Reasons of Blackening after Vacuum Brazing and Solutions
After the process of vacuum brazing, the work pieces are likely to become black. There are two kinds of blackening, one is high temperature black, the other is low temperature black.High temperature black refers to heating from 1080 ℃to 1120℃，and then naturally cooling to 1080 ℃. Low temperature black refers to naturally cooling from 1080 ℃ to 850℃, and forced cooling to 80℃. The reason why it becomes black as following :
(1)In the brazing process, the furnace of some part appeared not sealed (such as the bottom of the furnace door, the top of the furnace door,electrode seal ring ,safety valve and so on )
（2）Whenpositive and negative pressure gas in, the nitrogen pressure is less than the required or not getting ormal air compensating, which is resulting the black color change.
Solution: We can change the black color to white by , keep the temperature perservation 35 mins in the temperature of 1060℃. However, If it is high temperature black, it depons on the the degree of sealing leakage and the copper solder loss degree.If the copper solder is in big loss, then the work pieces have to be discarded.But the vacuum leakage for a short time, the work pieces can be rebrazed. For rebrazed the work pieces, the technology process temperature must set 5 degree higher than first batch.re-welding process temperature and time curves are provided.
Edited by Jessica
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