Brazing Tungsten Carbide Components

Details: Category: Tungsten Information; Published on Thursday, 11 September 2014 16:17

Tungsten carbide components are commonly joined to steels and other materials by brazing. Brazing involves placing a metallic braze alloy along with a fluxing agent between the components to be joined and then heating the assembly until the braze alloy melts and flows to fill completely the small gap between the two components. Soldering is similar to brazing, but is performed at lower temperatures with lower melting point alloys. Soldered joints, however, generally lack the mechanical or thermal strength to satisfy the requirements of many applications.

Although many variables determine the quality and strength of the bond between two brazed components, attention to a few important principles usually leads to a satisfactory result. Both the carbide and the steel components must be clean so that the molten braze alloy wets their surfaces completely and forms a strong chemical bond with each. Tungsten carbide components are often grit blasted, sanded, or ground to create clean, new surfaces, or plated or treated in salt baths to prepare the surfaces for brazing. Similarly, steel components are vapor degreased or cleaned with solvents or caustic solutions. The presence of any residual grease, oil, oxidation, dirt, or other surface contaminants adversely affects the wetting of component surfaces by the braze alloy and results in an inferior joint. Various tests of the flow of molten braze alloys over component surfaces are commonly used to assess the “brazeability” of the components. The relative importance of clean surfaces will, however, vary with the design and application of the brazed assembly.

Under the right conditions all common braze alloys from pure copper to silver alloys will readily wet cemented carbide surfaces. The most popular braze alloys (American Welding Society designations shown in parentheses) consist of approximately 50% silver and include alloys with cadmium (BAg-6), without cadmium (BAg-24), with manganese (BAg-22), and with tin (BAg-7). These alloys possess moderate melting points in the range 1150 to 1300ºF and can be purchased in wire, rod, or ribbon form or as a trimetal braze in which a copper shim is “sandwiched” between two layers of silver alloy. Braze alloy manufacturers are a good source of detailed information on the selection and use of these products.

For high temperature applications copper is typically used as the brazing material. Although copper has a lower tensile strength than the silver alloys at room temperature, copper retains much of its strength to temperatures approaching 1000ºF . If brazing is carried out in an oxidizing environment, borax is an effective flux for copper. Normally, however, copper brazing is performed in a hydrogen atmosphere where no flux is needed. Other high temperature brazes include high nickel alloys containing some chromium, boron, and silicon. These alloys flow at temperatures exceeding 1800ºF. It should be noted that high brazing temperatures may cause grain growth or other unwanted changes in the steel component.

Fluxes are generally used in combination with the braze alloy to minimize the oxidation of surfaces to be joined during the heating of the assembly. Both “white” and “black” fluxes are commonly used in combination with the silver alloys listed above. The two are similar except that the “black” flux has a higher boron content and therefore is more effective at higher brazing temperatures.

The basic brazing steps follow.

Lightly apply flux to the steel surface.

Position a precut piece of braze alloy on the steel and lightly coat with flux.

Position the tungsten carbide component and coat the outside surfaces with flux.

Heat the assembly evenly throughout its volume to the proper temperature.

Once the braze alloy is molten, jiggle the carbide slightly to allow any flux or fumes to escape. Do not press too firmly or the braze alloy will be forced out of the joint.

Allow the assembly to cool slowly. Make no attempt to cool the assembly quickly.

Wash off any excess flux with hot water.

The optimal thickness of a braze joint is believed to be about 0.004 inches. This thickness represents a compromise between the high strengths associated with very thin joints and the superior ability of thicker brazes to absorb thermal and mechanical strains acting on the joint. Brazing strains are minimized by brazing only one surface between the carbide and steel components. Cemented carbides expand and contract only about one-half as much as most steels. If the carbide component is constrained during either heating or cooling, excessive stresses can develop and failure by cracking may occur. If the braze joint consists of two or more surfaces, the design of the joint must allow the carbide component adequate room during heating and cooling. The brazing of a carbide ring to a steel core, for example, is a very tricky case. Brazing strains become more significant in larger or longer joints. In such cases, various design considerations are utilized alone or in combination to avoid the problem.

Methods of heating the assembly to be brazed include hand torches, batch furnaces, and high frequency induction coils. Torch flames should be somewhat reducing to minimize oxidation of component surfaces. It is important to heat the entire assembly uniformly to minimize thermal gradients and stresses and to reach, but not exceed the proper brazing temperature. Underheated brazes will not melt and flow properly. Overheating may cause low boiling point constituents in the braze alloy to boil off. This alters the properties of the alloy and may result in entrapped gas bubbles in the braze joint. Any environmental factor that influences the rates of heating and cooling of the braze joint must be controlled to insure that the quality of the braze joint does not vary.

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Brazing Tungsten Carbide Components

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