Coloring of Stainless Steel GTAW welds

Date 09-25-2001 20:42

Metals (Steel & SSs) change color with enough heat and when they cool rapidly the color stays near the highest temp that area reached. There is also the fact that many SS are poor conductors of heat which makes the color band travel slower.

It would seem what you are seeing are welds that had little or no preheat and the color bands show the gradent of the heat traveling out ward from the puddle.

There are charts available (Pipe fitters book is one) that show the color of different metals at different temps.

By pipewelder_1999 (****)

Date 09-25-2001 23:17

The closer they are to the original color of the base metal, the less oxidation occured while cooling. I am basing this on experience with various thicknesses, gas cup dias and welding parameters. A pretty blue weld has oxidized more than a straw colered one.

If you have a machine with remote current/contactor control, set your post flow to max or leave the valve on the torch open, run a bead and when you get to the end slope the current down and out but do not move the torch. Keep it there for a minute or so.

Do the same thing on another bead on similar thickness and when you get to the end move the torch away as soon as the arc is broken.

There should be a difference .

Have a good day

Gerald Austin

By dasimonds (**)

Date 09-28-2001 07:54

So your saying the coloration is more of an indication of what temperature the base metal was when the shielding was removed. This would explain why a purged root has no color to speak of.

Is it the chromium oxide that gives the coloration, in say, 316 Stainless? And if it is, how does the "colored" chromium oxide differ from the film of chromium oxide that gives stainless its corrosion protection?

Dale Simonds

By TimGary (****)

Date 09-28-2001 13:27

While fabricating Stainless Pressure Vessels and piping for the Pharmaceutical Industry, we had to got to extreme measures to eliminate discoloration from welding. We were working under the ASME BPE Code which would not allow any discoloration beyond a light staw color. The color is caused by Oxygen an/or moisture being absorbed into the metal while it is above a certain temperature. (I forgot the exact temp) The hotter the metal is, the more absorbent it becomes. That is why the color changes as it moves farther from the weld. Oxygen gives pretty shades of yellow and blue while moisture generally causes brown and black. AWS has put out a discoloration chart which shows the different colors and gives an indication of how much (a percentage) Oxygen was in the purge gas while welding. When Oxygen has been absorbed, it deteriorates the Stainless properties of the metal and promotes the phenomenom known as "Rouging" not only at the discolored location, bust down stream on non-discolored portions as well. Therefore, discoloration is not allowed on Pharmaceutical or Semi-Conductor piping systems in order to reduce the possibility of product contamination. The way to eliminate discoloration it is to purge with only 99.999% pure Argon (from Liquid) via clean, dry stainless or polyethelene supply lines that have absolutely no leaks (which draw in Oxygen and moisture) and can supply enough CFM to displace the purging atmosphere completely several times per minute, along with a miriad of other procedural steps which I can't include here. I can e-mail a complete procedure if that's what you're looking for.
Tim

By Lawrence (*****)

Date 09-28-2001 18:16

Dale,

Regarding your excellent question about coloration of stainless steel after GTA Welding. The short answer is that the colors you see are oxides, which form on the surface of the stainless steel when atmosphere or surface contaminants come into contact with it at elevated temperatures.

Practically what does this mean? We must consider two different points at witch contamination/oxides can color stainless and the performance risks of each.

First and most critical is contamination of the shielding gas over the molten pool. Contrary to popular opinion and shop legend, the amount of contaminant it takes to cause marked loss in performance is NOT necessarily enough to cause discoloration of the weld face. This means that if your torch connections are somehow sucking oxygen, your cover gas is insufficient or moisture has entered your system from dew or a leak in your water cooled torch system, contaminants may be present when the weld metal is most vulnerable <soluble> and your project could be at risk. The point of this lengthy bit of prose is that a weld can look perfect and even Xray perfect yet have less than ideal mechanical properties. The amount of contamination it takes to foul a weld may not cause a color change. (This fact is deadly true with Titanium) By the way, this same risk is present when surface contaminants such as Lead; sulfur, oils and carbon are not removed during pre-weld surface prep operations. Also remember the old stick welding rule of thumb applies-Moisture = hydrogen=embrittlement.

Second. Oxide color caused by hot metal exposed to atmosphere after the trailing gas envelope has expired. This is the more common item and the performance requirements of the part will dictate what color if any is acceptable. Here the weld metal has solidified and as the welding torch progresses forward the argon coverage moves as well, now if the metal is still too hot, oxides will form on the weld face taking the form of discoloration. Most procedures require that oxides are to be brushed off for corrosion prevention and appearance. Also mechanical removal of oxides are generally required if multi pass welds are being produced. Welding multiple passes over the oxides can lead to embrittlement or lack of fusion at the toe of the weld causing "wagon track" or other linear indications.

Stainless steels and super alloys come in literally hundreds of flavors; each different alloy will react in its own fashion, some are virtually impervious and remain tough under the worst of conditions (347), other alloy formulas are far more sensitive (Inco718).

So what can we do? We can pay attention to our specifications and go the extra mile to actually understand why a given specification is requiring specific post weld appearance. We can also attend to process control, especially with manual GTA welding- Operator inputs (Craftsmanship) can make all the difference. Torch angle, cup walking technique with pipe, custom gas trailers as well as measures taken to baffle external breezes from the hot zone will all add to consistent integrity in output. Parent material cannot be too clean, your electrode cannot be too clean, the extra effort to employ the optimum cups or gas lenses can make a difference.

To further editorialize; We all have experienced ourselves or heard tales of welders hanging off scaffolds, hooked up to an R2D2 and scratch starting with a tungsten that has not been sharpened in 6 months and a chipped cup with a mirror in a rainstorm. Or *special* field welding techniques for flammable containers. If only we had an equal passion to know the best way to do things and build that knowledge into habit. Being versatile and making the best of a poor situation is the mark of a true pro. None the less, that same pro must enter the situation knowing the absolute best way to do the job.

More specific source material on stainless steels can be gotten from the "ASM Handbook" Vol. 6 or

AWS "Welding Handbook" Volume 3 Materials and applications part 1 Eighth Edition.

My fellow eggheads who may be interested in the colors in Titanium might enjoy A full report entitled "Development of Titanium Weld Color
Inspection Tool" it can be found in the March 2000 issue of The Welding Journal, Vol. 79, No. 3 (Pg.35-38). This article is a real eye opener for many welders. EWI also has good stuff on their website.

By RonG

Date 09-28-2001 18:36

Gee and all this time I thought it had to do with how fast and uniform you ran your puddle. Chrome & Nickel content not with standing.

By Lawrence (*****)

Date 09-28-2001 19:54

Ron,

Heat input controls are of enormous importance, so travel speed and heat sinks can be vital, especially when welding precipitation hardenable alloys. A weld with perfect gas coverage that is subjected to too much heat build up via poor technique has been the cause of many premature part failures, especially when dealing with the more heat sensitive alloys.

Also important to know when welding PH stainless, even when welds are produced with excellent color, the refractory oxides (aluminum & niobium, titanium) must be removed in multi pass welds or buildups.

By dasimonds (**)

Date 10-13-2001 20:20

What do you call "excellent" color?
And are the refractory oxides discernable by eye?
If so, what do they appear as?

By Lawrence (*****)

Date 10-15-2001 15:55

Excellent color could be no color at all via oxide or maybe a light straw appearance. For example, 321 stainless welded with 347 filler wire will often show a bluish tint, Not a big deal unless you are welding multi-passes in which case it should be removed. However, many high nickle alloys will never turn blue, oxidation appearing as straw or gold. With the great number of alloys out there (Each responding in unique fashion to oxygen and nitrogen) its important to hammer home this point. When making multi pass welds or even a restart on long weld. Its imparative to mechanically clean any area to be remelted. In this case what you can't see may hurt you. Not every alloy is this sensitive, none the less its just a matter of craftsmanship to put into practice this precaution. This extra bit of work won't have a great negative time impact on production but very well might improve your reject rates.

Excellent color for a 3 inch 308 stainless Schedule 5 fieldwelded pipe might be a little redish. The weld is sound and will perform its function. Still the welder brushes off that oxide eh?

By dasimonds (**)

Date 10-16-2001 10:45

Is no color a realistic possibility? Perhaps on a purged root, if the purge is maintained through out the welding process. But on the OD? I agree that brushing is definitely a must.
What would be a good heat input for achieving such a reddish color?(kJ/in)
As far as the mechanical cleaning, I assume your refering to grinding. Are you refering to the weld metal itself, the adjacent base metal, or both?
Is there a minimium temperature that one should wait for the base metal to reach before brushing? I've noticed that brushing too soon will result in a gold appearence for the entire fitting, say for example, a 3000# socket weld 90 degree elbow, especially if one does not wait for the piece to go through it's thermal cycling. There's also the issue of waiting too long, and not being able to remove the heat tint, or rather oxidation, by brushing alone.
I'm totally with you on the craftsmanship. As far as I'm concerned, there are no welds that don't matter. The most important weld I have ever done is the one that is right in front of me. It doesn't matter that it might not be pipe. It might be stitch welding. Cladding some boiler tubes below min. wall. Possibly some FCAW work. If it wasn't important, it would be bolted! I have found that it's the small details that can make a large difference in the end. And I do incorporate the information I aquire in this forum into my daily work activities. I used to try for perfect welds, but I'm not sure that's an attainable goal. But I think excellent surely is. And I think applying all this information spells-excellent! For myself, chance favors the prepared mind.
So many questions. It seems as though every time I get a question answered, several more arise. But I'm going to keep asking questions anyway. I truly miss working with gentleman such as yourself. Thank GOD for this forum.
And thank you.
Dale Simonds

By Lawrence (*****)

Date 10-16-2001 16:13

Dale,
Is no color a realistic possibility? Yes! At least sometimes. It may require a big cup or a trailer. Think again about Titanium. Now Everybody these days seems to have an opinion about manual GTA welding of Titanium and more specifically the colors or lack of color which may or may not be acceptable for a finished weld. No matter what the newest study comes out saying, or what story you hear from a guy making mountain bikes, one constant remains. The better the shielding and trailing coverage, the lower the reject rate and the fewer encounters with embrittlement occur. Remember things can appear fine during routine inspection only to later manifest embrittlement or other problems under the cyclic loads of heat, vibration or even exterior contamination.

Back to Stainless. Many of those cute new fangled orbital welders have been thoughtfully constructed with integrated argon trailers that keep coverage over the hot zone long enough to make what appears to be a perfectly colored finish on the OD right? As to removing oxides that again is kinda dependent on the function of the weld and the alloy involved. For instance the newly published "AWS D17.1:2001 Specification for Fusion Welding for Aerospace Applications" comes right out in Table 6.1 and says that in the matter of Discoloration of Stainless Steel welds that All oxidation colors are Acceptable in all classes. Food processing and Bio-stuff remove every trace that might be left even in the heat-effected zone. Now there are reasons for these differences. Those tenacious oxides that could be a gathering place for bacteria in a food service situation are actually a corrosion preventative and protector in some aircraft engine hot section components. And as to method, again if you're working to a standard it's prolly going to call out for specifics. I've seen everything from stainless wire toothbrushes to power wire brushes to 80 grit disks, chemicals or bead blasting.

This is fun Dale tho I fear we are the only two geeks who enjoy such discussion J

PS. The reddish color was from my own hand, long ago when I was learning how to "walk the cup"

By dasimonds (**)

Date 10-16-2001 19:06

Lawrence,
I had thought about Titanium (which I have absolutely no hands-on experience with) and the use of trailers, but I wasn't aware that anyone had applied it to stainless. I always use a jumbo cup for GTAW, whether it's carbon or stainless, just for coverage reasons. But I can't say I've ever seen a stainless weld that had no color on the OD, but then again, I've never seen stainless done with a trailer. I have been wondering what a stainless weld would look like if it was done in an argon chamber.

We routinely use 316L and 304L pipe. Are there any interbead cleaning issues during a multipass weld, schedule 40 say, that I should be aware of with these alloys?
Dale Simonds

By Lawrence (*****)

Date 10-16-2001 19:27

Dale,
Lets see how small these little boxes can get!

Visualize a stainless tack weld with a 20 sec postflow. It would be an example of no color eh?

As far as trailers go for stainless, they are commonly employed with semi-automatic or automatic GTA or mircroPlasma-arc welding buildups in turbines such as wear areas and knife edge airseals. Since it is not feasible to break the arc and remove oxides from the surface of welds on a component that may take up to 15 or more 360 degree passes, a gas trailer is incorporated to a large cup to make sure as good a coverage as possible occurs. We also employ heat sinks or specially cooled fixtures so that the hot zone is small and of a short duration. A trailer for handwelding of stainless seems to be a bit of overkill even to me J

In my experience with fieldwelds of 308 Stainless, schedule 5 all the way up to Sch. 160, I was taught to brush tacks before welding over them and to brush start and stop areas whenever I could.

By dasimonds (**)

Date 10-16-2001 21:22

Lawrence,
What about the oxidation between starts and stops? Or is it not an issue with 308L?
What of the refractory oxides, Aluminum, Noibium, and Titanium, you mentioned in an earlier post? Are these oxides specific to certain alloys of stainless? Which alloys?
What does the "J" mean?
Dale Simonds

By Lawrence (*****)

Date 10-17-2001 20:43

Dale,
I was taught as a rule of thumb on any stainless to always brush off oxides before welding over them. (But thats subjective) Weather or not it's a real worry with 308 is a matter to be addressed by the fellows who write the standards.

The refractory oxides mentioned above were tied to an example of Precipitation Hardenable alloys, some of which have high percentages of Alum and Ti which makes oxide removal critical in multi pass operations. The most readable source material and commentary on these phenomena I have found to be the good ol AWS Welding Handbook Vol. 3 Materials and applications. The ASM Handbook Vol. 6 also provides good data and much more of it, but is at a level of scholarship I must admit at times exceeds my ability to digest quickly.

The J is what the webhost translates a little smiley face thingie I keep trying to post :)

By dasimonds (**)

Date 10-17-2001 20:49

Lawrence,
Thanks for the clarification.
Happy welding!
Dale Simonds

By dasimonds (**)

Date 09-28-2001 18:55

Thanks to all for the excellent responses.

To Lawrence, Chance favors the prepared mind!

Dale Simonds

By qamanager Date 03-12-2008 16:50

Hi guys,

The company I am working for has been asked by the Client to supply a discoloration "Atlas" as the Danes call it . Can anyb ody tell me were I will get one . I have tried the ASME site but it will not let me as I am in the UK.

Thanks
Andy

By chall

Date 03-12-2008 20:21 Edited 03-12-2008 20:25

AWS has one for use in High Purity systems. I don't have access to mine, but I think it has 10 different shades of "coloration".

Charles

edit - It's AWS D18.2

By Jim Hughes (***)

Date 03-18-2008 12:08

Dale,
stainless steels are materials with great corrosion resistance. Chromium is the alloying element that gives stainless steels their ability to resist corrosion by combining oxygen to form a thin film on the surface of chromium oxides. This is called passivity. The oxidation that you see after welding in the form of color inpedes this passivity process and in its extreme form on the back side of a root pass which we call sugar. By simply taking this oxidation off the weld by brushing or grinding the thin film of protection is restored. Even in its exstreme form oxidation does not have much effect on the mechanical properties that I personally have seen, but I'm sure some might beg to differ with me on that point. Remember one of the main reasons we use stainless steels is the corrosion resistance. This invisible film can be restored very easy and quickly by brushing.

Thanks
Jim

By jon20013 (*****)

Date 03-18-2008 13:58

I once got into a heated argument with QC over stainless steel colors.

We had placed root tacks in a vessel and the QC guy looked inside and saw that the internal tacks were essentially the same color as the base metal (a GREAT thing), he rejected the tacks because they didn't have that "pretty" rainbow color, claiming our purge was improperly established (idiot)... well, he won and the tacks were cut out... to make a long story short, I'm no longer with that Company...

All too often nowdays, Companies are taking the advice from their QA/QC instead of their Engineering Staff, probably because they don't want the hassles.. NOT a good thing.

The rainbow effect most people seem to rave about is nothing but chormium oxides and should be removed when the welds are completed.

By MDG Custom Weld (***)

Date 03-18-2008 14:28

Amen to that jon!! Everyone thinks that the pretty rainbow means the weld is perfect, but we know it's NOT. Chemical treatment or wire brushing at the least is required to ensure the weld has good resistance to attack.

By jon20013 (*****)

Date 03-18-2008 20:24

It's really somewhat shocking to hear how many welder's have been taught that the rainbow color is a good thing though, isn't it? I know that's what my instructor told me when I was apprenticing.

By jon20013 (*****)

Date 03-18-2008 20:28

Andy, try TWI: The Welding Institute or perhaps the Nickel Development Institute (NDI is here in the states and has a wonderful technical selection free for downloading). You might even try ASM, American Society for Metals.

By vittorio (*)

Date 03-28-2008 11:05

I suggest these articles.

"Acceptance Criteria for Oxidation of Stainless Steel Weldments", Stainless Steel World, Vol. 8, Issue 10, December 1996. SMITH L.M., and KLEIN M.
http://www.intetech.com/images/stories/papers/Paper34.pdf

Welding and Post Fabrication Cleaning for Construction and Architectural Applications
http://www.bssa.org.uk/cms/File/Chris%20Baxter%20IOM%20paper.pdf

hope this help
V.

http://www.corrosionist.com

By 3.1 Inspector Date 07-11-2008 14:21

www.force.dk

Its not really an atlas, more a reference chart.

how did I find this tread?