S.S nozzle on C.S vessel of over 2" shell thickness should be SET-ON & Full Penetration weld type.

The preferred option could be , do a Stainless(309L) or In-82(as recommended by MetEngr) weld build-up/buttering and machine the weld Lip on circumference of the drilled nozzle & perform PWHT. Weld the Stainless nozzle to the build up and machined weld lip after PWHT using the same welding consumables as sated above.

The other option could be weld 2" nozzle prior to PWHT. Put sufficient stiffeners around the circumference of the nozzle so as to prevent any distortion/warping during PWHT.

Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
Ontario,Canada.
ca.linkedin.com/pub/pradip-goswami/5/985/299

 

[DesertEagle01]

I would also suggest to go with Incoloy buttering and weld the 304L nozzle after PWHT. For hydrogen service you should go with full penetration welds. Performing PWHT with the nozzles welded should be prevented since your vessel will be subject to temperatures of about 600-700°C for some hours during PWHT and preticipates may form in the 304L (=sensitization) even if the carbon content is very low.
I've made good experience with buttering and PWHT as long as the buttering is big enough in size and thickness for further welding so that should be taken care of and checked in the PQR.

BR

 

[Trestala]

The 304L SS clad itself would be exposed to the PWHT of the CS part.

The OP mentioned that the clad has 0.02% carbon content, lower than the 0.03% carbon specified for 304L grade. So sensitization is no longer expected to the cladding?

The solid SS nozzle is also 304L grade. Can I ask if you expect sensitization to the solid SS nozzle, even if it is low carbon grade, why would the clad itself be exempted? Or is it ok for that to happen to the cladding? TIA.

Regards,
RaymondN

 

[metengr]

The 0.02%C low carbon content of the 304L clad will help regarding severity of sensitization, but it will still occur. This is why corrosion testing is performed on occasion to determine if the clad material will have corrosion issues in service after fabrication (e.g., PWHT of the vessel).

 

[r6155]

I do not recommend weld overlay in bore of SA 516-70 to install 2 NPS 304L
Inspection of weld overlays has typically included liquid penetrant examination, ultrasonic testing, ferrite testing, and
weld chemistry verification.
Full penetration weld 2 NPS (say sch 80S 5,5 mm set-on nozzle) may be difficult to weld free of defects or imperfections, and not easy to repair and inspect the root weld.

I insist with Fig UW-16.1 (j)

See NACE 8X194 Materials and Fabrication Practices for New Pressure Vessels Used in Wet H2S Refinery Service.

Regards
r6155

 

[honourable]

Thanks a lot for your kind guidance....

 

[Paulettaa]

The following is from ASM handbook volume 13. under the article of "corrosion of stainless steels":

I recommend you to first calculate the time required for the PWHT of the equipment for the material and thickness based on UCS-56. Then considering the solid stainless steel carbon content enter the above diagram. If the carbon content is such that the time to sensitization is more than the PWHT time then you can weld the nozzle before PWHT and there will be no problem. But if the time to sensitization is less than the PWHT time then you can follow one of the above advice.
It would be a great decision if your solid nozzle was of a low carbon grade like the cladding material since with less than 0.02 percent of carbon content the sensitization starts only after 100 hours of exposure to temperature of about 595 degrees but I do not know the carbon content of your nozzle.

Warm Regards