Interpass Temperature for Welding 17/4 PH

There is an application where we are welding 17/4 PH in the hardened condition. Typical geometry includes a 1.5 inch V-groove depth with a .25 inch land. The Part diameter is roughly 7 inches, with a pin/box connection. The bevel from each side making an included angle of 40 degrees.

There is not an opportunity to heat treat after welding the material.

The process we use is GTAW with an austenitic filler material, and to keep interpass temperatures below 300F we experience extended periods of NPT. We would like to be able to weld continuously (without interruption). Copper fixturing with water circulation is an idea that has been kicked around.

Is there anybody out there who has experimented with other methods, perhaps cooling with Nitrogen or CO2 gas while welding. What mechanical properties can be expected with rapid cooling (in the substrate and the weld). Any information would be greatly appreciated.