Question:
We are brazing an M-50 tool steel with a copper paste in a vacuum furnace at about 2040+/-25˚F for 3-15 minutes. After brazing, there is an apparent lack of brazing alloy inside the joint. We sent some pieces to be evaluated by our customer, and they would like us to re-braze them for lack of bond. They physically peeled the pieces apart and said there was less than 80% coverage in the joint. Brazing the parts in either a high-vacuum or in a partial-pressure atmosphere of 150 microns does not seem to change the outcome of the copper to seemingly vaporize or disappear. Do you have any suggestions?

Answer:
You have encountered a problem with using copper brazing paste in a vacuum that is common in the industry, and you’re situation is potentially further complicated by the M-50 tool steel. Here are some things to consider:

M-50 Potential Concerns
M-50 contains about 4% chromium and about 1% vanadium. Both of these elements can cause problems in brazing furnaces if they are not dealt with correctly. Both of these elements in the M-50 require clean, dry, oxygen-free brazing atmospheres to braze effectively, especially the vanadium content. Vanadium is highly sensitive to oxidation, much more so than either chromium or manganese and, therefore, requires a high-quality, clean, leak-tight vacuum furnace for good brazing.

By brazing in the 10-5 area, in a vacuum with a leak-rate of less than 10 microns per hour and above 2000˚F (1100˚C), you'd still be barely in the safe zone for getting rid of vanadium oxides. But if you backfill the furnace with dry argon gas in order to build up a partial-pressure atmosphere in the chamber to prevent outgassing of copper, problems may occur. The backfill gas will have a lot of moisture in it compared to straight vacuum, which could then result in the formation of vanadium oxide and perhaps enough of it to prevent full wetting inside the joint by the copper brazing filler metal (BFM).

Two other issues will be considered in our next blog.