Fig 1 Large all-metal, vacuum-furnace hot zone

The incorporation of high-performance materials in place of traditional pure molybdenum refractory metal of choice greatly improves the performance and longevity of large vacuum furnace hot zones. Hearth components tend to remain straight and true, heating elements resist sagging and distortion, radiation shielding is less likely to break and gas-cooling nozzles typically maintain their shape and last the life of the hot zone. Improved performance stems not only from fundamentally solid mechanical design, but also from dramatic improvements in the properties of the high performance materials of construction. Alloyed and doped molybdenum outperforms pure molybdenum in high-temperature vacuum-furnace applications. In addition, Plansee Ag.'s (Reutte, Austria) blackbody radiation heating elements offer significant electrical energy savings over conventional designs.

Approximately 25% of all large vacuum furnaces (Fig. 1) currently in use have an all-metal internal hot zone. The majority of these furnaces operate at a maximum temperature of 1315C (2400F), with a few operating at temperatures to 1650C (3000F). Generally, all-metal hot zones consume more electrical power than their insulated counterparts (i.e., graphite board/felt, graphite foil/ graphite felt, molybdenum/ graphite felt and molybdenum/ceramic fiber).

New blackbody radiation heating elements (combined with special preparation of the all-metal radiation shield pack) yield a 15 to 20% energy savings over conventional all-metal designs. This enables an all-metal hot zone to have nearly the same energy consumption as an insulated hot zone. Processes requiring ultimate quench rates, quick pump down times, high vacuum levels and an ultraclean process environment no longer suffer a high-energy penalty using an all-metal hot zone (Table 1). In addition, using high-performance TZM moly (titanium and zirconium alloyed molybdenum), ML moly (lanthanum-oxide doped molybdenum) and preoxidized PM 2000 superalloy (in braze applications) throughout an all-metal hot zone can dramatically improve service life over that of traditional pure-molybdenum hot-zone construction.

Emissivity and blackbody concept

A hot object, such as a molybdenum or graphite vacuum-furnace heating element, emits energy in a range of wavelengths based on electrical power input. The emitted wavelength becomes shorter, and the total energy output increases as the heating element becomes hotter. The wavelengths emitted from a heating element include some from the visible light spectrum and infrared (IR) band.

The heat-up phase of a vacuum furnace process cycle is readily observable via color changes with increasing temperature. Maximum heat-output from the heating element begins within the infrared spectrum.

In general, highly polished (reflective) metals have low emissivities. Emissivity increases with increasing temperature. Most nonmetals have higher emissivities than metallic materials and have wide variations of emissivity dependent on surface condition. This is the basis of new high-emissivity blackbody heating-element coating developed by Plansee. The black-body coating takes advantage of the principles of emissivity with its reflective backside (outside diameter) and blackbody front side (inside diameter).

Theoretically, a blackbody has an emissivity of 1.0, making it both a perfect emitter and absorber of radiation (it absorbs all the radiation that strikes it regardless of the wavelength of the radiation). Performance of blackbody coated heating elements is based on this principle. This condition is ideal for a heating element in a vacuum furnace because it is desirable to reflect energy off the backside of the element (a reflective surface) to the radiation shielding (also a reflective surface). More importantly, radiated energy is focused (via the blackbody coating) into the center of the hot zone where it is needed to heat the workload (Table 2).

Conventional heating elements (both graphite and molybdenum) radiate equally (50/50) in both directions. With the high-emissivity blackbody coating applied only to inside diameter of flat ribbon style heating elements throughout the hot zone, it is possible to change this ratio into a more favorable one for faster heat-up rates and to realize significant electrical energy savings. The coating enables approximately 75% of the radiation energy to be directed into the center of the hot zone (toward the workload). The other 25% is directed to the radiation shielding, where the majority of the energy is carried out of the furnace through the cold wall. A significant energy savings can be realized during the holding, or soak, time because almost all the energy during a soak time is lost energy, being carried away to the cold wall by the cooling water via heat losses in the shield pack.

In addition to energy savings, blackbody-coated heating elements offer a significant advantage on heat up rates with 75% of the energy radiating toward the workload instead of the cold wall. The blackbody element offers a 25% increase in the radiated energy based on the same electrical power input over uncoated conventional heating elements.

An added benefit of blackbody-coated heating elements is an increase in hot-zone life. This is realized because internal components, such as radiation shielding, element supports, gas nozzles and hearth assembly, see less radiated heat, which reduces temperature gradients compared with conventional uncoated designs.

Fig 2 Plansee high-emissivity blackbody radiation coating on a refractory metal after 12 hours exposure in hydrogen at a temperature of 2000C (3630F) Fig 3 Plansee high-emissivity blackbody radiation coating on a refractory metal after 112 hours exposure in hydrogen at a temperature of 2000C (3630F)

Extensively tested at Plansee in both research and production furnaces, the high-emissivity blackbody coating is both mechanically and thermodynamically stable at operating temperatures to 2000C (3630F), in high vacuum and in typical process gases such as nitrogen, hydrogen, argon and helium (Fig. 2 and 3).

Fig 4 Comparison of tensile strength of pure molybdenum and TZM (Ti-, Zr-alloyed molybdenum)

TZM moly performance

Increasing demand for a further improvement in high-temperature properties of molybdenum led to the development of TZM moly alloy, which contains small additions of finely dispersed alloying additions (0.5% titanium, 0.08% zirconium and 0.01-0.04% carbon) to inhibit the grain growth of molybdenum at elevated temperatures.

Fig 5 Relative creep resistance and ductility of Mo-base alloys at an operating temperature of 1800C (3270F). Ductility is at room temperature after 100 h at operating temperature. Creep resistance based on stress for a steady-state creep rate of 0.0001%/h.

Alloying substantially increases molybdenum's high temperature tensile strength (Fig. 4), ductility and creep strength (Fig. 5, 6 and 7).

Fig 6 Relative creep resistance and ductility of Mo-base alloys at an operating temperature of 1450C (2640F). Ductility is at room temperature after 100 h at operating temperature. Creep resistance based on stress for a steady-state creep rate of 0.0001%/h.

In addition, TZM has a higher recrystallization temperature than pure molybdenum (Fig. 8). In vacuum furnace hot-zone applications, TZM has significantly higher strength than pure molybdenum at typical vacuum-furnace process temperatures.

Fig 7 Relative creep resistance and ductility of Mo-base alloys at an operating temperature of 1000C (1830F). Ductility is at room temperature after 100 h at operating temperature. Creep resistance based on stress for a steady-state creep rate of 0.0001%/h.

Therefore, it is a very suitable material for subcomponents within a hot zone in applications where high temperature strength is required, such as a hearth assembly consisting of rails, posts and caps.

Fig 8 Recrystallization temperatures of pure molybdenum, TMZ moly alloy and ML-doped moly alloy

ML moly performance

Plansee ML moly is used when it is necessary to avoid brittle fracture after recrystallization and where dimensional stability at high temperatures is required, such as in vacuum-furnace heating elements and radiation shields. Doping molybdenum with small amounts of lanthanum oxide increases the recrystallization temperature to 1400C (2550F) as shown in Fig. 8. In addition, due to the resultant overlapping fiber structure of the lanthanum oxide within the grain structure, ductility is partially retained even after recrystallization (Plansee MLR in Fig. 5, 6 and 7).

Fig 9 Sag behavior of pure molybdenum, and TMZ and ML alloys

Furthermore, the creep rate of doped molybdenum is significantly lower than that of pure molybdenum and the sag resistance is much greater (Plansee ML in Fig. 9). Good ductility and sag resistance are very desirable properties for vacuum furnace heating elements and radiation shielding. ML components offer a dramatic improvement over previous designs, which typically use pure molybdenum construction.

PM 2000 ODS superalloy

Furnace hot zones used for nickel brazing require ultimate vacuum levels and high cleanliness to achieve satisfactory braze results. Thus, an all-metal hot zone frequently is selected for this thermal processing application. Unfortunately, excess nickel braze-alloy runoff damages molybdenum shielding (pure Mo, TZM and ML) immediately upon contact, resulting in premature failure of the hot-face shielding in the hot zone.

In the case of a vertical bottom-loading furnace, the radiation shield pack on the bottom head (below the hearth assembly) fails before the main hot zone (plenum) and may have to be replaced and/or repaired several times before failure of the main hot zone. In the case of a horizontal loading furnace, the main hot zone radiation shielding (on the inside of the plenum) simply fails. Radiation shields no longer radiate energy into the hot zone and excessive heat losses occur to the cold wall resulting in high electrical energy costs. This situation requires a costly replacement of the entire zone or an extensive repair.

Plansee PM 2000 is an iron-base ODS (oxide dispersion strengthened) superalloy having the chemical composition: 19.0% Cr, 5.5% Al, 0.5% Ti, 0.5% Y2O3, bal Fe. The material is ideal for use in nickel-braze hot zones because it forms a dense, strongly adhering Al2O3 protective film on the surface, which resists the corrosive nature of the high temperature braze process. PM 2000 also has high creep strength and toughness. It has good physical-damage resistance under normal conditions and has relatively high ductility compared with refractory metals including alloyed and doped molybdenum. In addition, it offers excellent hot-gas corrosion and oxidation resistance. A hot zone constructed entirely of PM 2000 could be exposed to air at furnace temperatures to 1300C (2370F) without undergoing damage to the hot zone.

An alternative to 100% PM 2000 construction (due to cost considerations) is to build the vacuum furnace hot zone using preoxidized PM 2000 for the hot face. This includes the entire bottom head of the hot face shield on a vertical bottom loading furnace and the bottom third of the radiation shielding on a horizontal furnace. Such a construction markedly extends service life over that of a molybdenum-base radiation shield assembly in high-temperature nickel-braze applications.

For more information: Thomas P. Farrell, Jr. is vice president, Sales and Marketing, Schwarzkopf Technologies Corp., 115 Constitution Blvd., Franklin, MA 02038; tel: 508-553-3800; fax: 508-553-3823; e-mail: tom.farrell@stc-ma.com