How to Braze the Tungsten Carbide?

    Tungsten Carbide products are used in many applications, including cutting tools, drilling, punching, and numerous other applications. Cemented Carbide provides superior wear resistance and extends the life of these various wear and cutting tools. Cause its main component is tungsten, which is a rare and non-renewable resource, the price of cemented carbide is relatively high. In addition, cemented carbide has a relatively high hardness, and its bending strength is much lower than that of other metals, such as steel. For these two reasons, cemented carbide must be connected to other metals in different ways, such as through threaded connections, clamping, and welding. This article is talking about how to braze the tungsten carbide on other metals.

1. What is tungsten carbide?

    Tungsten carbide (WC), also referred to as cemented carbide, is a composite material manufactured by a process called powder metallurgy. WC powder is mixed with a binder metal, usually cobalt or nickel, compacted in a preform tooling, and then sintered in a furnace. The term “cemented” refers to the tungsten carbide particles being captured in the metallic binder material and “cemented” together, forming a metallurgical bond between the tungsten carbide particles and the binder (WC-Co), in the sintering process. The cemented carbide industry commonly refers to this material as simply “carbide”, although the terms tungsten carbide and cemented carbide are used interchangeably.  Carbide exhibits high compressive strength, resists deflection, and retains its hardness values at high temperatures, a physical property especially useful in metal-cutting applications. 

2 . Two points to ensure the success in brazing the tungsten carbide

A.Managing the stresses caused by differential expansion

B.Contraction rates of parent materials and wetting of the carbide by the braze alloy

    During heating and cooling, the base parent metal will typically expand and contract at a higher rate than the carbide.  Tungsten carbide has a thermal expansion rate of approximately 1/3 to 1/2 that of steel.  When the brazed assembly cools, residual stress may build within the carbide.  Slow uniform cooling of the carbide is always recommended to avoid stressing and possible cracking. Quenching is not recommended as it can cause cracks in the carbides due to the rapid contraction of the parent base metal.

3. How to choose the braze alloy

    Tungsten carbide is difficult to wet. Silver braze alloys with small additions of Nickel (Ni) are typically used to braze carbides to steel. Of course, both the carbide and steel must be clean so the molten braze alloy can wet the mating surfaces completely. Grinding the carbide surface to create a clean surface for brazing is necessary. Grinding also has the advantage of flattening the surface topography of the carbide ,which can aid with braze alloy wetting and adhesion. Steel components, similarly, would need to be cleaned to remove any residual grease, oil, dirt, or other surface contaminants.  

    ·Commercially available silver braze alloys with small additions of nickel (Ni) and Manganese (Mn) will readily wet cemented carbide surfaces. These braze alloys typically exhibit good wetting of tungsten carbides.  It is recommended to select a brazing filler metal with the lowest possible brazing temperature to diminish the residual stresses within the joint.  

For applications involving the brazing of large carbides, a sandwich braze alloy is often used.  If small carbides (1/2 inch2) cannot be utilized, a sandwich alloy is beneficial in preventing cracking and warpage of the carbide. These trimetals are clad with a braze filler bonded to both sides of a copper core.  

    ·Although much of the discussion has been around brazing of tungsten carbide (WC), we would be amiss if we didn’t mention polycrystalline diamond or PCD. The brazing temperature for PCD is generally kept below 1382°F (750 °C) to avoid degradation of the diamond.  Often, manufacturers of PCD tips to steel bodies will use a low-temperature, high-silver braze filler metal, such as a BAg-24 braze alloy.  Some manufacturers use a braze alloy without nickel or manganese, such as the BAg-5 or BAg-7 braze alloy, with lower melt temperatures and less wetting properties of the carbide and steel.

    A brazing flux is used to prevent the oxidation of the surfaces to be joined during the heating of the assembly.  Flux powder is used with the common silver braze alloys.  Black flux powder is typically recommended by braze and flux manufacturers as it has the addition of boron and is more effective at higher temperatures.

    There are several brazing alloys used for carbide. The classic is BAG-3, 50% silver with Cadmium.  This is an excellent product, but it 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 additio,n 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 heat until the carbide settles in place.   

    The standard should be that the carbide ruptures or the steel rips before the joint fails.