Please forgive me if I omit critical details, but I am relatively new to the welding industry. I am grateful to the AWS and people like you for sharing their knowledge with me.
I work as a fabricator/ maint. tech.in a further processing plant that serves the poultry industry. Our plant utilizes T304 sanitary tubing to transport everything from cooking oil to raw product. The tubing ranges from 1-6"(approx. 0.050"-0.120" wall respectively). I am currently using the GTAW process, with 0.062" (Th 2%) electrode, 308L filler, and Argon purge to repair and fabricate our tubing systems. This has been producing excellent results with regards to internal and external weld appearance on our 1-2" tubes. The trouble I am experiencing is on the larger material. To achieve full penetration and thus the smooth "Wedding band" on the I.D., more current is required. Unfortunately, this leaves a dull gray finish on the outside of the tube. I find this unacceptable and want to reduce/eliminate it. I am using a relatively small nozzle on the torch head, and I have ordered some .750" diameter lenses as a result. Furthermore, I am utilizing the "Pulse Programmer" on our Synchrowave 300 in an effort to increase travel speed. This has helped slightly but has not eliminated the problem completely. We are passing USDA inspections with flying colors; however, I feel there is room to improve my work.
Any information on this subject, including recommended techniques, would be greatly appreciated.
Mr Milligan,
It sounds to me like you need to increase your travel speed or turn down the heat. I see that you are taking advantage of the pulse from your machine and using a gas lens on the rig. Most welders have more difficulty welding the smaller diameters than the larger ones. But in this case you are different. Does the larger pipe have a thinner wall? The best advice I can give you is dont get frustrated, stay within the ranges of your WPS, and keep tyring to improve yourself.
Must say that everything you are saying in your post sounds pretty good to me. The dull gray appearance on the outside of the pipe sounds like a possible purging issue. The following are things to keep in mind:
1) Too low a shielding gas flow rate will not give enough coverage. Too high a flow rate can result in a venturi effect, which means that it "sucks" air into the gas stream. A flow rate of 10 - 15 Litres/min tends to do quite a good job. The flow rate should not really need to change between the larger and smaller diameter pipes, so I do not believe that your problem lies here. (Unless you are changing this when you are increasing your current.)
2) If you hold your tig torch at too acute an angle to your workpiece, you can also "suck" air into the shielding gas. Do you change this angle between the pipe sizes?
3) Obviously you need to stop the purge gas from exiting from the root gap between the pipes. To achieve this, most people use masking tape to close the gap. How you do this, and also how you remove this while welding can have some effect on your shielding and possible contamination into the weld area. (Some people just weld and let the arc burn the tape away. - This may give you some discolouration of your weld.) How do you do it, and is there a difference between the different pipe sizes? Do you have a higher gas flow rate for your purge gas with the larger pipe size. If so, it is possible that you are getting some "turbulence" around the weld pool due to the interference in flow between the purge and shielding gas. (Only an issue if the purge gas is coming out of the gap around the arc with some speed.)
Hope this helps
Regards
Niekie Jooste
PS. As wildturkey said, it is unusual that you are having problems with the larger diameters.
Thanks for the advice;
My apologies for omiting a few of the details. First, the larger tubes do have a heavier wall. As far as the gas flow is concerned, I ran weldcraft spec. of 18 Lph. with the original nozzle; however, the large lens requires nearly 40. With regards to parts fit, the ferrules are lathe cut, and total gap is below 0.010". I haven't been cutting a bevel on any of the tubes, and maybe I should start. Since the ferrule side is already in the lathe, how does a single groove (30 deg.) sound? Furthermore, I am guilty of excessive lead angle (45 deg. from tan.) Correcting this may even improve penetration, weld speed, finish etc. One more quick question: I know only enough about taping the root to be dangerous, but do any of the chemicals in "Duct" or "Masking" tape pose a threat to the integrity or corrosion resistance of the finished weld? At the moment, I am relying on aluminum foil and tie wraps to cap the tubes, and purging from a "Y" connector on my torch bottle. Running a seperate purge system could allow better control of gas flow exiting the root area. Espically since I would like to experiment with a few Ar/H blends. These are details I may have overlooked If not for your input.
Thanks;
Chris in North GA
I’ve always preferred separate cylinders but your Y-branch connection is a common occurrence and useable but only if it has two flowmeters’s/valves.
You’re asking a difficult question concerning the use of tape because all service requirements are different. Niekie3 will have some interesting comments I’m sure concerning the tape chemicals but I would offer you this. I come from a chemical manufacturing industry which includes quite a few fermentation applications (similar to yours but different) where taping 300 series SS joints with masking tape is as common as breathing. I have never been made aware of a failure or a corrosion problem that was traced to the use of masking tape on the joint.
Duct tape though is not used for any part of joint coverage. If it has come in contact though fixturing or other uses, the affected area is washed with acetone prior to welding or cutting. Contaminates are contaminates, just some are far less problematic then others!
Regarding the use of masking tape, I can only give the same comment as Seldom, that it is THE material used most widely. I do not know what is in the glue, but so far have never come accross failures caused by the use of masking tape. (If there is somebody out there that has heard of this, please let us know.) I have never seen duct tape used for maintaining shielding gas. Besides the possible contamination problems, it is also more difficult to work with and more expensive. I would recommend you stick with the masking tape.
Seldom makes a good point about possible contamination of the shielding gas. We have had problems with welds in GTAW, and then sent some gas in for analysis. It came back as 99.5% Ar. This was not pure enough. Typically you need 99.99% purity to be absolutely sure you are OK. I know some fabricators that only purchase analytical grade Argon (99.998%) for GTAW.
Regarding the wall thickness and prep issues, I believe that you will be better off with a 1mm land and a 60° included angle. This will allow you to use amperages that are not quite so high. You will just have to increase the amount of filler you add. Welding a 3mm wall without a prep is however not a problem, as long as you have the skill to do this while achieving the desired penetration.
Just a quick question: Are your welds radiographed 100%? If so, how do the radiographs look? Contaminated shielding gas can sometimes result in porosity in the weld.
Regards
Niekie Jooste
While Wildturkey and Niekie3 make some good points, but I'd like to offer a few additional options:
1. I notice you mentioning, "using a relatively small nozzle (cup)". Many times the surface condition you've explained as "a dull gray finish" can be the result of too much cover gas velocity. You may or may not have increased your cfm but as cup size gets smaller so does the gas velocity. As the velocity increases so does the venturi effect. This correlates into pulling in air behind the gas stream that contaminates the back edge of the puddle giving you the "dull gray" appearance! The use of a lens is always a good thing but it won't have an affect on the gas velocity, a larger cup certainly does though.
2. A pinhole leak in your hose or torch will allow air (venturi effect) to mix with your gas and a similar effect as you described will be noticed.
3. Have you changed gas cylinders recently? Many times you'll get argon cylinders contaminated with CO2. This happens when some customer using the GMAW process is using a cylinder of argon and one of CO2 and is mixing his percentages with only a Y in the line instead of a regulated mixer. Generally speaking, the GMAW welder is going to use about three cylinders of argon to one of CO2. Think about what happens when the welder runs out of argon but his cylinder of CO2 is still flowing along! The empty argon cylinder goes back to be refilled but it has a small amount of CO2 in it. The cylinder is filled and what happens, somebody using GTAW to weld SS is sent the contaminated cylinder. The GTAW welder wonders why he is getting this dull gray finish when he's been getting a nice shiny dime or light gold before!
These are a few prominent examples of what I've found over the years "trouble-shooting" for the cause of exactly what you've described.
Maybe one of these pertains to your situation and maybe not but good luck!
My turn,
A more simplistic explanation may be something as simple as Technique/torch angle. Remember if the same setup is producing good color in small diameter/thickness material the torch setup is prolly good.
Many folks employ a technique called "Walking the cup" when GTA welding pipe. In this technique the cup rests on the pipe while a rocking motion of the wrist weaves the bead in forward progression. Filler may or may not be added. Now, if the tungsten stickout is a little too long your torch angle can become such that very little argon covers the weld after freezing, an indication of this is the grey outer appearance you mention. With 304/308 the stage before grey is a reddish hue. Hotter welds on thicker material will obviously produce a larger heat affected zone which is more difficult to cover.
I would think that poor fitup with leaky purge gas would only leave sporadic bad spots and roasted tungstens while welding rather than an unacceptable finish on the entire outer bead.
If your pipe does not have a deep bevel you may be able to employ a slightly larger cup with a slightly shorter stickout, this may begin to solve your problem.
Lastly. Design specs dictate what is acceptable and what is not. If all of the above advice fails, A nice power wire wheel may be the best remedy. If the welds are passing mechanical muster and your engineering documentation does not forbid brushing the outside of the weld you may want to go that route. 300 series stainless is mighty tough stuff and oxidation on the outer surface is often acceptable if followed by mechanical cleaning.
Keep us posted on your progress
Lar
just a little note on lawrences post. Addition or deletion of filler metal is an essential variable so beware of "washing" your welds.
Right on Mr. Turkey.
My only point was that one can "walk the cup" with or without filler wire. Not everybody is familliar with that non standard term, and to avoid confusion by the casual reader I indulged in a brief explaination. Back in the day I welded miles of 304 sch. 5 with what our forman called "Brewmaster" welds. A cup walked fusion weld with no filler. Now one can also walk the cup on Sch. 180 pipe but of course filler wire is necessary. I just assumed somebody working in food grade condations is working to some kind of code. So to be extra clear. Addition or deleation of wire must always be done with the support of proper engeneering documentation.
While welding on SS tubing for Pharmaceutical industry use, weld area discoloration and contamination is a big issue which is addressed by the ASME BPE Code.
If your not having discoloration issues on the inside of the pipe then your purge metod is fine. A point about the tape though is that ordinary masking tape adhesive contains chlorides that are known to be detrimental to the corrosion resistant properties of SS and may aid in "rougeing". Therefore we use to use a special purging tape that was chloride free for joint sealing. I din't recall the name or manufacturer but I know it was supplied to us by Briggs & Weaver.
The discolration on the outside is a result of the base metal being heated over a certain temperature that causes it to act like a magnet to attract and solidify impurities from the air. As you are having to apply more "current", the metal has not cooled below the certain temperature by the time your sheilding gas has moved along with your cup. If a large gas lens cup is not sufficient to fix your problem, you may want to install a trailing gas fixture that will continue to shield the weld as you move along.
Good Luck!
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