Adequacy of FCAW process 14

[fakhfakh]

Hello every body,

For butt welding of material A283GrD thickness 22mm, we use FCAW process with E71T1-MJ filler metal with gas 20%CO2+80%Argon. UT is used for weld inspection and defects found are up to 3%.
Based on what we heard about this process, I have a doubt about its adequacy even if the UT results are satisfactory. Can somebody help me to know about the risk of this process against quality and if any failure can occure for pressurased items in service.
For exemple, can some types of lack of fusion produced by this process not be identified by UT?

Thank you

 

[MOB1]

Flux cored welding processes do not suffer from the lack of fusion defects which can characterise MIG welding.
It is my experience that FCAW approaches MMAW in ability to weld without defects but has the advantage of more operator appeal.
Lack of fusion defects cannot be hidden from a UT operator with the right probes and experience.

 

[fakhfakh]

Thank you.

But why the most known codes and standards Ex. AWS D1.1 has some reserves about this process for exemple in pre-qualified process...
We understand that this proccess is not treated like the others (SMAW, SAW, GTAW...)
Europeen standards EN 13480-4 states in a note that "lack of fusion can occur, particularily at the start of the welding"

 

[metengr]

The main benefit of FCAW is the rather high deposition rates, and it has been used for pressure vessel applications. As an owner/user of ASME boiler and pressure vessels, I would not have any reservations about permitting the use of FCAW.

I would place the risk of LOF type weld defects as rather low. The fact that your using a shielding gas on top of the flux cored wire is good. I really see no problems with the selection of your process.

 

[HgTX]

fakhfakh--where in D1.1 are the restrictions on prequalification of FCAW? All I see are restrictions onf GMAW-S (which isn't appropriate for anything but very thin material), ESW, EGW, and GTAW.

Hg

Eng-Tips guidelines: faq731-376

 

[swall]

In my experience, LOF defects CAN be a problem with FCAW. If you have a poorly designed butt joint and a poorly trained welder, the FCAW process can allow the welder to "pour" so much weld metal into the joint so quickely that it won't all fuse.We found this out the hard way with boom arms on front end loaders. Examination of field fractures showed plenty of weld metal, but poor bond. Corrective action was to redesign the butt weld joint and train the welders.This was in the 1970's, when FCAW was relatively new.

 

[JB2001]

Swall!
I have to comment...
With a poorly designed joint and a poorly trained welded, ANY welding process might give LOF or a range of other defects!

 

[fakhfakh]

I need to go deep in the analysis of this process even if no serious "problem" yet occurred with it.

Spray transfer mode gives a large rate of metal deposition but I'm not sure that there is enough energy to produce a good weld dilution which gives a sound weld with a good mechanical properties.

 

[swall]

JB2001--I don't disagree at all. My experience was in a manufacturing plant with lots of familiarity with welding plate structural members with stick welding and some MIG & TIG. It was assumed that the transition to FCAW on this particular product would be a no-brainer. It wasn't.

 

[StoneCold]

fakhfakh
Why don't you do some bend tests on some coupons with your current process settings to get some level of comfort in the fusion quality and durability of the joint.

StoneCold

 

[weldtek]

Switch to stick ( SMAW) and I'd be willing to bet your defect rate rises well above 3%. In the hands of a trained welder, who is working with an adequate joint design, FCAW is a great process. You didn't say what position the joints are being welded in, but, if it's flat, 1G, and if the length of the weld is sufficient to warrant it, I'd suggest SAW as first choice, but FCAW is a close second choice, and if the joint is in a position other than flat, in my opinion, for the materials you describe, it's the only choice that makes sense.

 

[stanweld]

fakhfakh,
What type of defects are you encountering; porosity, slag inclusions, lack of fusion? Also why are you using such a high quality weld metal for A-283 D, which has very poor toughness as a general rule? The "MJ" fluxes have been formulated with additional basic minerals, which can lead to an uneven bead appearance.

 

[unclesyd]

Don't be too hard on me.
I have to agree and disagree at the same time concerning the utility of FCAW. First as stated self shielded wire, in my opinion, is useless in any application other automobiles or fence posts. Gas shielded FCAW is somewhat better but still carries all the attributes of any MIG welding. That is the very probable lack of fusion weld defect. Granted this isn’t a problem in a cold static or very low cyclic process but becomes very deleterious in any cyclic service that involves heating and cooling. The afore mentioned lack of fusion defect usually is of such a nature that it is extremely hard to detect by UT by all but the most proficient technician. What I’ve found is the lack of fusion that gives the most trouble is the one where only about 10% to 20% of the surface is unfused and occurring in small islands. This will confound most UT technicians by hiding in the grass and will be missed if using the normal UT procedures for welds. We have tried with over twelve different UT technicians, with about as many different machines, including four three level 3's. We have seen and still seeing manifestation of primary lack of fusion in high pressure polymer filters welded anywhere from 10 to 15 years ago with both self shielded and gas shielded. Another aspect of inspection of the FCAW weld is there is no surface penetration of the LOF defect. The only way I’ve found it is by grinding to about 1/2 thickness of the weld cap. The PT or MT indications are normally discontinuous and very small. The tell tale is the straightness as one would expect.
These vessels are cycled about 5 or more times per year form ambient to 600°F operating for about 8 weeks then cooled to ambient and then cycled to 900°F for12 hrs for cleaning with cooling in air. These housings were originally welded with spray and transfer arc and then the switch to FCAW. About 20 of 40 have been completely rewelded with SMAW. All of the above is with CS welding. The problems with all types of MIG welding processes including FCAW on SS would fill a book Again in thermal cyclic service at FV to 250 psig ambient to 600°F .

I have recently found out that FCAW sans shielding gas was proposed in the fabrication, integral nozzle welding, of 16 new PSA vessels to replace the originals that were failing from Hydrogen Accelerated Fatigue. Fortunately the project engineer nixed this at the first proposal, glad I wasn’t at the meeting probably would have had the big one. This would have made the History Channels Engineering disasters program.

weldtek,
If I saw a 3% weld defect rate from our shop or on site contract welders I would fire everyone. Our standards call 100% RT with grading in excess of code, in three of the largest areas at our site. The historical repair rate is less than .5% with very very few cutouts. I can’t remember the last one.
Granted we have had tremendous contractor problems, one with a 100% cutout on 21 6" Sch 10 304L SS butt welds and a 40% reject rate across the board with an internationally renown welding firm, and better than 50% on a very large project using CS socket welds.

 

[pipewelder1999]

Here is a picture of the LOF spoke above by a few. This was done with E71T1 with the Electrical Characteristics well within the range of the WPS.

Gerald Austin
Iuka, Mississippi

http://www.weldinginspectionsvcs.com

 

[unclesyd]

I'll have to admit that Blind Man could have found that one. A second though is that he must have welded it too.

pipewelder1999,
You'll have to admit that this is in the extreme, reminiscent of the early days of MIG or the Alaska Pipe Line. It’s hard to tell but it looks like when they cleaned the root pass for the inside weld something would have jumped out at the inspector.

 

[pipewelder1999]

I agree about it being extreme. It was the result of just carrying too much metal. I have also seen the same thing in GTAW welds in Schedule 10 SS.

Gerald Austin
Iuka, Mississippi

http://www.weldinginspectionsvcs.com

 

[andyenergy]

I've been interested in this thread. I like others have faith in the testing carried for Procedure Qualification, after all that's what it is there for. What bothers me most is what happens when the welder gets hold of the job. I'm not convinced that procedures are always followed, parameters are often changed with no real knowledge of what the effect is, there is often a lack of preparation and sometimes a general arrogance about knowing best. I could point you in the direction of bulletin boards/forums where some of the welder's discussions are astonishing.

I think that pipewelders' last comments are pertinent - carrying too much weld metal - if it was welded within the electrical characteristics of the WPS this must have been an approved procedure so what changed? Looks to me like the prep wasn't wide enough...

I'm beginning to change my mind about FCAW....

 

[fakhfakh]

Thank you for all.

I would clarify that in the thread I've posted I don't look for conventional welding defects such as lack of fusion, inclusions, etc... which are common for all the known welding processes. What I look for is the weld performance in service. That is, how much are we sure about the mechanical propreties of a weld made by FCAW after all qualifications, tests, NDT, ... are performed according to standards or construction Codes.

May be the tests required by those standrads or codes are not enough for this welding process. Wy not to go beyond by more accurate tests such as cristallographic analysis, micrographic analysis, fatigue tests or others to see deeply what happens.

Believe me, that I'll perform an engineering investigation for this matter.

 

[stanweld]

fakhfakh,
Literally, tens of thousands of welds in low temperature carbon steel pipe have been welded with the gas shielded FCAW process for pipe systems located in the arctic - both shop welds and field welds. Millions of shop welds have been made in more standard operating temperature pipe systems with the gas shielded process.

Millions of welds have been made in structural steel materials with the FCAW process. The primary failures have been associated with seismic activity and the use of self shielded electrodes, many of which could not pass the bend tests of AWS D1.1 welder qualification (extremely poor ductility); qualification had to be done with radiographic testing.

The success of FCAW is predicated on the knowledge of the welding engineer or supervisor responsible for implementing the process, training of the welders in its use, and surveillance to assure that the welders are conforming to their training and the parameters set forth in the welding procedures.

 

[unclesyd]

stanweld,
I agree that the welds have been made and probably 95% are OK for intended function and will last the lifetime of the equipment. As you probably know a good percentage of the welds in Alaska pipeline were totally unacceptable by any standard and were supposedly corrected, this was mostly done by graphite analysis, IE pencil. As long as the root and cap were good let the middle fall where it may. I was privy to but not part of the NDT fiasco perpetuated on the weld inspection. In the end they took the McNamara approach: we know 10% are bad, but only 2% of customers will complain about the defects and of this only 1% will demand satisfaction and hopefully we can talk half of them out of that. No one will ever know how many repairs were made then or since to the pipeline. I know that all the problems can’t be from bullet wounds or a bull moose in rut.
There is an interesting aspect of the problem with FCAW in the Northridge Earthquake, was the the rewrite of the initial/preliminary report to the final report. It was like a bunch of moles digging out from under a sheep’s foot roller, a tremendous amount of CMA. A goodly number of the welds had for all intents and purpose failed prior to the big one. The qualifications were to ASME D 1.1, no mention was made of the test method being RT until the final report. Bending is a lot cheaper than RT.

I agree with your last paragraph in every respect. The only problem that I see is the implementation of such a program in today’s penny pinching enviroment. My philosophy was to hug a welder everyday, but management has killed all incentives for welders to consistently perform at there peak efficiency. This requires more knowledgeable supervision which isn’t available to insure that all welder controlled parameters are in order.

I may be out in left field due my experiences and our applications with their attendant conditions. As stated before we jumped on the MIG band wagon at the onset and spent a tremendous amount of time and money to implement it into our fabrications both onsite and offsite with every conceivable aid and consultant available . Everyone we delt with from the worlds largest to the smallest shop contributed to our rework pile. So far we have only had some very expensive shutdowns but no catastrophic failures and no one hurt, only bruised egos .



Anecdote:
I just rented a 30ft hydraulic lift that was completely welded out with FCAW, presumed. The design was elegant, no sharp corners and very few straight edges on the welded components. One side of the machine had very pretty welds, quite acceptable, but the other side of the machine had the best undercut I have ever seen. I measured 1/8", the channel was 5/16", deep for a complete weld on several end connectors. This undercut was on both toes of the weld and very consistent. Further examination revealed that the upper boom was beginning to buckle. As I wasn’t going to load it anywhere near it’s capacity I passed on it. I pointed this out to rental company and their comment was that this very common with this equipment. This is one case where FCAW welding is probably fused .