gas question tyler evans friday nights

[Virgin Steele]

hi again i have seen another pic in bike fryday tyler evans building a stainless steel bike,

it seems that he has three set of gas tubes plug to the bb and vertical tube, what is the aim of this set up? is it just to gather fumes from welding or is another reason cos it seems very well seal just for collecting fumes.
cheers

[edoz]

It's because the back side of the welded area needs to free from oxygen, so the inside of the frame is filled with an inert gas such as argon. It's called purging, or back purging. The torch creates the inert gas zone on the front side. If you don't do it, the oxygen reacts with the stainless above a certain temperature and oxides form and it looks like a blind man was welding moon rocks.

[Tyler Evans]

Hey Virgin Steele,

What Eric said. Stainless must be back-purged to avoid contamination. The gas lines plug into heatsinks that distribute the argon gas inside the frame as well as suck the heat out of the weld to minimize distortion. There are small holes throughout the inside of the frame that allow the argon to travel trough the frame... essentially filling it up with argon displacing all of the other ambient gasses. Hope this helps. Tyler.

[Virgin Steele]

thanks both! good to know , i am learning a lot in the forum

[Mark Kelly]

There are small holes throughout the inside of the frame that allow the argon to travel trough the frame...

Tyler and everyone else: you need to watch the "small hole" business.

We do a lot of gas blanketing on a somewhat larger scale, eg excluding air from surface contact with wine in a 300,000 litre tank which is only 3/4 full. Wineries with an abundance of cash use argon, the rest of us use nitrogen* which is less effective but a lot cheaper, especially when you run an on site nitrogen generator.

One thing we have learnt is that flow restriction creates turbulence and turbulence creates mixing. The end result is that you need to flush several volumes of gas to get adequate inerting. If you eliminate the flow restrictions or otherwise reduce the turbulence and mixing, you can get away with a much smaller ratio of gas volume to void volume.

It may not sound like much but it might save you a few tanks of argon per year and they're not cheap.

* For others including OP: you can't purge welds with N2 it reacts with the metals at elevated temps. It doesn't react with the oxidisable compounds in wine.