Refractory and reactive metals include beryllium, boron, chromium, cobalt, columbium, hafnium, molybdenum, niobium, rhenium, tantalum, thorium, and titanium. Refractory metals such as tungsten, tantalum, molybdenum, niobium and zirconium have melting points above
1750° C (
3200° F) and are used in high temperature, structural, electrical, and other specialty applications. Reactive metals such as beryllium, titanium, and zirconium combine readily with oxygen at elevated temperatures to form very stable oxides. Finely-divided reactive metals can react explosively with oxygen and are often added to rocket fuels or other combustible mixtures. The formation of a highly stable oxide film on the alloy surface provides protection against further oxidization or corrosion at low to moderate temperatures.
There are several major types of refractory and reactive metals. Cobalt and cobalt alloys are non-ferrous magnetic alloys with high strength and toughness, excellent corrosion and oxidation resistance, and high temperature strength. Tungsten, tungsten alloys, molybdenum, and molybdenum alloys are refractory metals used to fabricate light bulb filaments, physical vapor deposition (PVD) evaporation crucibles, electric discharge machining (EDM) electrodes, electrical contacts, and in other high temperature components. These metals have very high melting points and relatively high densities. Titanium and titanium alloys provide excellent corrosion resistance, good fatigue properties, and high strength-to-weight ratios. They are used in aircraft or air frame parts, jet engine superalloy components, corrosion resistant chemical process equipment (e.g., valves, piping and pumps), medical devices, and marine equipment.
Many refractory and reactive metals meet the compositional standards of the Unified Numbering System (UNS), a specification established by the American Society for Testing and Materials (ASTM), the Society of Automotive Engineers (SAE), and metal trade associations such as the American Iron and Steel Institute (AISI). The UNS assigns metals and alloys a lettered prefix and a five-digit number. Refractive and reactive metals belong to the UNS R category and have designations such as UNS R03620. Other standards for refractory and reactive alloys include casting grades, European Norm (EN), American Society of Mechanical Engineers (ASME) standards, and U.S. military specifications (MIL-SPEC). QQ and QQS prefixes are used to designate specific MIL-SPEC metals.
Suppliers provide refractory and reactive metals in many stock shapes and forms. Semi-finished stock shapes are suitable for part fabrication by machining, assembly, or other processes. They are also used as feedstock for casting, forging, and spinning. Common stock shapes and forms for nickel and nickel alloys include bars, rods, tubes, plates, profiles, sheets, strips, shims, spheres, foil, wire, billets, slabs, and blooms. Materials are also supplied as billets, ingots, powders, fillers, and reinforcements. Round, hexagonal, coil, and hollow stock are also available. There are two basic types of anodes. Plating anodes are in used in plating or electroplating processes. Sacrificial anodes are used to protect stainless steel or other metal structures from corrosion.
Selecting refractory and reactive metals requires an analysis of dimensions, production processes, and performance features. Outer diameter (OD), inner diameter (ID) overall length, and overall thickness are important dimensions. Most materials are cast, wrought, extruded, forged, cold-finished, hot-rolled, or formed by compacting powdered metals or alloys. Electric arc furnaces are used to produce very clean metals and alloys with fewer inclusions and lower variability. Performance features for refractory and reactive metals include resistance to corrosion, heat, and wear. Super alloys provide elevated temperature strength, creep properties, and oxidation resistance.
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