Use of proper electrode

[sjuju]

We have manufactured a pressure vessel for testing purposes (no ASME U stamp) out of Ni alloy 800H.
All welds were welded using the ERNiCr-3 electrode. The vessel was PWHTed as per ASME requirements (~890 deg C).
After operating for short time (2400 hours) some repairs are necessary and one of the pressure retaining welds is to be cut and re-welded. No PWHT can be performed this time, as this will damage the structural integrity of the test vessel. One of the engineers suggested using the ERNiCrMo-3 electrode for this repair.
Both electrodes are classified in the same category and this can be done with the available PQR.
The material manufacturer suggests the ERNiCr-3 (no Mo) and after doing some searching API RP 582 suggests ERNiCrMo-3 for welding the same material (although no Mo exists in the parent metal).
There are some concerns on the high temperature properties (500 to 750 deg C) and the danger of embitterment by using the ERNiCrMo-3 electrode.
The fact that this weld cannot be PWHTed also raises some concerns.
I would appreciate any thoughts / feedback on this.

 

[metengr]

Follow the material manufacturer's advice for this material because of dilution concerns between the original weld metal and ERNiCrMo-3. In fact, Special Metals would be the folks I would talk to about recommended filler metal.

 

[pradipgoswami]

Hi SJuju,
A few querries in addition to the response of metengr.
-What;s the design code, ASME,SecVIII, DIV-1 or DIV-2
-What's the service environment:- involves Sulphur or No Sulphur
-Is the PWHT(looks to be a stabilization anneal) a service or process licensor's requirement ?
- What's the extent of repair, the entire seam has to be cut and repaired or only a part of of it would be repaired.

Appreciate your response.

Thanks

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

 

[sjuju]

Thanks metengr & pradipgoswami for your replies.

Some additional information:
-The vessel is designed as per ASME VIII Div.1 (but vessel not U-stamped)
-The service involves no S, is typical combustion environment (fuel + air)
-PWHT as per ASME VIII(see UNF-33(f): 885deg C for 90min)
-The entire pressure retaining seam has been cut and re-welded (with no additional PWHT)

 

[pradipgoswami]

Hi SJuJu,

The correct Sec-VIII, DIV-1 clause is UNF-56. PWHT is required as specified in the code(the extract is as below):-

-pressure boundary welds in vessels with design temperatures above 1000°F fabricated from UNS No. N08810 (Alloy 800H) shall be postweld heat treated. The postweld heat treatment shall consist of heating to a minimum temperature of 1625°F (885°C) for 11⁄2 hr for thicknesses up to 1 in. (25 mm.

-Except as permitted in Clause (3)(of UNF-56) vessels or parts that have been postweld heat treated as stated above shall again be postweld heat treated after welded repairs have been made. Clause (3) refers to only part repair during fabrication of new vessels.

I would say that PWHT is required after complete seam welding as you stated above.

You may refer this link below to get more details about this alloy.
--

http://www.specialmetals.com/documents/Incoloy alloys 800H 800HT.pdf

Welds in this alloy are prone to Stress- relaxation cracking, which could happen on service exposure. UNF-56 has the provision of solution annealing also in lieu of PWHT. You may discuss this option with your design/process licensor as required. Note wheather PWHT or Solution anneal , field PWHT witrh resistance heating band would be advisable.

Thanks.



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

 

[EdStainless]

I would start over. This unit can't be repaired if you can't PWHT.
Make a new one and do it correctly to avoid this failure.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[metengr]

sjuju;
I would like to add some additional information regarding PWHT, since I only addressed the filler metal concern. ASME Section VIII, Div 1 code committee revised the PWHT requirement for Incoloy 800 after several vessel failures were reported some time ago. Originally, these vessels were not required to be PWHT after welding fabrication. The failures were caused by stress relaxation cracking, which occurs between 1,000 deg F and 1600 deg F.

Since this is a non-code vessel, you can take exception to any requirements in Section VIII, Div 1. However, I would caution you in taking exception to the PWHT requirement unless the vessel operates below 1,000 deg F or the design internal pressure is below 15 psig.

 

[sjuju]

As explained in my original email, PWHT cannot be done period.
Other ways to avoid PWHT acceptable by code exist and this is what will be done.
My question is really only about the filler material properties.

And sorry about the mistake - you are of course correct - the correct reference to ASME 8 is UNF-56 for this material.

Thank you for your feedback.

 

[pandithan]

This Filler metal ERNiCrMo3 if subjected to PWHT leads to embrittlement. Best is ERNiCr3
Pandithan