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Refractory-metalsfor heat and corrosion resistance.Solutions with Effective, Powerful Advice.
Refractory Metals Refractory-metals, widely agreed to be those with a melting point greater than 2000 oC (3,630 oF) include Niobium (formerly known as Columbium), Tantalum, Molybdenum, Tungsten (also known as Wolfram), and Rhenium. Furthermore in this group are also Osmium and Iridium (two of the platinum-group metals), Rubidium, Hafnium and Technetium which are rare and of limited use. The first four of them have the largest commercial importance. These Refractory-metals and their alloys are available in mill forms (sheet, bar, tubing and wire) as well as products such as screws, bolts, studs. They are used in demanding applications requiring high-temperature strength and corrosion resistance. All the Refractory-metals are mutually soluble and form solid-solution alloys with each other in any proportion.
Powder metallurgy processes are widely used in the fabrication of Refractory-metals. These are produced via PM when the alloy components have substantially differing melting temperatures, limited mutual solubility, and different densities. Finer grain size of powder products facilitates hot working operations following compaction and sintering. Conversely Refractory-metals are never fabricated by casting because of their high melting point.
The Refractory-metals are readily degraded by oxidizing environments at moderately low temperatures, a fact which restricts the applicability of the metals unless they are properly protected from oxidation by suitable coatings, vacuum or inert high temperature atmospheres. Selection of a specific alloy from the Refractory-metals group is often based more on fabricability than on strength or corrosion resistance. Tantalum Tantalum is one of the Refractory-metals most corrosion resistant to a wide spectrum of reagents. Out of several commercial grade tantalum alloys, those containing tungsten, niobium, and molybdenum generally retain the corrosion resistance of tantalum and provide higher mechanical properties. The most common alloying additions to tantalum are tungsten and niobium. Small amounts (1 to 2%) of hafnium and/or rhenium are also sometimes added. The corrosion resistance of tantalum can be compared to that of glass but at higher temperatures, and with fabrication advantages. Tantalum is also used extensively to repair damage in chemical equipment. Tantalum is usually used as a lining over a stronger, less expensive base material because of its high cost and lack of strength compared to its easy fabricability. Chemical equipment has been fabricated from steel plate explosively clad with tantalum. Forming and welding methods have been developed for fabrication of the clad plate into reactor vessels, tanks, and other types of chemical equipment. It is used in prosthetic devices and in surgical implants because of its biocompatibility and corrosion resistance to body fluids. Niobium Niobium can be a less expensive Refractory-metal alternative to tantalum. However, its corrosion resistance is more limited. This is because it is sensitive to most alkalis and certain strong oxidants, but it may resist to highly corrosive media within specific temperature and concentration limits. Even though the mechanical strength of niobium is less than that of tantalum, it can be used economically where the extreme inertness of tantalum is not required. Niobium and its alloys are Refractory-metals often selected for high temperature use like for propulsion engines in fabricated systems. Niobium is also available in Refractory-metals alloys containing tantalum, tungsten, molybdenum, vanadium, hafnium, zirconium, or carbon. Alloys provide improved tensile, yield, and creep properties, particularly in the 1,100 to 1650 oC (2,010 to 3,000 oF) range. Protective coatings or atmospheres are mandatory for fabrication or usage of Refractory-metals at elevated temperature. Tantalum and niobium are Group V Refractory-metals, ductile in the pure state. They have a high solubility of interstitial elements like carbon, nitrogen, oxygen, and hydrogen, and a low DBTT (Ductile-to-Brittle Transition Temperature). Niobium alloys developed for superconducting magnets include Nb-46.5Ti and Nb3Sn. An Nb-55Ti alloy is used for fasteners for aerospace structures. Fabrication Niobium, tantalum, and their alloys are the most easily fabricated Refractory-metals. They can be formed, machined, and joined by conventional methods. Tantalum and niobium are easily weldable Refractory-metals without the brittleness problem encountered when welding tungsten and molybdenum. Niobium and tantalum begin oxidizing at 200 and 343 oC (390 and 650 oF), respectively. Below 1370 oC (2,500 oF), the oxides are nonvolatile, but the ductility of the parent metals is reduced because of oxygen absorption. Most sheet metal fabrication of niobium and tantalum is done in the thickness range of 0.1 to 1.5 mm (0.004 to 0.060 in). Niobium, like tantalum, can be welded to itself and to certain other metals by resistance welding, gas tungsten arc welding (GTAW) (either autogenously or with matching filler metals in inert atmospheres). A vacuum-purged chamber backfilled with argon and helium is recommended but, if not available, trailing inert gas shields may help reduce atmospheric contamination. Solid state diffusion bonding, friction welding, explosive bonding and brazing, are also applicable to Refractory-metals. Electron beam welding and laser beam welding can be used, particularly for joining to other metals, provided there are not too great differences in melting point. However, surfaces that are heated above 315 oC (600 oF) during welding must be protected with an inert gas or vacuum to prevent embrittlement. Welds in tantalum alloys have excellent bend ductility at room temperature, but the formation of brittle intermetallics makes them difficult to weld to most structural metals. Grain size do not appear to affect ductility, but impurity concentration has an adverse influence. High interstitial contents in the alloys can lead to a large concentration of micropores in the welds. Atmospheric contamination is a major cause of the embrittlement of niobium alloy welded joints. To avoid significant increases in the ductile-to-brittle transition temperature (DBTT) of welds (caused by interstitial contamination), welding atmospheres should be as pure as possible. The DBTT of Refractory-metals welds is also influenced by microstructural changes resulting from weld thermal cycles. The size of the Heat Affected Zone and grain growth in this zone are major factors that affect the ductility of welds. Furthermore, alloys with different strengthening mechanisms respond differently to weld heat input. To reduce the tendency of grain growth in the HAZ and fusion zones, a high energy density welding process, such as EBW, is often more advantageous than GTAW. For niobium and tantalum Refractory-metals alloys, joint design is particularly important. Weld tooling should be of hard chromium plated copper. Tantalum and niobium alloys generally retain greater than 75% joint efficiency after gas tungsten arc welding. Preheating is not required, but postweld annealing can restore large amounts of ductility and toughness to commercial alloys. Niobium and its alloys may be silicide-coated with a chromium- and titanium- containing material before being subjected to temperature-oxidizing environments; the preferred brazing filler alloy is Ti-33Cr because it is compatible with the coating. Foil, 0.13 mm (0.005 in.) thick, is fit metal to metal to ensure good wetting of the joint. The foil should be held in place with resistance spot welds. Cleanliness of the mating surfaces of Refractory-metals will ensure good flow of the alloy. A clean vacuum furnace is heated to 1315 oC (2,400 oF) and held for 5 min, then increased to 1480 oC (2,700 oF) and held for 8 min. The parts must be furnace cooled to 205 oC (400 oF) before exposure to air. After brazing, the hardware should be pickled and diffusion treated at 1315 oC (2,400 oF) in a vacuum for a period of 16 h. If possible, the parts should be wrapped in tantalum foil to minimize contamination. Diffusion bonding also is used to join Refractory-metals, primarily niobium and tantalum. The same precautions are required as for brazing. Vanadium foil, 0.05 to 0.08 mm (0.002 to 0.003 in) thick is placed in the joint and put under load using molybdenum or tungsten tooling. Diffusion bonding with vanadium is considered superior to brazing because a bimetallic system is not necessary, and the joint is microstructurally clean because vanadium forms a continuous solid solution with niobium and tantalum. Other Refractory-metals Tungsten and molybdenum are Group VI Refractory-metals and have a lower solubility (than Nb or Ta) for interstitial elements like carbon, nitrogen, oxygen, and hydrogen, and a high DBTT (Ductile-to-Brittle Transition Temperature). Tungsten and molybdenum are specifically processed and alloyed to control the microstructure to ensure a sufficiently low, as much as practical, ductile to brittle transition temperature. For both tungsten and molybdenum, a warm-worked structure has a lower DBTT than a recrystallized structure, and alloys have been developed to retard annealing. Annealed molybdenum has a DBTT that is near room temperature, whereas annealed tungsten may be brittle at temperatures below 315 oC (600 oF). These Refractory-metals and their alloys suffer losses in ductility and increases in ductile-to-brittle transition temperature when welded. Molybdenum Molybdenum provides corrosion resistance that is slightly better than that of tungsten. It particularly resists non-oxidizing mineral acids. Molybdenum is relatively inert to carbon dioxide, hydrogen, ammonia and nitrogen to 1100 oC (2,010 oF) and also in reducing atmospheres containing hydrogen sulfide. It has excellent resistance to corrosion by iodine vapor, bromine, and chlorine up to well defined temperature limits. It also provides good resistance to several liquid metals including bismuth, lithium, potassium, and sodium. Molybdenum is most versatile and is an excellent structural material for applications requiring high strength and rigidity at temperatures up to 1650 oC (3,000 oF), where it can operate in vacuum or under inert or reducing atmospheres. Unalloyed molybdenum and its principal alloy, TZM, are produced by powder metallurgy methods and by vacuum-arc melting. Both are commercially available in ordinary mill product forms. Compared to unalloyed molybdenum, the TZM alloy (Mo-0.5%Ti-0.1%Zr) develops higher strength at room temperature and much higher stress-rupture and creep properties at all elevated temperatures. At 980 to 1090 oC (1,800 to 2,000 oF), TZM can sustain a higher stress for longer times than unalloyed molybdenum. The TZM alloy is used for cores in die casting of aluminum, and for die cavities in casting of brass, bronze, and even stainless steel as it retains usable strength at elevated temperatures, has a low coefficient of thermal expansion, and resists erosion by molten metals. Heavy dies of the TZM alloy are used for isothermal forging of superalloy components for aircraft gas turbines, and die inserts made of TZM have been used for extruding steel shapes. Piercer points of TZM are used to produce stainless steel seamless tubing. Molybdenum and TZM are readily machined with conventional tools. Sheet can be processed by punching, stamping, spinning, and deep drawing. Some parts can be forged to shape. Molybdenum wire and powder can be flame sprayed onto steel substrates to salvage worn parts or to produce long-wearing, low-friction surfaces for tools. Molybdenum oxidizes at high temperatures, except that some parts have been used successfully for brief exposure time to very high temperatures gases in propulsion systems. Molybdenum starts to oxidize at about 500 oC (930 oF). At temperatures over 778 oC (1,430 oF), which is the eutectic temperature of MoO2-MoO, oxides become particularly volatile and the oxidation rate accelerates rapidly. Tungsten Tungsten is similar to molybdenum in certain properties. The two metals have about the same electrical conductivity and resistivity, coefficient of thermal expansion, and about the same resistance to corrosion by mineral acids. Both have high strength at temperatures above 1090 oC (2,000 oF), but because the melting point of tungsten is higher, it retains more strength at higher temperatures than molybdenum. The elastic modulus for tungsten is about 25% higher than that of molybdenum, and its density is almost twice that of molybdenum. Tungsten oxidizes rapidly at temperatures over 500 oC (930 oF), but not as rapidly as molybdenum. All commercial unalloyed tungsten is produced by powder metallurgy methods; it is available as rod, wire, plate, sheet, and some forged shapes. Several tungsten alloy shapes are produced by sintering of compacted bodies of tungsten powder with binders of nickel-copper, iron-nickel, iron-copper, or nickel-cobalt-molybdenum combinations; tungsten is usually the major part of the alloy by weight. These Refractory-metals alloys are often called heavy metals or machinable tungsten alloys. They can be machined by turning, drilling, boring, milling, and shaping. They are not available in mill product forms because they cannot be wrought or forged at any temperature. The heavy metal alloys are useful for aircraft counterbalances and as inertia weights in gyratory compasses. Heavy metal inserts are used for kinetic-energy penetrators in military projectiles. Filaments in vacuum metallizing furnaces, X-ray targets (anodes) and shielding are other important applications, as are contacts for automotive distributors. Tungsten electrodes are used for GTA welding and water cooled tungsten tips for
nonconsumable electrode vacuum arc melting of alloys.
The accepted specification for tungsten electrodes is: AWS A5.12/A5.12M:1998 Filaments for incandescent lamps are usually coils of very fine unalloyed tungsten wire. Heaters for electronic tubes are made of tungsten, sometimes alloyed with 3% rhenium. A thermocouple rated to 2400 oC (4,350 oF) consists of one tungsten wire alloyed with 25% rhenium and another wire alloyed with 5% rhenium. Nozzle throats of forged and machined unalloyed tungsten have been used in solid fuel rocket engines; throats were machined from porous bodies of tungsten powder infiltrated with silver for exposure to gases at temperatures near 3500 oC (6,300 oF). Cutting tools and parts that must resist severe abrasion are often made of tungsten carbide. Tungsten carbide chips or inserts, with the cutting edges ground, are attached to the bodies of steel tools by brazing or by screws. The higher cutting speeds and longer tool life made feasible by the use of tungsten carbide tools are such that the inserts are discarded after one use. Tungsten carbide dies have been used for drawing wire. Inserts of tungsten carbide are used in rotary bits for drilling oil and gas wells and in mining operations. Fused tungsten carbide is applied (by Hardfacing) to the surfaces of mining and earthmoving machinery subjected to severe wear. Fabrication Molybdenum, molybdenum alloys, tungsten, and tungsten alloys require special fabrication techniques. Fabrication involving mechanical working should be performed below the recrystallization temperature. Molybdenum parts can be welded by inertia, resistance, and spot methods in air; by Gas Tungsten Arc and Gas Metal Arc welding under inert atmospheres, by electron beam welding in vacuum, and by diffusion bonding. The best welds are produced by inertia (friction) welding and electron beam welding; welds produced by the other techniques are less ductile. Welds in molybdenum and tungsten are brittle (<50% joint efficiency), and thus these metals are difficult to join. Before welding, molybdenum and tungsten must be preheated above their ductile to brittle transition temperatures to prevent fracture. Heavy sections of molybdenum should be preheated and postheated when welded to reduce thermal stresses. Sections in thicknesses of 0.64 mm (0.025 in.) and less present serious cracking problems. Welds are always brittle, and joint efficiency depends on the reinforcement of the weld bead. Welding done in pure inert atmosphere normally shows good results. The filler metal compositions should be the same as the base metal. The base metal in the heat affected zone becomes embrittled by grain growth and recrystallization as a result of the welding temperatures. The ductile to brittle transition temperature of molybdenum welds is significantly increased after GTAW and EBW processes by recrystallization so that molybdenum welds tend to be brittle. Molybdenum is highly notch sensitive, craters and notch effects must be avoided. Tungsten is the most difficult of the Refractory-metals to join for satisfactory high temperature service. Welding, especially the EB process, offers the best solution for joining tungsten for those uses. Tungsten is welded in the same way as molybdenum and has the same problems, only more highly so. It has greater susceptibility to cracking because the ductile to brittle transition temperatures are higher. The preparation of tungsten for welding is more difficult. The gas tungsten arc welding process is used with direct current electrode negative. Welding should be done slowly to avoid cracking. Preheating may assist in reducing cracking but must be done in inert gas atmosphere. All the Refractory-metals are susceptible to weld porosity and sometimes to weld cracking if not properly chemically cleaned before welding. Weld cracking usually can be avoided by preheating to above the DBTT. Welding tungsten-rhenium alloys or using the EBW process makes it possible to avoid preheat requirements. Mechanical joints are unsatisfactory unless molybdenum fasteners are used. Diffusion bonding is impractical because of severe tooling problems. Brazing for relatively low temperature applications is done using precious metals (silver, palladium, and platinum alloys) and transition metals (nickel and manganese alloys) as filler metals Rhenium The rarity of Rhenium makes it the most expensive of the refractory metals. Rhenium welds made by inert gas or Electron Beam methods are extremely ductile and can be formed further at room temperature. Care must be taken during welding to protect rhenium against oxidation.
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