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HEAT TREATING/THERMAL EQUIPMENT Ideal for use in enowned for their high melting is brittle, very expensive, points, the refractory metals and difficult to fabricate, so it is mainly high-temperature furnace , , and tan- used as an alloying element with tung- talum are widely used in the con- sten or molybdenum for high-tem- . applications struction of high-temperature perature thermocouples and some furnaces as elements, insulation high stability components in the elec- by David Rowe* R screens, and furnace furniture. tronic valve industry. is H.C. Starck Ltd. Various applications using the extremely hard, brittle and difficult to Brentwood, United Kingdom materials are discussed. Unfortu- fabricate, so its uses are limited to an nately, all refractory metals oxidize in alloying element with and air above 250oC (480oF) so their uses palladium to increase resistance, are limited to vacuum or inert atmos- with to improve phere applications. Nevertheless, resistance, and for electroplating onto tungsten elements can be used up to other materials for corrosion resis- 2800oC (5070oF) in vacuum. Reactions tance. is found as an impu- with insulating materials are rity in and its main use is discussed, and methods for the con- in the nuclear industry due to its struction of hot zones are reviewed. ability to absorb neutrons. In the future, available anti-oxidation Because of their affinity for coatings may allow usage of these ma- — oxidation occurs above 250oC terials in oxidizing conditions. (480oF) for and , above 300oC (570oF) for molybdenum, Refractory metals defined and above 400oC (750oF) for tungsten The term “refractory metals” is gen- — their use is confined to protective erally applied to metals that have atmosphere or ap- melting points greater than 2000oC o plications, where they may be used (3630 F). Metals in the are for resistance or induction heating el- shown in Table 1. Principal materials ements, radiation screens, and furnace in the group are tungsten, tantalum, furniture such as skids, supports, and molybdenum, and niobium, all of boats. These metals have good elec- which are routinely used in the fur- trical properties and a high vapor nace industry. pressure making them suitable for a Although its is 1852oC o wide range of applications, such as an- (3365 F), zirconium is also included in nealing, , melting, and the refractory metals group. Other re- . Due to their low electrical re- fractory metals are rhenium, , sistance, refractory elements op- hafnium, and . Iridium and erate under high current/low voltage osmium also belong to the platinum conditions requiring a variable output group of precious metals, and all three transformer or a thyristor-controlled are very expensive to produce, pre- transformer with a current limiting cluding their use to very specialized device to limit the power surge when applications such as nozzles for ce- a cold element is switched on. ramic fiber production. Table 1 – Melting points of refractory metals Selection criteria Metals commonly used for furnace Melting point, o interior manufacture are tungsten, Element Symbol C Remarks molybdenum, and tantalum where Tungsten W 3410 they are used as resistance heating el- Tantalum Ta 2996 Also used in the chemical industry ements and screens/shields. The Molybdenum Mo 2610 metals are also used for the racks, Niobium Nb 2497 Also used in the chemical industry skids and the boats used for material Zirconium Zr 1825 Mainly used in the chemical industry processing such as sintering, powder Osmium Os 3045 Also a member of the Iridium Ir 2410 Also a member of the platinum group reduction, annealing, and vacuum Rhenium Re 3180 Brittle, difficult to fabricate brazing. Advantages of refractory metals include good electrical prop- *Member of ASM International erties, low vapor pressures, low elec- 56 HEAT TREATING PROGRESS • NOVEMBER/DECEMBER 2003 1203htsb.qxd 11/17/03 3:45 PM Page 27

HEAT TREATING/THERMAL EQUIPMENT REFRACTORY METALS Table 2 – Compatibility of refractory metals with selected atmospheres Atmosphere Tungsten Tantalum Molybdenum Niobium Air >400oC1>800oC2 >250oC1,4 >300oC1>600oC2 >250oC1,4 Dry hydrogen No reaction Hydrides No reaction Hydrides Moist hydrogen <1400oC no reaction. Hydrides, <1400oC no reaction. Hydrides, >1400oC O/R5 Oxidizes>250oC >1400oC O/R5 Oxidizes >250o(C Cracked ammonia No reaction Nitrides, hydrides >400oC No reaction Nitrides, hydrides >400oC Hydrocarbons <900oC3 <700oC3 <1100oC3 <700oC3 Nitrogen <2300oC No reaction >700oC Nitrides <2200oC No reaction >300oC Nitrides Argon No reaction No reaction No reaction No reaction Helium No reaction No reaction No reaction No reaction Vacuum 10-2mbar <2000oC No reaction Embrittled/ Oxidized <1700oC No reaction Embrittled/ Oxidized Vacuum <10-4mbar Evaporates >2400oC Evaporates >2200oC Evaporates >1600oC Evaporates >1950oC 1Oxidizes. 2Sublimes. 3Carburizes. 4Embrittled. 5Whisker production; >5g/m3 oxidation/reduction process occurs. trical resistivity, and low thermal Table 3 – Refractory metals compatibility with common capacity. This latter property is of advantage as refractory metal Refractory Tungsten Tantalum Molybdenum Niobium o o o o furnaces have a low thermal Al2O3 <1900 C <1900 C <1900 C <1700 C mass leading to rapid heat up BeO <2000oC <1600oC <1900oC <1500oC and cooling. Energy savings re- Graphite <1400oC1 <1000oC1 <1100oC1 <800oC1 sult since heat is not wasted MgO <1500oC <1800oC <1600oC <1600oC o o o o heating up masses of refractory ThO2 <2200 C <1900 C <1900 C <1700 C o o o o insulating material such as ZrO2 <1600 C <1600 C <1900 C <1500 C bricks. Tungsten and molyb- 1 Carburizes. denum also have excellent resis- tance to molten glass and quartz, so are used for dies and mandrels as well as in the melting process. When choosing a refractory metal for a furnace application, operating conditions as well as the properties of the product being processed should be considered. Table 2 shows the com- patibility of the most frequently used refractory metals with furnace atmos- pheres; Table 3, compatibility with common refractories. All refractory metals are readily fab- ricated by conventional techniques. Tantalum and niobium are the easiest, Fig. 2 — Furnace assembly with braided Fig. 1 — Tungsten mesh heating element. molybdenum element. and tungsten is the most difficult. Tungsten is useful in very high-tem- susceptors in induction furnaces. Typ- perature applications up to 2800oC ical high-temperature heating ele- (5070oF) in high vacuum or protective, ments made of refractory metals are reducing atmosphere. At these high shown in Figures 1–3. temperatures, insulation of the hot • Thermal insulation. The choice of zone is a problem and it is normal to thermal insulation depends on the op- use tungsten in high vacuum or a low erating temperature and furnace at- partial pressure of hydrogen. mosphere. Hydrogen is a good pro- • Heating elements. Hot zones usu- tective atmosphere for tungsten and ally consist of a refractory metal ele- molybdenum, but unfortunately it is ment (single, two, or three phase) con- an excellent conductor of heat, ren- structed of sheet, plate, rods, stranded dering radiation screens ineffective. wire or woven wire mesh surrounded When a hydrogen atmosphere is re- by radiation screens. Rings of tung- quired for applications such as sin- Fig. 3 — Furnace with tungsten rod heating sten or molybdenum can be used as tering, it is normal to use refractory elements.

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HEAT TREATING/THERMAL EQUIPMENT REFRACTORY METALS as insulating materials. Simi- All metal hot zones rely on pre- from tungsten, the next six from larly, bricks or ceramic fiber insulation venting heat loss by radiation only molybdenum, and the outer screens should be used with other gaseous at- and can only be used in vacuum or from stainless . The degree of in- mospheres. low partial gas pressure applications. sulation depends on the screen sepa- • Hot zones. In certain applications Screens are made from , ration and interscreen contact where — especially in the production of car- which can withstand the temperature heat is transmitted by conduction be- bides — fiber felt can be used. at that particular point and can be tween screens. Separation can be small However, if this material is used with made of tungsten, molybdenum, tan- spacers of sheet metal bent into a tube. moist hydrogen or if water vapor is talum, or . The number Another method is to rigidize the formed as part of the process (for ex- of screens required depends on the op- metal so that the screens only touch at ample, a reduction process of metal erating temperature. A useful “rule of the peaks of the ridges. A typical ), the oxygen in the water vapor thumb” is to use one radiation screen rigidized tantalum element and radi- reacts with the in the felt to for each 200oC (360oF). For instance, a ation screen pack is shown in Fig. 4. form carbon monoxide, which in turn furnace operating at 2400oC (4350oF) reacts with the product being would require 12 radiation screens, Which refractory metal? processed with possibly disastrous re- separated by refractory metal or ce- In selecting a refractory metal for sults. Consequently, when processing ramic spacers between the element use in a vacuum furnace, the degree products that react with carbon and the water-cooled wall of the of vacuum should also be considered monoxide at high temperatures, all- vacuum chamber. In this case, the in addition to the operating tempera- metal hot zones should be used. three inner screens would be made ture. For instance, molybdenum Table 4 – Selected properties of refractory metals* Property Unit Tungsten Tantalum Molybdenum Niobium g/cc 19.2 16.6 10.22 8.58 Expansion coefficient, 20–1500 oC × 10-5 4.4 6.5 6.5 9.5 oC Resistivity 20oC mWm 0.055 0.125 0.05 0.152 1000oC 0.33 0.54 0.27 0.60 1500oC 0.50 0.72 0.43 0.90 2000oC 0.66 0.87 0.60 1.15 W 130 54.4 155 52.3 moK Specific heat at 20oC Joules 0.138 0.152 0.260 0.27 goK Surface loading <1800oC W/cm2 10–20 10–20 10–20 10–20 >1800oC 20–40 20–40 20–40 20–40 Tensile strength 20oC MPa 1200–1500 250–330 650–700 330–400 1000oC 230–650 120–170 180–200 75–130 1500oC 120–180 35-60 20–50 30–50 Vapor pressure 1500oC Pa <1×10-12 <1×10-12 2.5×10-7 <1×10-12 1700oC 1×10-10 5×10-9 2.5×10-5 5.5×10-7 1900oC 1.5×10-8 4×10-7 7×10-4 3.5×10-5 2100oC 1×10-6 1.5×10-5 1.5×10-2 1×10-3 2300oC 3×10-5 3×10-4 0.20 1.6×10-2 2500oC 6×10-4 4.5×10-3 15 0.45 Evaporation rate 1500oC g/cm2sec Nil Nil Nil Nil 1700oC Nil 4×10-11 1.2×10-7 3.3×10-9 1900oC 1.3×10-10 3×10-9 3.3×10-6 1.7×10-7 2100oC 5.8×10-9 1×10-7 6.7×10-5 7.5×10-6 2300oC 2×10-7 2.3×10-6 6.7×10-4 8.3×10-5 2500oC 3.5×10-6 1.3×10-5 0.15 8.3×10-4 *Values for evaporation rate and vapor pressure are from various sources and are based on the latest information available. They are guidelines only. 58 HEAT TREATING PROGRESS • NOVEMBER/DECEMBER 2003 1203htsb.qxd 11/17/03 3:46 PM Page 30

HEAT TREATING/THERMAL EQUIPMENT REFRACTORY METALS should not be used at temperatures Table 4. Properties like surface loading greater than 1700oC (3090oF) in a depend on the actual operating con- vacuum higher than 1 × 10-4 mbar be- ditions, such as mounting the element cause of evaporation problems. When as free radiating or attaching it to a re- produced, all the refractory metals ex- fractory . The density of the ele- hibit fibrous microstructures, but these ment material is important especially change to an equiaxed crystalline when designing current lead-ins, since structure on heating above the re- tungsten mesh elements can be rather crystallization temperature. Tantalum heavy. Fig. 4 — Typical rigidized tantalum heating and niobium remain ductile in this element and radiation screen pack. condition, but molybdenum and tung- Future trends sten become brittle. This ex- One of the major problems of using plains why welding should refractory metals as heating element be avoided during furnace materials is that they all oxidize at low construction using these temperatures, so use is limited to an materials especially for inert atmosphere or vacuum. Today, load-bearing components, however, anti-oxidation coatings which should be of riveted based on and are now construction. available to prevent oxidation by The recrystallization tem- coating the surface with a layer of perature of tungsten and silica. These can be applied by a slurry molybdenum can be in- dipping process, plasma spraying, or creased by doping with lan- by chemically reacting the metal sur- thanum or zirconium ox- face with silicon tetrafluoride. This ides. The former raises the latter process is carried out by H.C. Fig. 5 —Rack and boats manufactured from recrystallization tempera- Starck Ltd under the proprietary name lanthanated molybdenum. ture of molybdenum from about 1000 of Muride “SP” and “T” coatings. to 1900oC (1800 to 3450oF). When re- Being integral with and chemicaly crystallization occurs, the material re- bonded to the surface, the coating re- crystallizes into long interlocking sists and spalling. Pri- grains and not normal equiaxed marily developed for use in the aero- grains, thereby maintaining a good space and glass industries, it may degree of . prove useful in coating element ma- Tantalum and niobium screens can terials for use in air at temperatures be welded, as the welds in these ma- up to 2000oC (3630oF). Such elements terials are ductile, but tungsten and are currently in the experimental stage molybdenum elements or screens are and will require further investigation normally riveted or, in the case of before being offered commercially. screens of very thin sheet, “stitched” When heated in air to 1100oC (2010oF), with wire. Riveted constructions are a Muride coated glass melting elec- not very gas-tight so it is normal prac- trode outperforms an uncoated elec- tice to plasma spray the riveted areas trode at the same temperature (Fig. 6). with molybdenum to effect a gas seal. The uncoated oxidizes rapidly by subliming molybdenum Furnace furniture . HTP In addition to heating elements and radiation screens, refractory metals are used for charge carrying racks, skids, Fig. 6 — Coated molybdenum electrode re- and boats for containing the products sists oxidizing in air at 1100°C, while its un- being processed. Hooks for sup- For more information: coated counterpart readily oxidizes. porting molybdenum, tungsten rod, Charles E. David Rowe is Technical Man- or braided wire elements in refractory ager, H.C. Starck Ltd., Horndon Business brickwork are also made from molyb- Park, Brentwood, Essex CM13 3XD UK; denum rod or heavy wire. A typical tel: 441277814210; fax: 441277810255; e- rack and boats manufactured from mail: [email protected]; Web: www.hcstarck.com. lanthanated molybdenum are shown In the United States, Gary A. Rozak is in Fig. 5. Development Engineer, H.C. Starck Inc., 21801 Tungsten Road, Cleveland, OH Design considerations 44117-1117; tel: 216/ 692-4496; fax: 216/ Selected properties of refractory 692-0029; e-mail: gary.rozak@hcstarck. metals useful in design appear in com; Web: http://hcstarckus.com. 60 HEAT TREATING PROGRESS • NOVEMBER/DECEMBER 2003