I'm with Greg. Heat as always. But instead angle iron, how about large thick piece of aluminium or copper. It would tranfer the heat out from the welded piece. Pulse is nice on that kind of welds, just set it low p/sec.
Or fab the piece little bit different. Cut the opening but make hole smaller and make straight sections of the angle from it and only weld the cornerpieces.
Another cooling style is compressed air, blowgun, less mess and easier to continue welding than water.
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, Juhani
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