Brazing tungsten carbide with Multiplaz is a very easy and reliable process if a few simple steps are taken but the steps are not necessarily obvious or widely known. There are many additional articles on How to Braze in the Brazing index.
The carbide must be wettable, it must want to chemically and physically bond with the brazing alloy. Clean tungsten carbide brazes well. Tungsten carbide with any sort of oil, grease, oxide or free carbon on it will not braze. You can clean carbide by grinding the surface with a sander or a bench grinder. The simplest is just to specify that the carbide be wettable when you order it. There are also companies that will treat the carbide and pretin it with brazing alloy so that it is ready to use when you get it.
Test carbide parts for wetting or Brazeability. Above are parts for a farm implement. The manufacturer couldn’t get the carbide to stick to the steel. He thought it was the chrome in the steel. We put a small bit of brazing alloy dipped in Black flux in the middle of one steel part (left) and two carbide parts (right) and heated them with a torch. You can see that the alloy flowed well over the steel and not at all on the carbide.
The steel must be clean. See our article on Cleaning Steel. It used to be common to use vapor degreasing and very aggressive solvents to clean steal. Most of the really effective solvents are now illegal so it is now common to use legal solvents that don’t work very well.
The easiest way to test steel cleanliness is with an eye dropper or similar. Put a drop of water on the surface and see what shape it forms. The wider and flatter the puddle, the better. A high, round shape means trouble brazing.
The best steel cleaner seems to be a strong caustic substance. There is a wide variety of cleaners that work with varying degrees of success. We tested about a dozen. The cheapest and most effective was a barbecue and oven cleaner from the local janitorial supply. For small parts we use a can of spray oven cleaner from the supermarket.
There are several brazing alloys used for carbide. The classic is BAG-3, 50% silver with Cadmium. This is an excellent product but does have Cadmium. Commonly used is BAG-7, 56% silver with Tin, because it wets out readily however it is a very weak brazing alloy and joint failure is common with this alloy. The strongest non-Cadmium alloy is BAG-22, 49% silver with manganese, but it is a bit gummy in the flow. BAG-24, 50% Silver, is Cadmium free and is a compromise. It flows well but is about 40% weaker than BAG-3 and BAG-22.
We strongly prefer Black Flux although many braze successfully with White Flux. In both cases they are clearly high temperature fluxes. In addition we find that purified Black Flux gives better flow and stronger joints than ordinary Black Flux.
The final area where mistakes are common is in joint design. Trained welders commonly want to assemble the parts and then run a bead. When they braze they want to assemble the parts and then wick the brazing alloy into the joint.
When brazing carbide it is often much more effective to flux the sides and bottom of the notch then put pieces of fluxed alloy wire under the carbide. All you do then is to heat until the carbide settles in place.
The standard should be that the carbide ruptures or the steel rips before the joint fails.