Burn Mark Prevention

[AmitJ]

I was hoping that someone here may have some experience on preventing weld burns. We are welding two .25"x36"x60" 316L plates, that are each bent into a U, together to form a box with open ends. We then tack on support bars around the outside surface. However when these bars are tacked on, the heat produces weld burns on the inside surface of the box. Since there are approximately 50 tacks per box, cleaning up all these burn marks becomes a pain. I have looked into using those electrochemical wand type cleaners as well as conventional abrasive polishing but what would really help is preventing the weld burns in the first place. The size of the box makes backing material cumbersome. I have heard that purging the inside surface with an inert gas may solve this. Does anyone have any other suggestions? Are these marks caused by oxidation or something else? Any help is greatly appreciated!

We are currently TIG welding and have toyed around a lot with the current and speed without success. We want to remove the marks mainly for looks not performance.

Thanks!
-AJ

[littlefuzz]

Are you talkin about where the SS just turns color from heat?

[AmitJ]

Yes, we are welding on the top surface and getting discoloration on the bottom surface of the HAZ.

[pulser]

Yep, it is oxidation. Inert shielding should pretty much eliminate the discoloration. Instead of trying to purge the entire box interior, which could take a good deal of gas and time, how about rigging a large gas lens to the end of a length of pipe with a 90 degree fitting, and just purge the backside directly opposite of each tack weld. If you tack each part at the same regular intervals, maybe it would not be too difficult to position the purge device in a repeatable manner. Make marks, or put stops on the purge tube, so you know how deep to insert it for each tack weld. Either rig the thing to set in place by itself, or get a helper to position the purge as you do the tacks.

Plug the tungsten hole in the gas lens with a small screw, use a standard ceramic gas cup.

[aczeller]

when we get the oxidation marks at the back of the HAZ at work, we always pickle and passivate the SS. there is still some dark brown/grey/black marks on the bad parts, but those can be taken off with a grinder/flap disk. what is the part you are making for anyway? sounds like a directing channel of some sort. you could also try coating the back of your anticipated HAZ with a flux of some kind. from what i understand (Pulser, feel free to chime in if i'm wrong), as long as you barricade the O2 from the HAZ, you will get no oxidation. the flux would be your barricade.

just a thought.

Later,
Andy

[AmitJ]

Thank you everyone for all your help. This has given some things to think about. If all you need to do is get rid of the O2 does that mean we could seal both ends of the box with a lit candle on the inside? The candle would burn up all the O2 and then as long as the seal is air tight the metal couldn't oxidize. I know this sounds like a crazy solution but it seems to me to be by far the easiest solution. Any thoughts? Thanks again!

-AJ

[aczeller]

Originally Posted by AmitJ

If all you need to do is get rid of the O2 does that mean we could seal both ends of the box with a lit candle on the inside?

not quite... the candle burns untill there is insufficient O2 in the surrounding atmosphere to sustain the flame, yes. however, there is still some O2 inside the chamber, plus unless it is air-tight, you will have a CO2 leak at the bottom of your chamber. if that's what it came to, yes it woudl be better than nothing, but i think you are better off by back-purging. woudl it be possible to just turn your heat down enough to not send your HAZ all the way to the back? i have been able to succesfully fuse (2) plates of 304 SS that were 1/4" thick with no filler. i then beat the piss out of them with a 10 lbsledge in a 15 pound vise and nothing broke... things bent, but the weld was still good. when i look back at the weld, i did not use any back-purging and there was still no discoloration on the back side of the joint. keep the amps low and run slow... i bet you can get it without worrying about much oxidation.

Later,
Andy

[pulser]

The candle idea came up before in a post here, I'll see if I can find it later, but as I recall, the candle will extinquish at aroun 19% Oxygen, still way too high to prevent discoloration. 50 ppm O2 is enough to produce a light straw/gold color, and around 500 ppm starts to produce dark blue.

If you want to purge the whole thing in one shot, Argon might be too expensive for you, so you could us Nitrogen. N2 can be used as a purge gas on SS.

[olddad]

What about the solar flux I've read about here ? I've never used it but from what I've read it sounds like a possible solution ?

[Joe H]

Solarflux will prevent oxidation, but it's a real pain to remove. In some applications Solarflux is great, in some it's just trouble.

[pulser]

Also never used Solar Flux, sounds messy for this application, first you paint it on, then you have to remove it. Gas is alot easier to apply and clean up after.

[olddad]

Yeah, if it's a PITA to get back off then a moveable gas tube would be the way to go.

[hotrodder]

purging the whole thing will take time and eat a lot of argon. as Pulser said nitrogen would reduce the cost

solar flux is out for the reasons stated- time to apply and remove the residue afterwoods

assuming this is a production job i would make a purge tray to reduce the volume that needs purging. fold up some thin stainless to fit inside (add flanges so that when slid inside it's a snug fit in the box) and entry/exit points for the purge. this will reduce the volume that requires purging saving both time and money

regarding the purging. argon is denser than air- arrange things so that argon enters at a low point and exits at a high point to ensure the air is eliminiated. nitrogen is slightly less dense than air- keep flow rates low to ensure the air is slowly replaced or you can end up mixing the two and still have enough O2 around for oxidation

[AmitJ]

Hotrodder - Great idea. That is exactly the type of setup I am currently testing with. We have a sealed inner box with angle welded on to it to take up some "dead volume". We put angle onto the box to create a uniform space between the welded wall and the inner box. We have two ports, one at the bottom connected to the argon source and one at the top venting to atmosphere and the flow rate is regulated to maintain 2 psi in the purged volume.

We allow the purge to run for about 30 sec to push out all the air before starting to weld. The first few attempts did not prevent the burn marks but there are some process variables that we still playing with. I personally think that the argon may be mixing with the air but my coworker thinks that some air is "sticking" to the walls and not being purged. Either way we are still playing around. The plate we are welding is unpolished and starts with a very dull surface on both sides (I don't know if that would play a role.)

Any other thoughts are welcomed and appreciated. Thanks again for everyone's help!

-AJ

[aczeller]

i can't find it in here, but i know i've read somewhere that you need to back purge from the lowest possible point in your "gas chamber" area and fill it slowly. the faster you fill it, the more you will mix the air with the oxygen... remember, argon is heavier than air, but not so heavy that it falls like a rock through water. i tend to think of it like pouring oil into water. they will eventually settle out, but at first, there is a mixture of both... maybe let the argon fill for a few minutes before welding? just a thought... also, make sure the rest of your chamber is air-tight... the argon will still flow out of the hole sif they are low enough. use some masking tape and paper to seal all the other joints.

Later,
Andy

[hotrodder]

Andy, maybe you read it in my previous post (#13)

Amit, obviously i don't know the volume being purged but 30 secs is definately not enough time given the size of the fabrication. as long as the purge is at the lowest point and flow rates are kept low (say 3l/min) the air will be displaced. a positive pressure is not needed and could be causing the problem due to turbulance

i can't see surface finish playing a role