Hello again!
The fact that there's no gas protection when welding from the inside is the first part of your problem. the second part of your problem is the fact that you're backgouging is the second part of your problem in two ways.
One is excess heat input from the carbon arc or the weld passes themselves as GRoberts mentioned and Two is the addition of carbon from the gouging into the base metal and the previously welded side.
Upon further metallurgical examination this will bear fruit.
Now I'm not going to tell you what to do but, I can only recommend some changes that need to be made in order to minimize or even eliminate your problems.
First is to totally change your welding procedure whereby you do indeed protect the root side of the initially welded side of the joint with a high (less that 10ppm of O2) purity argon gas shielding (purging).
The same goes for the shielding gas for welding.
Get rid of the Carbon arc gouging on DSS PERIOD!!!
DSS= Duplex Stainless Steel
Get yourself an O2 monitor or gas analyzer so that you can constantly monitor the purity level (amount of O2 present in both the purging and shielding gas in parts per million) Do Not Weld unless the O2 content is at less than 10ppm.
On a side note: I do'nt trust blended gases for welding and shielding of DSS because of the fact that the purity levels cannot be maintained unless you blend them in real time and constantly monitor for O2 amounts in ppm.
Get yourself a DSS Weld/HAZ color comparison chart in order to "fine tune" your welding parameters (heat input) so that the amount of discoloration is minimized as much as possible. Avesta should be able to supply you with one.
Cleanliness is another TOP priority and should not be taken lightly especially with DSS, Ti and it's alloys or any other metal composition that would be extremely sensitive to any dirt, chemical residues, oils etc.
Passivating the welds/HAZ also needs to be performed after the welding is completed.
I wish I could give you some more recommendations but, due to the fact that a whole lot more information is needed from you regarding your situation, I'll stop here so, please let us know how you make out.
e-mail me at : hanklive39@hotmail.com so I can send you some .pdf's I have available on welding Duplex Stainless steels.
Respectfully,
SSBN727 Run Silent... Run Deep!!!