Here's a question:
Doing Tig on SS pipe. I have back purged the weld to prevent the sugaring on the inside. Run a nice bead and all. I turned off the purge once I finished the root pass to save gas. Go to do the fill/hot pass and then recheck the inside and some of the root pas is sugared up and drooping! Am I running too hot (color on the outside is light gold to silver) and too much filler? Or do I need to continue back purging. The sugaring is just oxidation from getting the Stainless molten, right?
Dean
Hello ason1965, the short answer is that you need to leave the purge on longer because you are getting the root above the critical temperature where it will indeed sugar. You could be running a bit hot or travelling too slowly after your initial root pass as you apply the fill passes. Barring making changes in your technique, an extended purge is the likely answer to preventing the sugaring and the simplest fix. Best regards, aevald
By Lawrence 
Date 05-05-2008 15:29
Edited 05-05-2008 15:31
Dean,
While you are currect that the sugaring is oxidation from getting the stainless molten.
Sugaring is usually a rejectable discontinuity/defect, which in the stainless pipe world most often requires removal of the entire weld and a replacement section "pup" put into place... This means 3 welds rather than one, lots of lost time and money and an angry lead man.
Purging requirements ought to be detailed in the WPS... If your working without one than you seem to have figured out for yourself that that purge gas must remain behind the weldment until it is no longer necessary :) The determination of what exactly is "necessary" is driven by the code or the performance requirements of the weldment. Sometimes a little discoloration is acceptable... in other applications like food grade work or biotech the standards are much higher.
Your passes that follow the root may also be causing rejectable discontinuities if the melt thru is found to be excessive. Again code or application demands dictate how much melt thru is allowed on the ID of your full pen stainless joint.
Leave that purge on even while your capping out on your thinner schedule pipes. Even your thicker schedules will color the root while you're capping without a purge on it and that can be bad like Lawrence said depending on what kind of service the pipe is intended for. Try to save the copany a buck and ya end up costing them a buck. Remember that sometimes going slower or spending an extra buck is actually going faster and saving ya a buck in the long run. Goes the same with racing, sometimes going slower through a corner is actually going faster because ya can keep it on two wheels.;-)
You didn't mention the size or schedule pipe you were welding but you could try putting the root in with 1/8" and the hot pass with 3/32". Weld faster and travel quicker across the root when welding the hot pass and concentrate/hesitate slightly on the walls to insure good fusion. Even if you try this you will need to keep back purge on the hot pass. You may be able to back off a little on the CFM's during the hot pass, but not much. As Kix stated, trying to save a dollar may cost you instead. You can tweak your technique when welding fittings to pipe by placing a clear lens on the opening and watching root, hot and fill passes to observe if you are re consuming the root or discoloring it too much. Then note your amps and approx. speed, argon setting, etc.
As a simple guide to proper purging, it was recommended in this month's Welding Journal that the purge should be sufficient to displace the volume of the vessel or pipe spool 6 times. The time necessary is dependent on the flow rate used. A better way to determine if the purge is sufficient is to use an oxygen meter and measure the moisture content of the escaping gas coming from the vent.
I have read that the purge should be maintained until at least 3/16 inch of weld has been deposited. I agree with the idea of maintaining the purge until the weld is completed. The flow of the purge gas can be reduced to a rather low level once the oxygen level has be reduced to a level compatible with the quality level required. The bottom line is, once the proper oxygen and moisture level is achieved, the cost of continuing the purge to ensure oxide free root surfaces is minimal.
Best regards - Al
