I have heard it's not good to weld over metal-core weld with a different welding process like GMAW or GTAW or SAW, etc. but no one has been able to tell me why and I haven't found it on the web why or I just haven't used the correct keywords in my search.
So, experts, why would someone tell me not to weld back over a weld made by metal-core weld wire?
I'm not an expert by any stretch of the imagination, but....
As far as the structural welding code goes, tack welds incorporated into final welds must be cleaned, and then made with electrodes meeting the requirements of the final welds. They both have to meet applicable quality requirements, be either qualified or prequalified with a WPS, be made by qualified personnel, and they should have at least the same tensile strength (E70xx, for example), and notch toughness where applicable. As far as I know, tack and final welds can be made by different processes provided they are not in contrast with applicable code requirements. Here are a couple good links by Duane Miller and Omer Blodgett….
Scroll down to “Tack Welds Within The Joint”….
http://www.jflf.org/pdfs/papers/design_file103.pdfScroll down to “Remelted Tack Welds” and Incorporated Tack Welds”….
http://weldingdesign.com/blodgett/tack-welds-joint-0409
I am not an expert either, but my limited experience with a few of the oil industry welding codes made me aware that some welding procedures control the thickness or maximum number of passes with any given welding process - whenever multi process procedures are qualified. If I am not mistaken, it is sometimes possible to vary the thicknesses with each process during the procedure qualification - but use caution here, because some specialized codes may control the maximum thickness for each process regardless of the qualification attempts (because some companies have their own proprietary welding specifications).
And, as Lawrence alluded to, thorough cleaning to remove slag between passes is vital to optimizing the quality. I have seen welders run new passes over slag without proper cleaning and removal - assuming the slag would just float to the top of the next bead!!!! No! No! No!
What you may have heard amounts to a misapplication of a phrase found only in D1.8 for seismic applications: Intermixing of filler metals.
D1.8 prohibits intermixing but only in one application, you can't use any other process without proper PQR testing and results after FCAW-S, better known in to most as 'inner-shield'. If innershield electrodes are used for tacking, root passes, etc then you can't weld over them with ANY other process unless properly tested and approved by the engineer.
I have never found this anywhere besides D1.8. That does not mean it does not exist. I know first hand the difficulty with tacking with innershield and then going over it with GMAW (-S or spray) or even SMAW. It does not play well with others. But code wise it appears to be limited only in the seismic codes.
I have heard more than one inspector claim that you can't intermix ANY welding processes, at least not without PQR's.
Another application, though not specifically by you in this thread, of not checking out what has been heard from others. This happens often and is a good example of why I stress the need to be sure of where something is found and if it in any way applies to your job or not.
Remember, check General Notes as well because the engineer may call it even if the applicable code does not.
He Is In Control, Have a Great Day, Brent
Thanks everyone but at this point i am not worried about codes. I am more concerned with feasiblity and potential defects. If I have pretty large tack welds (metal-cored) and weld over them with a automatic process like SAW and I start seeing weld defects...say porosity, could the metal-cored tack welds have soemthing to do with it?
I have an opinion.
Yes, if the metal core deposit had porosity you can expect it to show up again. If the deposit wasn't properly prepped to receive further welding, yes, porosity or further defect could result. But no, welding over a sound deposit of metal core should not result in further porosity IMO.
I'm not worried about codes either. I'm more concerned with understanding the question. I'm not saying right or wrong in application, but sound metal is sound metal and it doesn't cause porosity and thats what you asked.
You may not be interested in codes, but your OP stated you heard that this was the case. That means someone said something about it not being allowed. As such, I answered the question with circumstances that I see often when people hear something. What they heard has limited application and often doesn't apply to the situation at hand.
Now a new set of parameters has been added. I agree with others, if the original is clean, the cover 'should' be. The tacks should not cause porosity if they were clean on their own. Especially with SAW being the process asked about for the cover.
He Is In Control, Have a Great Day, Brent
I agree, solid metal is solid metal, and that is the practical answer.
I did not intend to throw off the discussion by brining up the code issue - my thought was that possibly more than one person might be interested in the response.
Thanks for clearing up the "exception(s)" to the code "rule(s)" that "related" to the OP question.
As an aside note, SAW might have lower impact values than the arc processes which run narrower beads with more opportunity for grain refinement (or that produce smaller grains) - but, I don't want to start a metallurgical debate because many others more qualified than myself have gone before me and developed the technology to meet the necessary requirements in many cases...
By lo-hi 
Date 09-16-2014 00:20
Edited 09-16-2014 00:26
Your work seems kind of high end production. Could you look for some kind of guidance in the procedure? Is this a fabrication or repair issue?
Hi everyone! I do not know the answer to the OP's question. I did run across some information recently, regarding deoxidizers and essential alloying elements. Long story short, the information stated different filler metal contain different percentages of these ingredients. Filler metal are specifically designed for the application. It is possible to end up with excessive alloying elements in the final weld, leading to defects. From process to process the deposited metal should share the same properties if the same type of filler metal is used. But if there is a difference between filler metal, between processes, the combination of the two, could result in excess of one or more elements. Like I said, I do not know if this is the problem you are facing. Just wanted to put it out there for you.
Remember too that heat input often plays a strong role in determining mechanical properties. Hence, the enforcement of maximum heat inputs by some welding codes. Not only do narrower weld beads trap less slag, but they may produce finer grains with better mechanical properties if lower overall heat input occurs. But, many modern SAW wires and fluxes are able to tolerate larger volumes of weld metal - provided that the maximum allowable heat input is properly documented, tested, qualified, and communicated to the production personnel. And, chemistry no doubt plays a key role as well.
Poor toughness due to large weld metal volume and high heat input is one reason the Electroslag welding process fell out of favor many years before our time, but maybe modern updates to wire and flux chemistry have overcome these concerns - because I have read about some European shipyards that still use the process....the base metal plate thickness is perhaps a key driver of the limitations....
Again, I'm not a "super-expert" in this area - just passing on some of what I picked up from others in my travels...
I hope this is not too far off track...
Have a nice afternoon and a restful evening.
The Koreans use Electro-slag widely in their shipyards as well and is one of the reasons why they pump out ships almost as fast as we once did during WWII before the liberty ships spoiled all of that fun...
With regards to this thread... This is why it's so important to become proficient enough in understanding welding metallurgy so that on can consider what OBEWAN (Steve) just posted...You don't have to get a degree in it, although it is nice to have so don't let some of the terminology intimidate anyone that may feel somewhat overwhelmed by not being able to understand every word's definitions, and everything else that one can learn from in a book, or website online that covers welding metallurgy, or metallurgy in general...
Metallurgy - my own personal brain food... The breakfast of Champions!
Respectfully,
Henry
Hello "IowaCWI", do you remember where you ran across the information excessive alloying elements?
I was trying to make my terminology understandable to everyone when I mentioned "alloying elements." I'm sure that wasn't the terminology used. I didn't find all of my information in a single place either. But if you research "single pass FCAW wire" you will begin to understand what I mentioned above, assuming you don't already.
By ssbn727 
Date 10-02-2014 08:36
Edited 10-03-2014 01:06
IowaCWI,
I tend to agree with what you're writing regarding excessive alloying elements and de-oxidizers... I also think that the as welded MCAW deposit will have a certain amount of alloying elements and residual de-oxidizers that when mixed and also reheated with GMAW, weld metal, micro-structural changes will occur as well as changes in it's carbon equivalency... And changes in certain mechanical properties resulting in the weld metal, and heat affected zone becoming weakened if care is not used in choosing the right GMAW filler metal to use... And even the differences in the shielding gases may play a role in achieving detrimental results if one isn't careful to consider this factor also...
Which is not to say that it cannot be done but, it requires intensely focused detailed matching of the chemical compositions between both filler metals as well as the overall heat input in order to control the final weld metal and HAZ (Heat Affected Zone) micro-structure... If one isn't familiar in doing this sort of practice, yet attempts do so without completely understanding the complex nature of this exercise, as well as not having the experience of previously executing similar situations then the resulting failure is practically guaranteed.
Respectfully,
Henry
rmd,
Are you finding the defects in the tack weld locations?
Tyrone
Perhaps the best answer is to use "best practices" across the board - like proper slag removal and cleaning between weld passes...this is often cost effective...
