I am trying to figure out why when we weld the other side of a tee joint porosity becomes a major problem. The weldments are cargo ship bulkheads AH32, 12mm thick with "weldable primer", using Kobelco E71T1 .045 wire using 75/25, 5-6mm fillet welds. After welding one side of the tee joint with no problems the other side of the joint gets welded, and thats where massive amounts of porosity occures. Does anybody have any idea why this happens?
i've never been a fan of that "weldable primer" crap. i'm not sure but i would suspect that when the first side is welded the heat is probably blackening and flaking the primer on the other side, which is probably changing how it reacts when welded. have you tried wire brushing the second side vigorously before welding?
JJ
Try it without the primer for sits and giggles. That stuff is junk anyway. I understand that you want to have the underside of the T-joint coated, but it's all going to burn out underneath the T anyway where the welds are. So why not prime it after welding. The only other thing I can think of besides the obvious stuff is that when going to the other side of the joint while the plate is hot, the puddle will stay molten longer and loose gas shielding as you move along. This is usually only found on larger single pass fillets, not 5-6mm fillets though so I'm stumped.
How are you priming the joints? Do you prime the joint and then weld or do you prime the joint, then weld, then reprime the otherside before you weld it? I really don't see the primer causing the porosity since it is only on the one side unless it's still wet when you weld through it. Can't really give anymore help without actually seeing the setup ya know. Let us know what you find out when you find out k.
Alan,
Two words, "weldable primer"!! Remove it from the weld area per code if there is one aplicable, if not one inch should be sufficient. Weld the joint, then prime as needed. I know, I know. They say it is made to weld over, but it should be removed, IMHO.
jrw159 :-)
How thick is the joint?
I'm thinking that on the 1st side you are burning off the paint in the weld zone, but that the gases can escape from the root side. I suspect that you are probably on the edge of getting porosity on the 1st pass, and probably would get porosity if the weld cooled any faster.
Then when you weld the second side, you are again burning off paint and generating gases, only now it is sealed off by the 1st weld - so the only place to go is back thru the weld you are making.
I would check that you are using the correct gas flow-maybe bump it up to maximun allowable, check the gun angle to be sure you aren't sucking in air into the weld zone that way. Then I would grind or needle-gun the weld joint to remove what paint you can. Finally, I would choose the slowest travel speed you can get by with to slow the cooling rate.
If that doesn't work, you could run a pass, gouge it out, then re-weld it.
Thanks for all the ideas and yes that "weldable primer" seems the be the likely culprit, but unfortunatly were getting all this material already completly primed. To top it off, all this steel is being shipped from Korea or China and who knows what else happens to it along the way. Believe me, Ive tried everything...grinding the primer off where i can, blowing out the joint, changing my volts, amps, travel angle...everything. I was thinking what CHGuilford wrote about the burning of the paint which generates gases and not being able to excape, except through the weld since the opposite side is blocked off with the previous weld. I do grind the primer off, but there is still that little paint left on the root face that i cant get to because its all already tacked together when the welders get it. (and try to tell the fitters to grind the paint off!) Oh well...thats the benefits of working in a shipyard!
Only sugestion I have sense the paint was not removed first, burn it out with torch without geting metal hot then hand wire brush the loose stuff out of seam. should eliminate the gases when welding problem.
Tom
Alan,
if the material is comig from Korea or China then there is a chance that what your seeing and calling weldable primer is lead based paint. We had the same problem with some Struct. steel coming from China that we had to remove the primer because of it having lead in it and was causing welding problems not to mention health problems.
Jim
I tend to agree with Chet in that you are creating a situation where gases developed in the root area, or in between the bulkhead and the deck have no place to escape to. Therefore, they follow the path of least resistance; through the soft, molten metal.
I am not at all familiar with "weldable" primer (except in the case of using E6011 and a little extra juice to burn it in), so I can't help out there at all. However, it seems to me that if you lay out your weld sequence so that you're backstepping and staggering, then you should be able to avoid the blowout issues. With that 12mm thickness, I would imagine you don't want to zip it up in one continuous pass so as to avoid distortion.
I am curious; is this a 100% weld on both sides? Also, are you required to use the "weldable" primer?
Use 100% CO2 instead of the 75/25. Much cheaper, deeper penetration, slag cools slower allowing gasses to escape. Not quite as good out of position as mixed gas, but it has eliminated alot of crap like this for me.
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