questions and opinions

By arcboy

Date 01-22-2007 20:03

anybody have any facts they would like to share?

By chuck meadows (***)

Date 01-22-2007 20:22

I'm not sure you can determine the extent of contamination based solely on color. "Color" primarily relates the extent of the chromium depleted oxide layer.

I don't think AC is the way to run GTAW. You might end up with a ruined tungsten.

I've never seen anyone weld aluminum foil, but I'm sure it can be done with some Hi-Tech welding.

By Lawrence (*****)

Date 01-22-2007 20:50

Arc,

The facts about contamination of stainless could fill several books, but here are a few facts that may get you started.

1. The oxides and nitrides that cause the color on the top of stainless GTA welds and there acceptability are going to be determined by the service/code requirements of the componant welded.

For instance. In the aerospace welding world, any color (discoloration) on nickle and stainless alloys is considered acceptable for a finished weld (*See AWS D17.1 Table 6.1). However the oxides should be removed between layers if there are multiple passes. The oxides and nitrides are so tough and tenatious that it is thought that they actually enhanse part performance in jet engine hot sections.

In other applications such as stainless piping or stainless food service, the discoloration may be a place for bacteria to begin to grow if it is inside a pipe or tank, or may be a place for corossion to start on the outside of a componant. Most standard practice manuals advise that the oxide layer be removed by brushing with a dedicated stainless steel brush or chemical removal methods.

My opinion is that if there is no code or standard practice to follow, oxides are better removed when possible.

As far as AC and steel with GTAW, the nature of an AC arc is less desirable than DCEN for 99% of applications. I took part in some trials with a Hybred Asymmetric power supply (Miller Aerowave) in an attempt to get a just a tiney puff DCEP cleaning action to help with field welds of Inco 718 that could not be surface prepped on both sides. After a promising start those trials never went past the scrap practice stage due to less than adequate results.

I think even with cheater lenses my foil welding days are over.

By JdB Date 01-25-2007 09:52

Aluminum is almost always welded with AC GTAW; you need the positive cycle to clean the Al

By CWI555

Date 02-13-2007 03:02

Lawrence,

I don't suppose you could quote the name of one of those "standard practice manuals?"

By Lawrence (*****)

Date 02-13-2007 04:09

155..... not sure exactly what you want to know

Are you looking for doccumentation that backs the cleaning of post weld oxides or doccumentation that is ambivalant to color?

Chucks Avista Manual gives all kinds of advice about corrosion prevention and sanatary applications regarding Oxides/passivation

Sandvic the same

Lincoln Electric has a pretty nifty little stainless guide that speaks to corrosion prevention.

ASM Metals Handbook Vol 6 has tremendous depth

Aerospace (Jet engine hot sections) often go the other way with stainless and super alloys.
Both Pratt and GE aircraft fusion welding standard practices along with United, American and Delta Airlines (who are strongly influenced by the OEM's) Have similar visual inspection critera for stainless and super alloys as far as oxide/nitride coatings on finished welds. All along the lines of D17

I did quote chapter and verse of D17.1 Now if you take a glance at the list of the authors of D17, who you will find? Pratt, GE, Boeing, DOD all branches, NASA, American, Delta, United, Rayethon etc. These big fish are the ones who set the visual color acceptence criteria I noted. As the welding instructor for Unted Airlines I worked with all of these organizations to a greater or lesser extent over the years. Being in the repair business we were pretty open about sharing stratagies for refurbishing components, good ideas were just too critical to keep to yourself.

So the argument for passivation/cleaning of weld surfaces is pretty dependent on service conditions... What is it you need to know about specifically?

By CWI555

Date 02-13-2007 17:15

In specific, the service is cryogenic, in a sea/salt air environment, 304L and 316L. Not necessarily related to the welding only either. If you have something specific it owuld be appreciated. (B31.3)

By Lawrence (*****)

Date 02-13-2007 17:37

This link is related to duplex...not austinetic.. but might be handy just to keep on the hard drive for later

http://www.imoa.info/FileLib/Duplex_Stainless_Steel.pdf

Most of this is over my head but wow.. Lots of data
http://www.valve-world.net/pdf/11021.pdf

By SWP

Date 01-25-2007 15:25

I belive it is a valid common practice to judge the quality of shielding based on the color of the finished weld and HAZ. I understand that the Navy uses color guidelines for Titanium welding. This is reference from TWI on titanium.
http://www.twi.co.uk/j32k/protected/band_3/jk24.html
Here is a link to a correlation of stainless steel color to oxygen content in the shield gas.
http://www.intercononline.com/welding/ypurge.htm
The correlation of color to oxygen level is most valid if welding is done inside of a closed welding chamber, since open welding is affected by travel speed, torch angle, gas flow rate, heat input, etc.

I have heard that some people do weld steel with AC, presumably because DC is not available. Technically, I see no issue with it, other than the reduced heat capacity of the electrode with AC, although I wonder if the power distribution or concentration of the arc is broader with AC vs DCEN. Also, realizing that the oxide "cleaning" action of the DCEP portion of the AC wave is not important for steels as it is for Aluminum, I wonder about the possible benifits that might be derived on alloy steels, nickel alloys, exotic alloys.

By aevald

Date 01-25-2007 15:50

Hello SWP, with regards to the welding of steel with AC current. Where I teach, it is common for many of the students to go into a booth after someone else has been welding on aluminum and set themselves down and start trying to weld on steel without first checking for proper machine set-up. As you stated, one of the first things that typically occurs is the lack of energy to make a puddle and melt the steel as it normally would with DCEN, one of the next things that they tend to notice is the relative violence in the way that the puddle forms up, obviously a lot of noise, lack of precise control of the focus of the arc, etc. For whatever reason, even the color of the finished bead, if they get that far, is a bit different than normal. I can only guess that over time, trial, and error and also some testing of finished bead performance that it has generally been deemed that steel should not be welded with ac/hf current, as dcen is far superior. Just my $.02 Regards, aevald

By medicinehawk (**)

Date 01-25-2007 22:32

Color of stainless after welding is going to happen because unless you are welding in a chamber full of argon gas, oxygen alters the heat affected zone and thus the finished weld. Color is proportional to the amount oxygen present, not heat although one can argue that the critical temperature and time at which the argon gas is removed from the HAZ is relevant. I don't know of a chart that exists referring to HAZ and stainless steel welds in general. What I do know is from observation and experience. IMO the less oxygen during and after a GTAW process weld, the "cleaner" it is, meaning the closer to silver (base metal color) you get. I say this because I have made thousands of welds with an orbital machine which uses GTAW and has a computer program which sets & runs the parameters to fuse the weld. There are of course a lot more specifics, but the key point I am making is that it has a head which clamps around the weld joint and is self contained where very little argon escapes the HAZ and this being the case, hardly any oxygen is present. The welded joint comes out nearly perfect with the actual weld being silver and maybe a slight straw color several millimeters away following the circumference of the weld.
Also, I have made stainless pipe welds (schedule 10 & schedule 40) where the client demanded minimum color on the root side of the pipe. Purging with the pipes with argon, it became apparent that the only way to get a "clean" weld was to eliminate oxygen. How do you know if you have got all the O2 out???? You use an O2 meter which measures oxygen. So down stream from the weld you need an outlet for the purging gas, here is where you use this instrument to measure how much oxygen is present in the line.
In our air we breath there is about 20.9 percent oxygen, the rest is mostly nitrogen (and pollution) and that is way to much to get a clean weld. What is the magic number of allowable oxygen to get such a clean weld???? The answer is two tenths of a percent of oxygen or .02 . It would be nearly impossible to weld pipes like this with an open root (and not get color) so fusing the root was the answer.
It is interesting to note that a weld can be passivated to remove some of the surface contamination due to color. This can be done with a stainless wire brush, chemicals or a finished weld can be sand blasted clean.

By aevald

Date 01-26-2007 00:06 Edited 01-26-2007 00:11

Hello medicinehawk, I believe you made a very nice and concise description describing many of the issues concerning color when welding on stainless steels. I would like to add an additional comment. When I make welds on stainless steels, in particular, with the GTAW process, the amperage, rate of travel, size of cup, use of gas lense or not, amount of filler being introduced, and filler diameter, can have a great effect on the colors that will be present upon completion of these welds. HAZ zone and color can vary greatly due to some or all of these variables. The example I would give is as follows: I make a 1/8" fillet weld on some 10 GA. material, in my first weld I use a 1/16" filler rod and run with sufficient amperage to provide the correct width of bead and it takes me 25 seconds to make this weld from one end of the material to the other. In the second example I make a similar weld, but due to an increase in the amperage, rod deposition, and travel speed, I am able to make the weld in 10 seconds. Between these two welds it is likely I will end up with a better HAZ zone on the latter weld, there is also likely to be less residual heat present in the weld that has been made more quickly, so there is less chance of surface oxidation of the weld, as it has a chance to cool below the oxidation point before the gas shielding has been removed by the progression of the weld. This might not be the best example that a person can make, but the majority of folks who weld stainless will probably understand what I am getting at. The main point I am trying to make here is that there is an optimum combination of variables to getting the best weld for a given situation, in most cases that is part of the reason for WPS's and similar documents and also the reason that most welders spend countless hours perfecting their techniques for these situations. My $.02, regards, aevald

By medicinehawk (**)

Date 01-28-2007 12:14

aevald, I agree with what you wrote about travel speed, weld wire size and all being relevant. This is true, even the angle of the tig torch can affect color for sure. Let me ask you this: Are the mechanical properties of the stainless altered after welding on a weld which ends up black (after tig welding) because of poor technique, too high heat or bad gas coverage and which has been sand blasted to remove such color??? Will the finished product still be stainless?
Just curious.

By aevald

Date 01-28-2007 18:33 Edited 01-28-2007 21:09

Hello again medicinehawk, I would tend to say yes, in an instance such as that, the welder has done the same thing to the stainless weld and haz zone that would occur due to excessive penetration through the joint without proper backing and caused a condition that many refer to as sugaring. Simply put, stainless oxides do not posess the same characteristics as properly welded stainless, the black color would definitely indicate a level of oxide that would not be good. The stainless would likely lose it's strength as well as many of it's corrosion resistance and wear resistance qualities. I have to cut this short right now, I'll see if I can expand on this further, later. Regards, aevald
Had to take care of a few things, back now. Stainless is one of many materials that are very heat and environmentally sensitive to welding processes. Incorrect filler material match and you can have a number of issues come up that will effect the soundness of the weld, strength, corrosion resistance, errosion resistance, heat resistance, to name just a few. Contaminants that are present in the welding environment can also have an adverse effect on the soundness of the weld, this could be in the form of chemicals, incorrect shielding gas, improper use of shielding gas, even oils from the skin(handling the filler rod with your bare hands) can cause issues with achieving a sound weld in some extreme cases of welding application. When I look at a completed weld on most stainless steels I don't want to see any black or gray, along with the color, I look very carefully at the surface condition of the fiinished weld, if it is smooth and doesn't exihibit any rough, scaly, or porous surface texture but still has the black or gray color you are probably marginal on whether it can be counted on to perform as it should(even in this case I would not accept this condition). If it is rough, scaly, and you can possibly also see some porosity and has the black or gray color as well I would feel that the weld will definitely fail and not work as intended. Many welds that are done on stainless require full penetration welding from one side only, in these cases, either a backing gas should be used or in some cases there are products such as solar flux which can be applied to the back side of the weld joint to achieve this protection. When I am welding on T-joints and trying to minimize the discoloration or oxidation on the backside of the weld I will clamp a piece of aluminum angle iron on the side opposite of the one that I am welding to help with this, it will also act as a heatsink and in some cases this will also minimize distortion issues. Hope this is some of what you were looking for. Allan

By medicinehawk (**)

Date 01-29-2007 10:37

Allan, thanks for the input and while I totally agree with you about what is acceptable and what you wouldn't accept, we both know that surface contamination in stainless steel due to welding is remove via the methods I mentioned and they are in fact in service today.
Thanks again for the replies.
Be well,
Hawk

By aevald

Date 01-29-2007 14:16

Hello medicinehawk, I would also agree with you that many times welds are taken care of as you described, when I was describing surface conditions in my post I was specifically referencing GTAW welds and not GMAW. The physical appearance in most cases will be considerably different and comparisons of the two are somewhat apples to oranges. Serviceability of welds will definitely have to do with their application and use, as you said many of these welds might survive reasonably well in some cases, but others will have a very short life. Regards, Allan

By arcboy

Date 02-04-2007 18:27

Well thanks for everyone's input, it is very informative and helpful!

By Milton Gravitt (***)

Date 02-11-2007 21:47

Yes, I have seen aluminum foil welded and it wasn't HI-Teck welding. I have seen people lose money on this trick.

By bozaktwo1 (***)

Date 02-12-2007 16:42

Never welded aluminum foil, but I have welded soda cans together in friendly competiton. 1/16" pure tungsten, argon set to a 14cfm flow rate. Can't really remember what amperage or the machine I was using, i had several to choose from then.