Molybdenum (Mo) and tantalum (Ta) are members of the group of refractory metals, so named because they melt at very high temperatures. Rhenium, osmium and iridium are sometimes included as members, as are chromium and vanadium. Table I lists some physical properties for several refractory metals.
Molybdenum and tungsten often are used in high-temperature applications, particularly in vacuum furnaces. Rhenium's availability and cost preclude its use in all but the most specific applications. Niobium and tantalum have a cost disadvantage, as well as low modulus and strength at high temperatures unless alloyed. Molybdenum and tungsten have good high-temperature strength, making them attractive for structural parts, and also have low electrical resistivity, allowing for efficient use of I2R heating in furnace elements. Molybdenum is the preferred furnace material, although tungsten is used in applications at temperatures above 2000C (3630F).
All of the refractory metals have good corrosion resistance in many environments, but niobium and tantalum are in a class by themselves. In particular, tantalum has become the material of choice in the chemical process industry, where pressure vessels operate under extraordinarily corrosive conditions. It has corrosion resistance similar to that of glass, and it almost is as inert as gold and platinum in the most hostile environments. This article discusses the use of molybdenum at high temperatures, and tantalum in highly corrosive environments.
Mo: HighTemperature Metal of ChoiceVacuum-Furnace Applications
Materials used in vacuum furnaces must be strong at high temperatures and must not contaminate furnace loads. Molybdenum has a low vapor pressure (~10-31 to 10-8 atm) over the temperature range typically used in vacuum heat treating (600 - 2000C or 1110 - 3630F), which means it will not contaminate workloads. Under these conditions, the evaporation of molybdenum is negligible, which is particularly important in critical applications such as heat treating aerospace alloy components.
Figure 1 shows the high-temperature strength of arc-cast molybdenum and Mo-TZM alloy (Mo-0.5Ti-0.08Zr-0.03C), and ODS (oxide-dispersion strengthened Mo-La (Mo-2 vol% La2O3). Pure molybdenum is preferred for use in furnace applications because it has the lowest initial cost, but its use is limited to a temperature of about 1200C (2190F) because of diminished strength at higher temperatures. TZM has a significant strength advantage to a temperature of about 1500C (2730F), but its strength drops rapidly at temperatures above 1300C (2370F). The strength of ODS Mo-La is lower than that of TZM at lower temperatures, but its strength is retained at temperatures well above 1500C.
ODS Mo-La has an advantage over conventional molybdenum alloys (and tungsten) in the creep regime. Figure 2 shows the stress-dependence of creep rate of ODS Mo at a temperature of 1800C (3270F) compared with Mo and TZM at much lower temperatures.
Figure 3 summarizes the rupture-life data for Mo, TZM, ODS Mo-La, and includes tungsten (W) for comparison.
Molybdenum is the most commonly used material for electrodes in electric glass melting due to its good combination of high temperature strength and corrosion resistance compared with other electrode materials. Some materials are used in specific applications (for example, tin oxide is used when glass color is critical), but molybdenum is unmatched for its versatility and economy.
Two factors to consider when using molybdenum electrodes are oxidation and bubble formation. Molybdenum trioxide sublimes at a temperature above about 800C (1470F), but the glass bath protects the immersed electrode from oxidation. The portion of the electrode that projects into the atmosphere typically is water or gas cooled. Coatings also can be used to protect the electrode in air. Container- and plate-glass manufacturers cannot tolerate the formation of bubbles that form due to a reaction between the glass and electrode impurities, because the bubbles produce defects in the glass products. Very low-carbon electrodes have been developed to prevent bubble formation.
The high-temperature strength of TZM (figure 1) makes it an ideal tooling material for isothermal forging. In this process, the tooling is heated to a temperature between 980 and 1205C (1795 and 2200F), and the superalloy workpiece is forged at slow strain rates. Die chill is eliminated completely, and parts can be forged at nearly superplastic strain rates. An inert atmosphere or vacuum chamber protects the dies from oxidation at these temperatures. The inherent cost in such a specialized process is offset by material and machining cost savings realized on the finished parts. For difficult-to-forge materials, iso-thermal forging is the only way to manufacture parts. Mo-TZM alloy is a critical material in this technology; its high-temperature strength is the enabling property that allows success.
Molybdenum, TZM alloy, and MHC alloy (Mo-1.2Hf-0.1C) also are widely used as extrusion dies and piercing points, where thermal shock and wear/erosion resistance are paramount requirements.
Ta Superior in Chemical Processing
Tantalum is susceptible to corrosion only by about 40 of more than 2000 reagents where its stability has been evaluated. Its excellent corrosion resistance is due to the presence of a passive tantalum pentoxide (Ta2O5) surface film, which imparts superior corrosion resistance. Tanta-lum is corroded only by reagents that attack the oxide film including are strong alkalis, fuming sulfuric acid containing free SO3 or SO2, fluorine, hydrofluoric acid and solutions containing greater than about 10 ppm (parts per million) fluoride ions. Sodium and potassium hydroxide solutions in concentrations greater than about 10% also attack tantalum and cause embrittlement.
Table II compares the mechanical properties of tantalum with those of AISI type 316 stainless steel, another commonly used material in the process industry. The stiffness of the two materials is similar, with 316 having a slight advantage. Tantalum is much softer than the stainless steel, and this has implications for maintenance and component design. Sharp tools used to remove surface deposits cause most failures. These can perforate the soft tantalum cladding, resulting in rapid failure when the component is returned to service.
Tantalum Production and Fabrication
Tantalum can be produced using powder metallurgy (PM) methods and vacuum arc and electron-beam (EB) melting. EB melting produces the purest material (>99.99%), but sheet produced from EB-melt ingots has a coarse grain structure, which can result in rough surfaces on drawn and spun parts. Although grain size can be controlled to some extent with rolling and annealing practices, finer grained material requires slightly less pure (>99.9%) PM or arc-melted feedstock. To improve specific properties, tantalum is alloyed with other metals, such as tungsten. Ta-2.5W alloy is widely used in strong sulfuric acid applications, and in applications involving high-pressure steam (especially for heat exchanger tubes). The alloy has 25% higher strength than pure tantalum, with very little increase in weight. Currently available alloys in descending order of importance to the chemical industry are pure tantalum, Ta-2.5W, Ta-40Nb, and Ta-10W. Tantalum also is used the electronics, furnace, and aerospace industries.
The metalworking properties of tantalum are excellent, allowing fabrication using standard metalworking techniques. Because of its high density and cost, only small components are made from solid material, such as the thermocouple pocket shown in Fig. 4. Usually, thin tantalum sheet is clad onto a less expensive base material. Tantalum is machined using high-speed steel or carbide tools with soluble oil as a lubricant. Tantalum tends to stick to tools during spinning and deep drawing, so lubricated aluminum-bronze tools and mandrels are used. Pure tantalum is annealed at a temperature around 1000C (1830F), and tantalum-tungsten alloys around 1200C (2190F). A high vacuum is required when heat treating tantalum to avoid oxygen contamination.
Welding is performed in a chamber back-filled with pure argon gas. Conventional tungsten inert gas (TIG) techniques are used without filler wire for thicknesses to 0.8 mm (0.031 in.). Above this thickness, appropriate metal filler wire, conventional weld preparation, and multipass welding techniques are used. Electron-beam welding also is possible, and is recommended for sheet thinner than 0.3 mm (0.012 in.). Welds made under the proper conditions are as strong as the parent metal, and properties of the heat-affected zone are not degraded. Residual gases in powder metallurgy materials lead to gassy, porous, and low-strength welds. Spot welding can be carried out without the use of a protective atmosphere if the cycle time is kept short. Use of an argon-gas shield or welding under water is recommended for spot welding thicker materials.
Chemical Industry Applications
Tantalum is used in a wide variety of components including heat exchangers, shell and tube heaters, bayonet heaters, condensers, thermocouple pockets, bursting discs, vessel liners and glass-vessel repair parts. As noted earlier, larger parts are made by cladding a thin layer of tantalum onto a less expensive material, such as steel, stainless steel and copper. Cladding can be achieved via physical contact or explosive bonding. The former method commonly is used when the cladding is under internal pressure and supported by the substrate. The latter can be used where pressure differentials could pull the cladding away from the substrate. Explosively clad materials present fabrication problems, especially during welding. To avoid contamination of the weld region with substrate material, "getter" strips must be inserted between the tantalum and substrate, increasing fabrication costs. It is more usual to "rigidize" the sheet-a forming operation that creates stiffeners in the tantalum lining to resist pressure-induced stresses.
Hydrogen embrittlement can cause disastrous failure in seemingly benign environments even at room temperature. Tantalum must never be allowed to become cathodic in the presence of hydrogen ions or gas, especially nascent hydrogen. It should not be used in electrolytic contact with nickel, stainless or carbon steel, or HastelloyR, even in quite low electrolyte concentrations. To survive under such conditions, tantalum must be electrically insulated from the other materials, or a small area of the tantalum must be plated with gold or platinum. The plated region causes the hydrogen ions to combine and form hydrogen gas that bubbles safely away. A plated area of platinum (~5 Km thick) having an area approximately 0.005%, or 1/20,000, that of the area to be protected is sufficient. Thus, a 15-mm (0.6 in.) diameter spot often is used to protect an entire four-tube tantalum bayonet heater.
Why Use Tantalum?
The cost of a graphite shell and tube heat exchanger is only about 20% that of a similar tantalum exchanger, but the service life of the graphite exchanger is between 3 to 5 years, while tantalum should last for at least 20 years. The majority of tantalum fabrications fail due to incorrect handling during service and maintenance. Graphite heat exchangers are necessarily thick walled, and respond slowly to temperature changes. By comparison, tantalum has good thermal conductivity, leading to better temperature control in chemical plants. Degradation of graphite also occurs during service, causing contamination of pharmaceutical products. Glass, on the other hand, is brittle, sensitive to thermal shock, difficult to fabricate, and has low thermal conductivity. Tantalum has none of these disadvantages. It also has a low fouling factor, leading to less frequent cleaning for peak operating efficiency and improved productivity. However, it sometimes is necessary to combine the advantages of glass with those of tantalum, such as in a glass vessel/tantalum coil construction.
Molybdenum's and tantalum's unique property combinations make them critical materials for use at elevated temperatures and in corrosive environments. Molybdenum allows the construction of today's high vacuum annealing furnaces, and permits the processing of alloys that cannot otherwise be processed. Tantalum gives increased chemical plant life, reduced maintenance, better process control and zero product contamination.