Written by Rick Pollick
Mgr., Strategic Marketing – Penn United Technologies, Inc.
March 24, 2010
Tungsten Carbide, essentially a powdered metal comprising the elements of tungsten and carbon, has long been used for mechanical components in applications requiring wear resistance, erosion resistance, toughness, and compressive strength; such as in mechanical shaft seals, pump bearings, valve trim, down hole oil and gas tools for the Oil and Gas, Petrochemical, Energy, and other industries. But what makes Tungsten Carbide perform so well in these applications are some of the same characteristics that make incorporating the material into mechanical designs more challenging—from both a mechanical and a cost perspective.
Unlike metals, Tungsten Carbide does not lend itself to common fabrication methods such as welding, nor common attachment methods such as threading and bolting. The material cannot be welded, and threading Tungsten Carbide requires costly grinding or EDM operations. Furthermore, as the material will not yield—instead fracturing when its transverse rupture strength is exceeded—bolting becomes a mechanical challenge. Further exacerbating design challenges is Tungsten Carbide’s mass, which is greater than two times that of 4140 steel.
To address issues of fabrication, assembly, mass, and processing costs, designers have utilized several joining processes to attach Tungsten Carbide to another primary component or to a transitory metal component such as a threaded insert; among these attachment methods are brazing, gluing (typically with high-solids epoxies), and shrink-fitting. While each of these methods of attachment has its place, if the parts are round, as with mechanical seal faces, valve trim, and many down-hole tools, shrink-fitting offers the broadest range of mechanical function.
Shrink-fitting Tungsten Carbide components into mating metallic components is a process by which the metallic component is heated to expand its bore beyond the diameter of the Tungsten Carbide components, and upon returning to room temperature results in a gripping force typically described as “interference”. Among the design considerations for the interference joint between the Tungsten Carbide and metallic component are operating temperature, differential pressures, and the mechanical properties of the alloy used.
For those new to designing for Tungsten Carbide components and assemblies, this serves as the briefest introduction. Service, design, and cost considerations are myriad, and your qualified Tungsten Carbide manufacturer can assist in designing for satisfactory service and lowest practical costs.
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