QUESTION: I am trying to furnace-braze a Monel K-500 thin plate to another thin plate of Monel K-500 using a nickel filler metal, but I am having problems. I’m using a very dry argon furnace atmosphere in the furnace, but the parts don’t braze well. They are also very discolored, even though I’ve cleaned the furnace thoroughly and the atmosphere appears to be good. What am I doing wrong?
ANSWER: Monel K-500, also called Alloy K500 or K-Monel, is a nickel-copper alloy consisting of about 65% nickel, 30% copper with the rest consisting of aluminum, iron, manganese and titanium. The fact that it contains a lot of aluminum (almost 3%) and some titanium (over 0.5%) in its chemistry in order to create a heat-treatable alloy also greatly affects its brazeability.
Unfortunately, it is virtually impossible to “wet” the surfaces of high-aluminum, titanium-containing metals because of the aluminum-oxide and titanium-oxide layers that instantly form when the Monel is heated. These oxides cannot be dissociated (removed) during the brazing process. Therefore, such surfaces must first be nickel plated, which can then make it readily brazeable. If you intend to braze the K-Monel to another K-Monel surface, then I suggest that you use an electrolytic nickel (pure nickel) to plate those surfaces to a thickness of at least 0.0006 inch or thicker. This will allow the plating, which diffuses away readily during brazing, to be thick enough to last throughout the entire brazing process and allow a good, strong braze bond to form between the two nickel-plated Monel-metal surfaces.
If these parts are critical surfaces, the failure of which (in service) could cause irreparable harm to product or to people, then I strongly advise that you perform a “blister test” on the plated surfaces before you commit them to brazing. This is accomplished by putting both plates in your brazing furnace, side by side, with the plated surfaces facing upwards (open to the atmosphere) and heating them to a temperature of about 1000°F (above about 500°C). Then bring back out of the furnace and examine the plated surfaces. If the surfaces are bright and very smooth, proceed with your brazing program.
But if those plated surfaces show any blisters, that indicates contaminated surfaces below the plating were not properly cleaned. Those blistered plates should be sent back to the plater for complete stripping of the nickel-plate, re-cleaning of the metal surfaces and applying fresh plating. Conduct another blister test on those re-plated plates to be sure that the plating is smooth, flat and tightly adherent to the metal surfaces. If so, proceed to your brazing program.
Since the Monel contains so much copper, I do not recommend brazing the components in a vacuum furnace unless you use a back-filling gas in the vacuum chamber to build up a partial pressure of about 100 microns or greater. This will suppress any outgassing of copper during the brazing process.