FCAW issues - cracks

[Phases]

Hello all,

Been investigating some issues a contractor has been having, using a rutile flux core, you name it they have had it, so ran some trials and made a number of changes to the process to combat porosity, LOF, inclusions etc.

In amongst all this were a number of transverse and longitudinal weld metal cracks.

A lot of the transverse cracks turned out to be LOF at stop starts which the NDT company reported as cracks. Some were clearly not and we investigated hydrogen controls etc etc but deduced there's very little chance of it occurring on 6mm S275 with a CEV of 0.27.

The longitudinal cracks also lead to a review of the most obvious causes, but it couldn't be definitively identified the cause. There was lots S and P, no untoward depth to width ratios etc etc.

Having developed a new PQR however, the contractor has now welded 6 off 1m seams, 6mm S275, using E71T1-M21A4-CS1-DH4 1.2mm wire, with 80/20 gas running around 220A and 24V with a ceramic backing tile.

On radiography of a full pen butt weld, it's been noticed there's one small, sub 1mm pore approximately half way between the weld centre and fusion line, with a longitudinal crack around 15mm long running away from this pore. The crack has a slight peak in it, rather than being perfectly straight like you'd see with hot cracking.

As I say, there's low CEV, 12mm combined thickness, 1.2-1.4kJ/mm heat input, and hydrogen scale D, with EN 1011 suggesting a 0°C preheat. Welders are applying around 20-50°C preheat however given the site conditions.

Anyone experienced similar and got any suggestions?

 

[o_grid]

There's a few things that come to mind, but the biggest one is welder skill.

First off, I hate FCAW for root passes, even if you have a backing plate. That alone raises some eyebrows.

Have you gone and audited the welding? Like actually looked at them doing their work? You mentioned problems with LOF at start/stops--why are they having a bunch of start/stops with a wire-fed welding process??

6 mm thick S275 shouldn't be this big of a problem.

 

[kingnero]

I agree, especially on ceramic backing problems can arise. Did you preheat/dry the ceramic as well? Is it possible to change for a permantent (steel) backing?
Depending on the diameter, 220 amps doesn't sound like a lot, they generally weld better at higher amps. Obviously if you go too high, the ceramic strip will be damaged on the root pass. Stick-out/CTWD is an important parameter as well. I insist of mentioning this on the PQR's for FCAW.

If you have porosity combined with cracks, chances are that the driving force is hydrogen. Comes from either the steel, the backing, the wire, or the atmosphere. If you preheat, and weld on clean, bare metal, you can easily exclude the base materials. Have you checked the gas hoses for leaks, have yoy tried wire from new (sealed) packaging, have you tried putting the ceramics in an oven?

If you use EN 1011-2, try the preheat determination method using calcs, rather than the graphs. They generally sit approx. 50°C higher, but either way in your case this shouldn't be the root cause of the problems you've got.

 

[weldstan]

Constant current or constant voltage power suppliy?

 

[kingnero]

@ Weldstan, would you care to elaborate on that?
I know the theory of both, I've been using the separate wire feeders from both Miller and Lincoln where you can change this setting,
I know how to use the voltage sensing clamp to regulate the wire feed when using CC,
I've been using these wire feeders from all possible sources (the big generators that pipeliners use, Miller XMT, Lincoln Invertec, "regular" CV MIG machines, ...)
but in the end I see very little difference, if any, during welding. Does this really impact the quality of the weld?

 

[weldstan]

In CC supplies voltage fluctuations can be highly detrimental to weld quality, especially with regard to inducing porosity.

 

[kingnero]

OK, thx for that. As we usually train using CV equipment, we haven't got much experience with these kind of defects and linked to CC sources.
But it's certainly something to keep in mind!

 

[gtaw]

Sounds like a welder skill issue provided you have already verified the welding parameters are those recommended by the manufacturer. As already mentioned, the electrode extension (CTWD) is one parameter that is often overlooked. FCAW is one process that should not be "set" by what feels right to the welder. Visit the electrode manufacturer's web site and see what they recommend for arc voltage, wire feed speed, and CTWD (or electrode extension). Amperage is fine for a quick check while the welder is actually welding, but amperage follows Ohm's Law. As the welder increases the electrode extension (resistance), the amperage is going to drop. Shorten the electrode extension and the amperage will increase when using a CV power source. Attempting to use a CC power supply complicates matters because the arc voltage isn't constant (FCAW is best when CV is used).

Best regards - Al