Back purging

Back purging, or flooding the back side of a weld with inert gas, is important for some situations, especially for reactive metals that will be damaged or contaminated by atmospheric gases during welding.  Of course, the very foundation of the TIG process relies upon protecting the molten metal pool from the atmosphere using argon gas.  However, the back side of the weldment might need protection as well.

Typically back purging is necessary on thin materials where the heat affected zone penetrates completely through the weldment.  This usually occurs with stainless steel tubing when making butt joints.  The inside of the tubing contains atmospheric gases and when the metal becomes hot enough on the back side, it will react with the oxygen and nitrogen in the atmosphere.  We must purge the atmosphere from the back side and replace it with an inert gas like argon.

For a video presentation of what happens with and without back purging, review the following:

How do you provide gas to the back purge setup?  You can accomplish this via a number of ways.  Here is a sequence of steps that will get you ready:

1. You need some means of extracting gas for the back purge apparatus.  You have several options:
   1. Buy a dual flow meter regulator and attach it to your inert gas cylinder.  One flow meter is for your welder, the other for back purging.  These can be somewhat pricey, and you will exhaust your cylinder faster by using gas for both functions from the same cylinder.
   2. Attach a running tee to your outlet gas line, with the flow meter for your welder on one branch of the tee and your gas purge line on the other branch.  Here is a video explaining how this works.  It can save you some money, but maybe not that much considering the convenience of the dual flow meter setup.
   3. Use a separate cylinder with regulator and flow meter for back purging.  You may not need it very often, but it is handy to have a backup cylinder to use for welding when your current cylinder runs down.  You can use the cheap regulator / flow meter that came with your welder and replace it with a better one, such as the Harris 355-2, or pick another one up for about $30.  This is no doubt the most expensive option, but if you live far away from your welding gas supplier or want to retain your own bottles rather than exchange them, you will have time to select the optimum time for your refill run, or even wait for the three week turnaround if you want to keep your own bottles.
   4. You need some way to seal the ends of the tubing and deliver the gas.  Here are a few ideas:
      1. Invest in some high temperature silicone plugs to match the size tubing you will be welding.  These can be expensive if you buy a back purging kit from a welding supply store, but these are available from other suppliers for powder coating and other applications.  Here is one supplier: High Temp Masking Supply.
      2. Use silicone gloves for the inlet and outlet plugs.  These don't need to be the thick ones for barbecues, unless your weld is close to the tubing end. This video shows how to attach the back purge gas hose and provide an exit path, as well as create an indicator that you have the right amount of back flow gas pressure:
      3. A lot of welders use aluminum foil to make plugs and shrouds to wrap around the tubing.  While this may work, it can be clumsy and fragile, especially if you are moving your weldment to obtain optimal welding angles and trying to keep the back purge gas outlet higher than the welding seam.