Problem with slag (mig/mag(gmaw))

Date 10-26-2005 08:25

The wire contains silicon as a deoxidizer. As it gathers the oxygen it is oxidized (to silicon dioxide) which remains on the surface as that glassy deposit. You have to chip it off. I don't know about MAG but I suppose that addition of some reducing gas to the mix (probably hydrogen but that introduces other possible problems) might scavenge the oxygen before the silicon has a chance to. If you clean everything super clean (no rust or scale) before you weld it should reduce the amount of slag.
Bill

By dlmann

Date 10-27-2005 10:43

Bill: I have heard some mention "non-metalic inclusions" when reading film of GMAW welds. Is this the same?
Regards, Donnie Mann

By billvanderhoof (****)

Date 10-28-2005 06:41

I would think that that would be either slag or bubbles but I hope that someone with actual knowledge of the subject will help here.
Bill

By jwright650 (*****)

Date 10-28-2005 10:22

Donnie,
Looking at bend tests where GMAW was used, I have found if the welder let his puddle roll too much or was trying to carry too large a puddle and had some LOF, it was not detectable to the eye when the sample was sawed...until you bend it, and then where the silicon was trapped or the LOF was, shows up in a big way. I'm assuming this is what the RT may have been showing as non metallic inclusions or possible LOF either between passes or at the groove faces using GMAW.
John Wright

By Lawrence (*****)

Date 10-26-2005 15:02

Bill is right.

Surface prep before the job may reduce what you see on top of the welds.

Chipping or power wire brush the silica off the top before paint.

Bill
The "MAG" in MIG/MAG is just a Euro term for GMAW since "MIG" is viewed as a non standard term to them because an Active Gas (usually CO2 or O2) is so often added to the inert gas. So 75/25 gas has both inert and active gas, hence MIG/MAG.

By jon20013 (*****)

Date 10-26-2005 22:39

I've noticed too that quite often the glassy deposit is simply "masked" by power wire brushing. By that, I mean it is simply discolored, making it much harder to discern by the brushing but remains in place unless really aggressive brushing is applied. Chipping is best for removal in my opinion.

By Lawrence (*****)

Date 10-26-2005 22:52

John.

Thats good advice and something I haven't noticed.

What do you recommend for something high production.

Needle scaler maybe?

By jwright650 (*****)

Date 10-27-2005 10:17

Lawrence,
The few times that we used GMAW here we used the same ole pneumatic chippers with a chisel point that we use on FCAW, rather than the needle. This will pop those silicon islands right off. Personally I think if you prep the material really good before welding, the silicon islands just about disappear. Again, my experience was getting the welder set up for that sweet spot will help with slag removal.
John Wright

By CHGuilford (****)

Date 10-27-2005 16:56

Very often, high production situations require blasting and painting anyway.
A hi-volume snow-plow manufacturer near us does their welding with GMAW and then blast and powder-coat their products.
On the medium volume side, structural fabricators may blast their pieces for multiple coat paint systems. Or when shop primer is all that is needed and blasting is not required, chipping slag and power wire brushing might be all that is done. (Shop primers are not considered a high quality coating and are not intended for long term protection anyway.)
Some shops find that whip-blasting is faster power wire brushing, but it is important to note that blasting usually does not thoroughly remove weld slag.

For higher volume, manual weld slagging, you are right in that needle scalers are fairly fast.
Chet