Major discontinuity at root of 5/16th LC-GMA F3 weld with hardly any sign on surface.

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Hi everyone,This is my first post. Thank you in advance for your help!We have a weldment at work showing small pores at the surface of some fillets. The joint is between ASTM A500C HSS that is  6" x 2" x 1/4" that is impact tested to get us int he ball park of 50WT CAT3. The plate is 2" thick CSA G40.21 50WT CAT3. The specified weld is LC-GMA 5/16TH F3. We produce these scissors regularily without difficulty BUT once in a while find porosity and end up reworking them. The below photo shows something that we have not seen before and is very alarming. When gouging through the weld to chase out the porosity, voids were found at the root that continued through the entire fillet even where there were no signs of porosity before gouging. We are using 0.045 L59 wire with mig mix gold running around 40 CFH (we think). We are completing this 5/16th fillet using one pass only. There is a 1/8" vent hole drilled in the middle of the HSS 16" from the either end of the tube. What is your best guess at the root cause of this. I am looking into sources of contamination ( ie. coolant from saw dripping inside HSS, corosion etc.), but wonder if this is a case of gas escaping the tube, or shielding gas flow, or voltage way too high. These welds passed mag particle NDE with the exception of the small pores ( one on each end of the HSS).Help and opinions are appreciated. I will try to provide all details I can. I am hoping someone will know exactly what this condition can be caused by at a glance ( wishful thinking?).Thanks,JamesLast edited by jiminy14; 10-28-2014 at 08:32 PM.
Reply:Hello James, from what I can see you have a piece of square tube welded to a base plate. A couple of possibilities from my perspective. Often there is identification paint that is present on the ends of tubes and other structural shapes. If this paint hasn't been removed before it is fitted (by way of a trim cut or grinding) and welded to a plate like this the vaporization of the paint during the welding can cause gases to be trapped in the weld metal before it has had a chance to escape while the puddle is still in it's liquid state. Another possibility is not cleaning the rust preventative from the tube or sawing fluids from the parts before welding takes place. Any compounds that are on the tube when it is welded (I'm assuming it is welded with the plate flat and the tube vertical) will possibly allow the rust inhibitor/sawing fluids to pool at the bottom of the part between the end of the tube and the plate prior to welding or during welding from the heat which makes the rust inhibitor runny and then causes it to run down on the inside of the tube and pool or wick in between the parts. These compounds are then drawn into the welding pool from the backside and cause off-gassing that can result in what you are seeing in the way of discontinuities. I noticed that you already addressed this in some of your comments and a very high likelihood in my estimation. Look forward to hearing other comments and also your sharing of what you have determined. Good luck and best regards, Allanaevald
Reply:jiminy14aevald's contaminates are the first consideration.What is your saw-juice - and do you hot-rinse?Is this: short circuit or spray transfer - manual or robotic?Because this is intermittent: gun angle and 'splatter load in the nozzle' may be worth monitoring - a larger vent-holewouldn't hurt, and;how good are your flow-meter/regulators?Opus