[sgtnoah]

The previous thread on stainless welding got me wondering, at what material thickness does back purging become necessary? I've been test welding progressively thinner stainless and at some point the sugaring starts (without a purge), just wondering what the experts consider to be the max thickness to weld without a purge?

Thanks,
Pete

[Otto Nobedder]

Pete,

If you intend 100% penetration, you need a purge at any thickness. You cannot have full penetration without sugaring unless you back-purge.

I've not worked sanitary pipe, but I worked alongside some of the best in the business (long-term contracts with Hershey Foods and M&M Mars), and they have elaborate jigs and fixtures to back-purge on anything from .035 wall tubing to sch. 80 pipe, whether they're fusion welding or filling a gap.

Steve S

[MinnesotaDave]

I'm interested in this topic. I added tig a few years ago and I just tried tig stainless for the first time - grabbed a scrap 2" tube about 0.050" thick.

Ran some beads to get the feel of it and the inside looked rugged.

Didn't want to purge for no reason on a scrap piece - but didn't expect it to sugar with just running beads.

I set my old Airco about 55-60 amps and used the pedal - so not huge amps - guess I'll purge next time

[Superiorwelding]

Pete,
I always try to either purge, tape or paste the backside of all my SS welds. It is best to start practicing this as soon as you can to get used to how this works and how to build the custom back purges.
As a example, You can have 1/4" plate with a 1/8" gap and require a back purge if you want a good weld on the back side. If it is food service work than you must always go for the back purge.
Steve is correct, you will not get a good pure weld on the back side without an inert atmosphere.
-Jonathan

[weldin mike 27]

Hey,

I do beleive that "full penetration " is the issue here (snigger) If as in a socket weld, the molten metal is not touching air, sugaring will not happen, but this is only above a certain thickness and there will still be discolouration and heat effected issues. So it is always a great idea to purge or protect the backside somehow. Such as with Solar flux.

Mick

[sgtnoah]

Thank you for all of your thoughts… Looks like I need to step up my game and get some purge equipment together! It's always good to read the replies backed with experience that can be found here.

-- Pete

[Superiorwelding]

Here is a topic I started on purging for your interest. I will add to this when I get a chance.
http://forum.weldingtipsandtricks.com/v ... f=5&t=4782
-Jonathan

[newschoppafowah]

In my experience, and in the experience of EVERYONE who taught me anything about Tigging SS, purging is as necessary to the process as having tungsten in your torch.

Seperate tanks, a splitter (you can achieve this cheaper at a big box hardware store than at your LWS), whatever, gotta make it happen.

Get o-on the bus.

[Rick_H]

I do food grade piping and like stated back purge is a must....however fit up is just as important you need square straight edges, must fit tight.

Since I do a lot with triclover fittings I have my own custom purge stuff, along with a dual Harris flowmeter. I'll be working on some stuff tonight I'll snap you some pics.

What welder are you using? What size tungsten and nozzle?

[sgtnoah]

I'll look forward to checking out those pictures of your welds and setups.

Right now I'm using a Miller Dynasty 200DX, with a "9" series air cooled torch, 1/16" 2% Lanth tungsten, and welding on .050 304 plate. Machine set to 50A, but probably far less than that used with the pedal.

Does this sound reasonable? I do not have a purge setup at this time, so I have to back way off on the amps to keep the other side clean. Trying to figure out the best way to keep gas on the back of longish irregular shapes. Possibly solar flux on the back for the time being?

-- Pete

[Rick_H]

Here's a 90 degree I did tonight

[Rick_H]

Here's some of the pieces I use...

The first on the left is the diffuser I use when I'm not using my special adapter for the triclover fitting. I use the plastic end cap that comes on a lot of the fittings, punch a hole for the 1/4" not fitting to go through and sand which it in between those washers....nice tight seal and a diffuser...then my tygon hose hooks to it from my flow meter.

The next item is a saw guide used when I'm tying in and need a straight cut where I can't use a gf or comparable saw. The next item is a "tigtack" clamp from Techsouth Inc. While it is not a necessity it helps align the pipes easily and gives you an area to tack them together....easier and more consistent then masking tape.

[sgtnoah]

Those are some beau-t-ful welds! Thanks for the weld and equipment pics. It's neat to see the quality work that skilled professional welders can do.

-- Pete