I'm TIG welding 2" x 1/8" 304 SS butt joints w/ partial penetration. Parameters are: 3/32" 2% thoriated tungsten, 1/16" 308L filler metal, 50 amps, #6 (3/8" dia. cup), argon 12 cfh.
I'm seeing the discoloration in the HAZ as follows from coldest to hottest: yellow, purple, blue, silver (same color as base metal), dark brown/black.
What I'm trying to understand is, what does it look like when you have carbide precipitation? After cleaning up the weld w/ a SS wire toothbrush there is a band of dark material right next to the weld. Is this normal or is it carbides?
It sounds to me like a sheilding gas/ contamination problem.
With SS, the more discoloration = the more oxygen absorbtion in the weld/base metal. The dark brown/ black colors are probably a result of dirty base metal and or excess moisture in the sheild gas.
If you really want it to come out clean, and retain it's corrosion resistant properties, you'll need to increase your sheilding gas cfh, increase cup size to a large gas lens, and sheild the opposite side of the plate as well.
Additionally, ensure the weld joint is as clean as possible before welding.
Use designated abrasives for SS only and wipe down with Alcohol just before welding.
Tim
Hi Welder-Guy
Some amount of discolouration while welding S/Steel (or just about anything else for that matter) is inevitable, as long as it is just not excessive. (Thre will always be some of the alloying elements that diffuse out at high temperatures, as well as the oxide layer "burning" due to the high temperatures.) To solve the problem, and restore the corrosion resistance, pickle and passivate the weld.
You can not visually "see" carbide precipitation, because it is something happening inside the steel. It's effects only become evident some time down the road.
Regards
Niekie Jooste
Fabristruct Solutions
I agree with everything Niekie says. If the intended service conditions require excellent corrosion resistance in the weld area make sure all discoloration is removed. By the way, if corrosion is a concern, Type 304L would be a better choice than 304. The L designates a low carbon content which helps minimize carbide precipitation. Keeping interpass temperatures low and using low heat input is also beneficial, although your thin metal may make these two issues less of a concern.
Are we talking about discoloration on the ID or OD?
Isn't the most important area the of the pipe the inside? I mean, this is the service environment, isn't it(as far as corrrosion resistant alloys are concerned)?
I have never heard of a weld failing because of discoloration on the OD.
Perhaps you can site some examples of weld failures due to OD discoloration(high-temperature oxidation).
Does the L designation indicate low carbon or low temperature service conditions? 316L isn't suitable for high temperature service, say in a power boiler, is it?
Can you define "excessive", in regards to high temperature oxidation?
I hope I'm not coming off as smart ass or anything even close to that, but I have a great interest in what you have to say, considering you work in a large chemical plant in an welding engineering capacity, probably utilizing a plethora of different alloys, and have experience and information that I just don't have access to.
thanks,
Dale Simonds
Hi Dale
The further post by welder-guy kind of places his application into perspective, but to answer your question: External (atmospheric) corrosion of S/Steel is a very real problem, depending on the environment. When the oxide is "burned", it is no longer an effective protective layer. It now tends to keep the oxygen out of the area, and oxygen is what S/Steel uses to make it "stainless". One therefore has a type of "crevice corrosion" type of situation where contaminants such as salt (or just about any electrolyte) can get in contact with the unprotected stainless. (Between the burned oxide layer and the stainless base metal.) This typically leads to pitting corrosion. This is the scourge of stainless, because while the bulk of the material looks good (no general corrosion) you have isolated holes in the pipe. (Bad news in the process industries.)
In fact, under certain circumstances, you can even get a rusted appearance in this area over time.
Due to the non-critical nature of this application, I would recommend that the discoloured area be brushed clean. Any further passivation will then again occurr of its own accord. If however it was in a process plant, I would still recommend pickling and passivation.
Regards
Niekie Jooste
Fabristruct Solutions
L designates low carbon content. The reason the L grades have a different allowable stress in the boiler&pressure vessel codes is that the lower carbon also results in some reduction of high temperature strength compared to the normal grades.
"Failure" is a relative term that has to be related to the intended service. In pressure retaining service a failure might mean corrosiuon pitting that results in loss of the contained fluid. In architectural applications a failure might be defined as unsightly rust staining. I have seen both occur as a result of leaving weld discoloration in place. Usually atmospheric corrosion won't result in much more than superficial corrosion and staining.
I think we are over analyzing a bit here.
I'm helpless but to continue the overanalyzation.
1/8 inch plate, partial pen butt joints at less than 50 amps are not going to be a risk for carbide precipitation unless the welder holds the torch in the same spot for half an hour.
While its true that its a good corrosion preventative measure to remove oxide from the top of the weld, just seeing the color after your weld has been completed is no cause for alarm in most cases. It can be simply brushed off.
The partial pen joint leads me to believe that the designers were not all that worried about cleanliness anyhow. Who knows what will eventually find its way into the back side of that groove.
D17.1 says something to the effect that oxide of any color is permissable on stainless, nickel and several super alloys.
Unless this is food service or Bio med (again doubtful with the partial pen) the only worry is basic soundness.
Good gas coverage with your torch angle near to perpendicular will allow the argon to evenly cover the weld; any more than about 10 degrees of push and you starve the back of the weld. I think somebody already mentioned gas lenses and a little higher flow rate, thats the trick. Tim Garry was also perfect in his advice to protect the back side of the weld, his post alone will solve your problem <g>.
What you shouldn't see with your setup is grey and black, that would be an indicator of poor torch angle or a possible draft blowing your argon coverage away from the weld before it solidifies.
I agree that sensitization is highly unlikely with the particular joint being discussed (thin wall, partial pen.). However, I don't agree that the only concern is soundness. The designers probably designed the joint for the required strength with little consideration of the probability of corrosion. In the handrail application the preferenctial corrosion and staining at discolored welds probably won't affect structural integrity, but very possibly could result in unacceptable, unsightly staining, if this is an outdoor application. I've seen some really ugly examples of it.
I increased the argon flow to 18 cfh and that seems to have cured most of the black discoloration next to the weld.
The SS tubing is for handrails...not a critical application, but I also don't want any rust showing up after somebody cleans them years down the road since they are being installed in a multi-million dollar house!
I just had no idea if carbide precip. was visible or not. I also didn't know how much heat it would take to precip. the carbides. Now I know I don't have a thing to worry about. Thanks!
Good job welder_guy. I kinda think you were looking for a basic answer for appearance sake.
The increased argon helps but too much causes problems too.
Watch your heat and the speed of your welding. Too hot leaves the wagon tracks if your hand speed cant keep up. Too cold does the same thing because you hae to weld so slow to melt any filler that you seem to burn up the pipe.
Find the happy medium. And keep your materials CLEAN.
good luck
Also, all of my welds are being grinded/polished out so you can't see or feel the weld.
I also got a #8 (1/2" dia.) cup. Life is now good. :-)
Better late than never I guess.
To directly answer your question and not get into analizing.
There is an Amerian National Standard called AWS D18.2:1999
Guide to Weld Discoloration Levels on inside of Austenitic Stainless Steel Tube.
You can get an idea as to what heat-tint oxides are in the HAZ.
The number 3 is typically the worst that will be accepted from an Sanitary standpoint
Good Luck
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