FCAW for vessel welding 1

[vesselguy]

Good day,

I have heard welding engineers say they don't want FCAW for welding pressure seams when the MDMT is -40°C. In fact they don't recommend FCAW for pressure seams, period.

From my research, I can only gather that one might not want to use FCAW process most likely is because of the possiblity of incomplete fusion as a result of the fast fill rate of FCAW. Other than that, I don't see why FCAW is not acceptable. Can anyone shed some more light on this subject or pass on a link to some resource so I can bone up on it?
Thanks in advance.

 

[metengr]

The concern is toughness of the weld metal. As with nay high deposition process you have rather coarse grained, columnar microstructures that can wreak havoc on toughness. There are heat treatments, such as normalization and normalize and temper to improve toughness. However, the heat treatments are more expensive in comparison to subcritical post weld heat treatment.

 

[vesselguy]

metengr,

Thanks for your input. I think you pointed out the key point.

 

[weldtek]

I'd like to add that certain wires come prequalified by AWS with impacts at -40F, eg. those classified as E7xT-9J. I'm a firm believer that everything has its place, and while not a panacea, FCAW is a viable process for many applications within the pressure vessel manufacturing industry. I find there is still some resistance in some companies to allow it, however, in several cases we have been successful in swaying the opinion, by showing the client certain advatages to the FCAW process. In one case the customer required us to perform some fairly extensive testing including impacts which was all successful but for us the effort paid off because putting it bluntly, in many cases FCAW is a process superior to SMAW.
Just my opinion.

 

[metengr]

weldtek;
I concur with your statement. Most of the misunderstanding related with FCAW is toughness, which you handled well in your post.

 

[zjliang]

I got the same unfavorable opinions on FCAW in field vessel welding from our clients but there is no exact explaination.

Now from your guys' expert post, I learned a lot.
Thanks.

 

[SJones]

Don't forget that you will then also find prejudices between self shielded and gas shielded FCAW. API RP 582 will help you further.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[vesselguy]

SJones,

Thanks for your input. We use API RP582 as the basis for our welding standard. It is an excellent welding standard that covered pretty much every thing that has to be said for welding pressure equipment. You're right, it has some excellent limitations on FCAW. However, as a non-specialist in welding, from what I have read I see that with carefull consumable material testing, limitation on hydrogen content, and limitation on travel speed, I don't see why we should not use FCAW for pressure seams more often. Like Weldtech said, it is a viable process. I am going to have to dig further to find out the history as to why welding people in our oil & gas industry do not like using FCAW on vessels. There must be some good reason for it.

Thanks all.

 

[vesselguy]

I thought I would share some comments I got from our Chief QA Inspector on this issue. Please read below an excerpt of discussion I was having with him.


It is one of those urban legends-- sort of. When I was with Amoco we had a pretty large welding/ research technology group and years ago they had issues with FCAW for use in sour service, but it was largely due to the segregation of the flux in the the wire, not the process persay. They claimed that they had experienced instances of the alloying elements segregating which resulted in inconsistent mechanical properties, particularly with isolated hard spots. They claimed that they never had the same experiences with SAW, even though both process can add alloying elements via the flux( They had some theory that the segregation happen in the wire itself after manufacture, which I personally really doubt)

About 10 years ago, Amoco removed any restrictions their welding specialists had with FCAW as they felt the manufacturing/ flux process issues had been resolved. I am not against the FCAW process and I think it can and should be used more. It is widely used in the most critical applications including nuclear/ submarines, etc.Like any process, there needs to be good procedures, trained welders, and adequate QC controls. if you manage those variables FCAW is fine.....

 

[weldtek]

vesselguy,
Your Chief QC Inspector stated the facts succinctly. I concur with his comments.
I don't want to offend anyone but, I honestly believe that in many cases the people that are deciding which welding processes are acceptable for use on their jobs, are unqualified to do so.