United Wolfram’s Cobalt-Chromium-Tungsten alloys (also Mo, Ni, Fe, C, Si and B can also be used), frequently called Stellites, are a group of Wear and Corrosion resistant alloys which are used for bearings, valve seats and pistons, mill inliners, and applications where a tough wear resistant material is required. They are applied either by hard facing as welding electrodes or powder spraying, or as massive forms produced by casting. The properties of stellite metal cutting tools are in the middle between those of high speed steels and cemented carbides, but stellite has lost its importance as cutting material due to modern cemented carbides. However, stellites have gained importance in the food industry due to their excellent corrosion resistance.
United Wolfram’s Tungsten Powders For Super Alloys are nickel, cobalt or iron based alloys with high contents of tungsten, molybdenum, Tantalum and, more recently, also Rhenium. Sixteen or more elements can be contained. Their important properties are: high temperature strength, high creep strength at high temperature, high thermal fatigue resistance, good oxidation resistance, excellent hot corrosion resistance, air melting capability, air or argon Re-Melting capability, Good welding properties and ease of casting. Tungsten accounts for solid solution strengthening, Strengthening by formation of intermetallic Compounds (e.g. Co3W), and formation of carbides. Super Alloys are used in Aircraft Engines, Marine Vehicles, and Stationary Power units as Turbine blades and Vanes, Exhaust Gas Assemblies and Burner Liners. They are also used as Construction Material for Furnace Parts.
TUNGSTEN HEAVY METAL ALLOYS
These are a group of two-phase composites, based on W-Ni-Fe or W-Ni-Cu, prepared by liquid phase sintering. Tungsten is the main component (90-98%). Ni-Fe, Co-Ni or Ni-Cu serves as the ductile binder phase for the brittle W grains. These alloys are characterized by a unique combination of high density (17-19 g/cm3), high strength and ductility. They are used as counterweights in aeroplanes, helicopter-blades, rotating inertia members, x-ray and gamma ray radiation shielding, as rigid tools for machining, for darts, for weights in golf club heads, as well as for ordnance purposes (high kinetic energy penetrators, fragmentation devices, etc).
In addition to the use of bulk parts made from cemented carbides, stellites or superalloys, the surfaces of big parts can be protected against wear by different surface modification technologies.
Plasma transferred arc welding (PTA) and laser cladding are both applied to prepare thick coatings of typically more than 1mm thickness with metallurgical bonding to the substrate. Predominantly mechanical mixtures of large particles of cast tungsten carbide and binder alloys, e.g. self-fluxing alloys, are used for coating preparation.
High velocity oxy-fuel (HVOF) spraying is one of the processes in the group of thermal spraying. It is nowadays the standard process for the preparation of highly wear resistant hard metal coatings. The coatings have a predominantly mechanical bonding to the substrate and a typical thickness in the range of 100-500μm. WC-based compositions with Co or Ni binder (mostly 20-27 vol%) are widely used. Chromium (up to 20 mass %) alloyed compositions are also widely applied, mostly to improve the corrosion resistance.
Alternatively, cemented carbide parts can be soldered onto selected surface areas. Chemical vapor deposition starting from tungsten halogenides or alkoxides is a less common method to provide hard, acid-resistant tungsten coatings on bearings, dies, rolls, gauges, etc.