We are trying to vacuum furnace braze deoxidized copper tubing (ASTM B-75 , C12200 ) to a 70-30 Cu-Ni block braze assembly. The tubes bottom out on counter bores approximately .4” into the block for each joint. There are thru holes on each joint. We are putting braze stop off on the small id right below the counter bores to prevent braze filler (BAg 8, liquidus=1435F) from running out the backside of the joint. The main issue we are having is we are getting small pit looking marks in the fillet of the braze on the top od of the tubes. See pic attached. I was wondering if anyone had any recommendations to eliminate these marks. The furnace cycle we are using is shown below:
Hard Vac
Ramp rate 35F/min
Ramp to 1200F, Hold for 5 min
Ramp to 1435F at 4F/min (visually confirm alloy melt/flow through furnace view port, if seen proceed to furnace cool, if not place on hold until seen)
Furnace cool to <1400F
Nitrogen force cool <250F
Additional Notes
We have also been seeing intergranular penetration of the fm into the copper tubing, which is why we are trying to minimize the time at braze temperature. We are held to a max IGP requirement of fm into the bm of 20% bm thickness.
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