I am trying to tig weld 14ga. ss for the food grade industry. All joints must be 100% welded on both sides, (or purged?). The pc. that we are fabricating is 12"dia. x 36"lg., one end has a flange turned out, which is no problem to get to and weld, both sides with no burn-through, but the other end, has a reducer/cone going from 12" to 2". I have had some luck in purging this pc., but I really think that I don't have everything set up just right. All openings are sealed off, and I have my additional argon line entering the fitting towards the bottom, which is running at about 50psi. I am only fusing the joint with about 60amps and moving pretty slow. The internal joint doesn't look too bad, but I really think it can be better. I would weld both sides of this joint only if I could get to it. Any additional suggestions would be greatly appreciated.
By -
Date 12-13-2000 12:17
It sounds like you almost have it. You also need to poke some holes on the areas that you have sealed. I'm assuming that you have the parts butted together tight with out a gap. These holes should be at the top of the assembly. This will give a place for the air to escape as the chamber becomes filled with argon. It is important that the holes are in the top, as the air is lighter than the argon and it will want to rise. I usually take a piece of sharpened tungsten and just poke a few holes through my sealing tape at. You should also turn down your purge rate to between 5-10cfh max. At 50 cfh it becomes very turbulant and the argon and the air become mixed up, it then takes longer to purge out all the air. If you purge it slowly you will have much better results. As well with 50cfh sometimes it will push the molten steel outwards and you will be left with a concave weld on the inside. Take your time and and have patients while purging and you will always have better results.
Good luck and let me know how it works out.
J Dryden
By -
Date 12-13-2000 18:20
It's been my experience that welders, even experienced ones, often get anxious and start welding before they have an adequate purge especially if they don't have access to an oxygen analyzer which is a small investment that will that the quesswork out and pay for itself if you do a lot of SS tig work.
The method that you are using to purge the 12"x3'lg pipe using a gas-flow rate of 50cfm and six volume changes will require approximately 17 minutes to get the internal atmosphere down into the neighborhood of 10ppm-25ppm. (shine like a dime)
You can cut your purge time and purge gas cost considerably by using an internal purge dam to cut your volume by 2/3-3/4 and it will allow for an initial, quick check of your internal bead without a complete loss of purge and a long wait to re-purge.
Also, I would recommend using a gas diffuser on the end of your purge hose instead using an open end. Without a diffuser, the gas swirls to the end of the weldment and bounces back mixing with the oxygen as it goes. That's why six volume changes have found to be required with this type of purging! I suggest using a "Speedaire" pneumatic exhaust muffler. Don't laugh! The pneumatic exhaust mufflers have proven to be very effective for enhancing and facilitating the internal purge when using GTAW & GMAW processes for piping as you are welding and nozzle attachments to pressure vessels. Usually 3/8" or 1/2" NPS is a good size and they can be found almost anywhere pneumatic supplies are sold (a couple bucks apiece, Granger catalog). They seem to blow-up and push the oxygen out the vents (in your case start with about a 2x2 hole and then close and use the punch hole vents and purge from the 2" nozzle) instead of acting as a "mixing-stream". It's OK to start with 50cfm but cut it back to 5cfm before you start welding.
