Alloy 625 Overlays on Carbon Steel - Cracking 1

[djwright]

It is a fairly common practise to overlay carbon steel componenents with alloy 625. Valve seat pockets and flange ring grooves are two applications which I have encountered many times in the petrochemical industry.

Recently, I heard about a crack like failure between overlay and carbon steel in a valve for sour gas service. The failure was described as "fusion line disbondment caused by hydrogen diffusion".

Does anybody else have experience or knowledge of such failures?

 

[mqas]

DJ,

It is very common in the Oil & Gas Industry to overlay specific areas of the valve with 316 and now more commonly 625. This is generally applied at the customers request where there is a seal in contact with the bare carbon steel. It is normally applied where there is a particularly corrosive environment, so that the area adjacent to the seal is not corroded providing a route for leakage and hence premature failure.

It can also be applied throughout all wetted areas of the valve and this is in preference to a solid 625 valve which can be very expensive. The route to go solid 625 or carbon steel is dependant upon the valve size.

As to premature failure of the coating i.e. disbondment caused by hydrogen diffusion I have not seen it in practise but believe it was a major problem on the BHP platform in Liverpool Bay project in the UK. The corrosion apparently started at the base metal and 625 overlay interface and caused bistering and eventual blistering of the overlay.

I trust this helps

 

[djwright]

Thanks for this.

It was the reported BHP failure(s) which had me worried.

The use of overlays is so common I assume that the BHP failure is specific to BHP rather than a generic problem.

In our sour service application, we have qualified overlay procedures in accordance with ASME IX and NACE MR00175. Weldments were tested (bend, toughness, hardness, chemical analysis). Hardness results in particular were good (180 HV10 typically).

Production welds were checked visually and with dye pen to ensure to machining deffects or cracks.

 

[Carburize]

Do you know who supplied the parts that had the problem in the BHP field?

 

[djwright]

I believe it was ITAG

 

[Carburize]

Interesting, I have not heard of any problems with 625 overlayed valves, wellhead etc from the "big boys" Cameron, FMC or ABB-Vetco Gray. Potentially the high pressure equipment using higher alloy base materials like 4130, 8620 etc would produce a worse case HAZ and have been used in more agressive environments.

 

[sayee1]

I have heard of this problem on Inconel overlays. This was in some reference with weld buttering on bevel faces with Inconel to carry out PWHT and then join the buttered surface with Inconel on the other side thus helping avoid PWHT of the butt weld. In fact, EXXON MOBIL has a Global Practice- Specification titled "Upstream Weld Metal Overlay for Load Bearing Applications For Pressure Piping systems." "GP-29-01-17" Note again, the emphasis on load bearing applications. This is same as the buttering I have mentioned above. (I had some references on the failure detailed above....Let me dig up the records.)

Thanks and regards
Sayee Prasad R
Ph: 0097143968906
Mob: 00971507682668
email: sayee_prasad@yahoo.com
The whole of science is nothing more than a refinement of everyday thinking!!!

 

[mqas]

The valves were in fact supplied by ITAG however, I don't know if it was a welding process problem e.g. hard HAZ or one specific to the Liverpool bay field. This field does have the most sour service in the UK Oil & Gas sector.

Some of the valves were replaced in service 1 - 2 years ago so there should be some sort of report available as to what caused the problem. If you have any contacts at Score Europe they might be able to provide some info. I will also contact Score and will publish any info. I receive.

 

[djwright]

Thanks for this.

Based on my investigations so far I believe that it is essential to ensure that overlay welding procedure provides hardness less than 22HRC / 248HV10 as per NACE MR-01-75.

I have contacted ITAG to establish the cause of the problem on Liverpool Bay and await a response.

djwright

 

[tgif]

In accordance with NACE MR0175-2001, Wrough Alloy 625 (UNS N06255), Hardnesses are acceptable to 35 HRC.

 

[Koz]

Hi all,

I didn't see any mention of UT in the list of NDE integrity checks. We typically UT'd our overlays to check for unbonding. It isn't hard to do.

Unbonding of the overlay would be a workmanship problem for us. It would be caused by tramp contaminants or lack of fusion during a process or operator related discrepancy. Alloy 625 to carbon is nice to weld.

The hydrogen would need some unbonded region to start blistering. Tramp oil might do that and it would be missed using VT and PT. Just a thought
Koz

 

[aybee]

disbonding of overlay stainless in alloy pressure vessels in hydrogen service is or was quite common. the practice to overcome this was to overlay first in 306 then 347

 

[aybee]

sorry 309 not 306

 

[SMF1964]

We've had problems with alloy 625 overlay on tubes where adjacent weld beads (longitudinal passes) are too close together when a final pass is made to close the gap between the two beads. You get lack of fusion at the tube/overlay interface and (in some cases) an axial crack running down the center-line of the final cap bead. Often, this is not discovered until service so the LOF/axial crack has time to develop into a hot-corrosion issue and fill up with oxide/sulfide from the flue gass environment.

 

[GRoberts]

Welding 625 in too narrow a gap is a problem. The depth to width ratio of the bead get too high and centerline cracking results. The gap between the beads needs to be opened up by grinding/machining/arcing, before the last pass is put in.

 

[mqas]

For those of you based in the UK, you may find the following event very interesting.

Liverpool and North Wales Materials Society
Programme 2003/04

06 Nov. Materials in Sour Service and other things that might crack

Presented by Dr Ed Heaver Liverpool Bay Asset, BHP

Location: Univeristy of Liverpool George Holt Building
17.30 Refreshments - 18.30 presentation

http://lnwmatsoc.liv.ac.uk

I will try and attend and provide soem feedback

 

[grampi1]

MQAS - Did you attend the Presentation by Heaver in Nov 03? Any notes?

 

[SJones]

As this one popped up again, I thought I would pitch in. Of course, we aren't party to the failure analysis reports but let's try to assess the root cause. If it is a process fluid generated "disbondment", surely that would require the process fluid to be in contact with the carbon steel as it will not be corroding the 625 to generate much hydrogen on the surface to diffuse through several mm? If this hypothesis is supported, then the implication is that the overlay was defective prior to exposure to the H2S containing fluid. Alternatively, it could be a function of the phases formed by the deposition of the nickel based alloy on the carbon steel and the welding parameters used. How about if hydrogen was introduced during the welding process by the presence of moisture or grease on the electrodes etc? Just my thoughts.

Steve Jones
Petroleum Development Oman LLC
Muscat
Sultanate Of Oman