Some stainless steel welding codes require gas shielding on the back side. Food service codes for example do not tolerate a bunch of germ trapping crevices in the weld area.
If you have ever welded stainless steel pipe, you know that purging the back side is part of the gig. But how do you get shielding gas to the back side of a gusset or doubler?
answer...aluminum foil and or copper tubing or back up boxes...or taping everything off and purging out the inside.
Here is one way to do it... with the aluminum foil you can just form it to make a dam and tape it to the area that needs argon shielding. Then poke a hole and stick an argon hose thru the foil. Poke another small hole for the argon to escape and then set the gas flow to where you can just barely feel it coming out the escape hole.
Copper tubing can be used to make tools for gas shielding also...Just get some pieces of 1/4" copper tubing and drill a straight line of holes every 1/4" or so using a 1/16 " drill bit. Crimp one end with a vise or pliers or just crimp and weld it shut. Get some 1/4" id automotive vacuum hose and run it from a dual flowmeter nipple. 1/4" copper tubing will fit nicely into 1/4" ID vacuum hose. You can then put the copper tubing with small holes drilled in it on the back side of a gusset T joint and shield it with argon. A little aluminum foil to surround it helps too.
no matter how you decide to do it, getting argon shielding to the back side of a weld will require a dual flowmeter. You can make one yourself with the right nipples and fittings but I just bought one from flametech and I also sell flametech dual flowmeters now at weldmonger.com. I have been using it for around 15 years and it is still working, a little bent,, but still good.
The next best thing to shielding the back side of a weld with argon is to use a piece of copper for backing.
Copper has an almost mystical property to it when you back up a weld. Copper removes heat and traps a small amount of shielding gas and makes the weld much prettier on the back than without anything.
Smart tig welders scrounge every odd size piece of copper they can get their hands on and keep it in their tool box for backing up welds in all kinds of places.
And a bunch of spring pony clamps come in handy too for holding the copper backup in place.
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