Delaying PWHT of a Pressure Vessel after Completion of Welding 4

[deics]

Is it inadvisable (or bad practice) to delay PWHT of a 1.25Cr0.5Mo Pressure Vessel with a wall thickness of up to 133mm (Hydrogen Service)?

Low hydrogen consumables have been used. Appropriate Pre-Heats and post heating / DHT have been applied.

Why is this even a concern?

Logically, since on large pressure vessels, welding takes place over 6 months, an extra 2 month delay after completion of welding, should make no difference.

On the other hand, 'Failure Analysis Case Studies II' by D R H Jones P373
'UNUSUAL CASES OF WELD-ASSOCIATED CRACKING EXPERIENCED IN A HIGH TEMPERATURE CATALYST
REDUCTION REACTOR' (1991) makes mention of the delay of 2 months in the PWHT of a 1.0Cr0.5Mo 133mm thick Pressure Vessel as being a possible factor in causing a major crack of a nozzle in the HAZ of the forging. The report concludes that, amongst other factors, that there was a possibility of some delayed cold-cracking in the HAZ that could not be ruled out in view of the protracted delay between welding and PWHT, and if present, would also have assisted in the nucleation of the reheat cracking. The delay was November through January (winter months? No indication is given about where this vessel was fabricated.)

I find nowhere else any mention of delaying PWHT being a problem for this low alloy steel.
However I do find lots of references for high stress levels in welds after completion of welding at or even over yield. If those stresses are not relieved, what happens to the stress levels in welds? Local plastic deformation resulting in a reduction of stress levels or cold cracks in the HAZ or both? If there are such levels of residual stress in the welds it would seem logical to carry out stress relief ASAP...yet as pointed out earlier welds may go 6 months or more before PWHT is performed on large low alloy thick wall Pressure Vessels.

Do Low hydrogen consumables,higher Pre-Heats and post heating / DHT effectively eliminate cold cracking in the HAZ so that delaying PWHT is now not an issue?

 

[cloa]

You do need strenuous cleanliness or else the low Hydrogen consumables means nothing because a bit of grease has supplied the hydrogen instead. The design allows for considerable weld stresses- you aren't going to remove those stresses with such great thicknesses without metallurgical effects like grain growth. yield level weld stresses plus thermal stresses is probably excessive. Handling while welding is better than any other point in its lifespan.

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[deics]

CLOA - Thank for your guidance on how to avoid hydrogen pick up during welding. Good point often overlooked.

Your other comments are appreciated.

My main concern remains:

Is it OK to delay by 2 months the PWHT of a 1.25Cr 0.5Mo Pressure Vessel with a wall thickness of up to 133 mm (in Hydrogen Service) after completion of welding?

 

[metengr]

Is it OK to delay by 2 months the PWHT of a 1.25Cr 0.5Mo Pressure Vessel with a wall thickness of up to 133 mm (in Hydrogen Service) after completion of welding?

In this case, I would not recommend the delay of PWHT. Even though you can reduce the risk of delayed cracking during welding, you still have the locally hardened base metal heat affected zones and in hydrogen service, this could mean stress corrosion cracking in service. So, I would not recommend delayed PWHT for your specific application.

 

[deics]

Metengr - Thanks for your reply. Vessel is still in Fabricators at this time awaiting a replacement for a part that cracked after welding.

 

[metengr]

deics;
The fact that the vessel is still in production versus being placed into service is a better scenario regarding delayed PWHT. My advise above was to make sure that prior to hydrogen service, all PWHT must be completed (this includes repairs). For fabrication, delaying PWHT is not an issue as long as the fabrication welds are not exposed to moisture and NDT is performed before PWHT.

 

[deics]

Matengr - OK - that is inline with my thinking too... however the report I mention in my original post is of a different opinion and it concerned me enough to seek the opinion and experience of others. Again sincere thanks.

 

[metengr]

decis
This is why NDT before PWHT is critical during production, if PWHT is sequenced out or delayed for other reasons.

 

[deics]

In this case there is MT/UT/TOFD-UT before PWHT, MT and UT after PWHT and same again after Hydro Test. Totally agree with you, but doesn't catch everything as past experience with this item and service has shown.

 

[weldstan]

Have seen a number of hydrogen induced cracks in heavy wall vessels due to inadequate preheating and cool down. The Code recommended minimum preheat is inadequate for the defined heavy wall. If you decide to perform final Code PWHT after full vessel fabrication, I would advise employing a dehydrogenation treatment after completion of each major seam.

 

[deics]

Weldstan - Thanks for your good advice

We are following API 934 Part C as well as ASME VIII Div 2 for this vessel. DHT is being applied for this vessel per my initial post and that is applied for all pressure welds apart from the SS/HA Tail Pipe which is welded on after PWHT to a 800H Transition piece welded to the CrMo stub of the bottom nozzle before PWHT.

DHT applied is 250~300 degC for 4 hours.

The concern was only about any possible ill effects of delaying PWHT for a couple of months. Nobody on this forum seems to share the concerns mentioned in 1991 failure report I mentioned in my first post. I am much reassured by that.

I'd like to thank all those on the forum kind enough to respond so far.

 

[weldstan]

I would only advise an increase in the minimum DHT to 280C.
The DHT practice for heavy wall vessels vessels has been in practice by the BETTER heavy wall shops for over 25 years. Previously intermediate PWHT at lower than Code mandated PWHT was performed.
Your proposed NDE before and after PWHT will provide much better assurance of crack detection than was available when the cited (faiulure investigation) vessel was manufactured.

 

[pradipgoswami]

Hello Deics,

The recomemndations of API-934C on DHT is as follows:-

The DHT should be performed at 570 °F (300 °C) minimum for duration of one hour minimum.
ISR is not required. However if the purchaser decides to require an ISR, the temperature and hold time should be 1100 °F (593 °C) for 2 hours minimum.

The default welding processfor 133mm thick 1.25Cr0.5Mo steel would be by SAW(single or Tandem wire). All high quality SAW fluexs meant for all hydro-processing equipments are guranteed to produce be at least H5 type of weld metal, if manufactured & baked properly as per manufacturer's guidelines. In additon to preheat and IPT control during welding, baking and control of welding consumables are important.

If there's a significant delay betwwen the first weld+ DHT and final PWHT, then ISR could be another viable option instaed of DHT, off-course this would impact the Bills.
Hydroprocessing reactors take upto a year for complete fabrication. Furnace PWHT of complete vessel in one setting or in sections is favoured by the fabricators. To facilitate this if required ISR could be another option.

If you're using the products of any of the big players for welding consumables for hydroprocessing reactors, e.g Bohler-Thyssen, Kobelco, Oerlikon, Metrode-Lincoln or any other reputed product , you may ask the supplier's opinion also.

Between 1991 and 2013 welding development for such steels have occurred by leaps and bounds.

Thanks


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

 

[deics]

Hello Pradip

Thanks for your input.

All welding is completed on this vessel apart from one rejected nozzle. Awaiting the replacement and weld in before carrying out the PWHT. We assume all welding has been performed properly.

Assuming everything has been done properly...

Is there any problem with delaying the PWHT given DHT has been applied, low Hydrogen consumables, adequate preheats and correct welding have been applied?

The answer I seem to be getting from this forum is 'None'. I believe you are confirming that.

Thanks

dEICS