cracking stainless steel

[redmule1154]

I was welding up a 45 degree butt joint on a s/s box where I bent in a 2 inch flange all around both ends of the box. The material is 16 ga I was using 1/16 tungsten and 1/16 filler rod. The first 7 joints came out all right but the last one kept cracking slightly around the weld. I would grind the weld smooth and there would still be a few spots that were showing crack lines. When I tried to weld again to fill the pit lines the metal would instantly melted like it was paper thin. I have never experienced this before on the same product and can't figure why this happened. I was using a Dynasty 200 set at 38 and my gas was set on 18 cfm. I ruined that box which is a a 2ft x 2ft x 2ft square. Does anyone have any ideas. I'm kinda gun shy to go back to the project. I'm fairly new at tig welding but have had great success up until now. Thanks Redmule1154

[Blacksmith]

Not a tig welder, but if it was the last joint was your gas bottle getting low or did the flow change? I just got burnback on my mig and found that all the times I slipped the tension off the drive rolls to check gas flow before welding had loosened the tension. Only a little bit, but I run mine light. It's the little things that get you.

[redmule1154]

There was no change in the gas. Redmule1154

[Northweldor]

"The first 7 joints came out all right but the last one kept cracking slightly around the weld."

So something changed on the last joint. Since you have checked gas flow (for air leaks?), I would suspect contamination from your tungsten, cleaning, or ss brush.

[SundownIII]

Redmule,

Read your description but still having a little trouble "getting my head around" exactly what you're doing and where you're having issues. Think this is a case where a picture may be worth a thousand words.

Two things come to mind.

First, with 16ga material, you may find that you need a backpurge to prevent the sugaring from "pulling thru" the weld bead. Lack of gas coverage on the backside of the weld can create brittleness (and the cracking you're seeing). Another option would be Solar Flux B, but that's going to require some cleanup if appearance is important.

Second option would be to try high speed pulsing to reduce the average heat buildup in the base material. Your Dynasty 200 is the ideal machine for this type weld. Thin gauge SS was the primary reason I installed the pulser option on the Sync 250. My Dynasty takes pulsing to a whole new level.

Bottom line. Think your "cracking" issues are coming from the backside of the weld (especially the case when you're getting full or close to full penetration) rather than the face of the weld.

[4sfed]

I agree with Sundown, probably oxidation coming from the back side. I would try backing up the joint with a copper bar before setting up a purge ... not better, just simpler.

[redmule1154]

I do use a strip of aluminum for backing. I do belive it was a contaminated tungston. I jumped right back in to the job after the weekend and all is going well. As soon as the arc changes color in the least bit I stop and regrind. thanks for the help. Redmule1154

[Northweldor]

I do use a strip of aluminum for backing. I do belive it was a contaminated tungston. I jumped right back in to the job after the weekend and all is going well. As soon as the arc changes color in the least bit I stop and regrind. thanks for the help. Redmule1154

If your score is 7 out of 8 on the same weld, always suspect contamination.