The latest material and process developments for refractory metals are discussed. Learn how proper hot-zone design leads to improved performance and productivity.



Fig. 1. Creep resistance of various high-temperature metals

Metals that offer a higher melting point than platinum (3222°F/1772°C) are generally considered to be refractory metals (Ru, Rh, Os, Ir, Pt, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Tc, Re). For functional components in furnace construction, only molybdenum- (Mo), tungsten- (W) and tantalum- (Ta) based materials are of significant importance. For several decades these materials have been fabricated and utilized for heating elements, radiation shielding and mechanically loaded furnace fixtures. The following properties, and their synergistic effects, highlight why refractory metals are frequently chosen for such high-temperature applications:

1. High melting point and low vapor pressure
Refractory metals’ high melting points (4730°F/2610°C for Mo, 6170°F/3410°C for W) do not permit any evaporation or contamination, and the treated materials gain an extremely clean and shiny surface. For example, the vapor pressure of molybdenum at 2300°F (1260°C) is below 10-8mbar whereas the values for Fe, Ni and Cr are in the range of 10-3to 10-2mbar.

2. High-temperature strength and creep resistance
Due to their high melting points, refractory metals offer very low creep rates at typical furnace temperatures. The curves in Figure 1 indicate that above 2000°F (1093°C) the high-temperature strength of pure molybdenum is approximately three times higher than that of INCONEL® alloy 601h. Advanced doped-molybdenum materials such as PLANSEE TZM and PLANSEE MLR can offer even more than 10 times the creep resistance of pure molybdenum.

3. Low thermal expansion and low specific heat
The thermal expansion of molybdenum is approximately one-third that of stainless steels. Because of this property, its low specific heat and its high thermal conductivity, parts made with molybdenum can be heated and cooled very quickly with low thermal stresses and, thus, low distortion.

4. Metallic character and low emissivity
Typically, metals are non-porous and do not absorb oxygen, humidity and other substances that will have a negative impact on the performance of a vacuum furnace. In comparison to graphite and ceramics, it is easy to form complex shapes with metal materials. In addition, there is no carbon or fiber contamination.

5. Behavior in protective gas atmospheres
Molybdenum and tungsten resist the most common protective furnace atmospheres, e.g., vacuum, argon, nitrogen, hydrogen and hydrocarbons up to high temperatures. In hydrogen, relatively high dew points of 120°F (50°C) can be withstood. Nevertheless, these materials start to sublimate in air above 1112°F (600°C).

Fig. 2. Ductility of pure and doped molybdenum after high-temperature exposure

Material Innovations

Improved performance of furnace hot zones and components stems not only from fundamentally solid mechanical design but also from dramatic improvements in the properties of the high-performance materials used in their construction. Enhanced metallurgical properties of high-performance materials allow for the longer life of hot zones:
  • Hearth components tend to remain straight and true
  • Heating elements resist sagging and distortion
  • Radiation shielding is less likely to break
  • Gas-cooling nozzles more readily maintain their shape
Most of these components typically last the life of the hot zone with little need, if any, for spare parts.

Fig. 3. Low-mass hearth assembly with MLR U-shaped rails

Doped Molybdenum (ML/MLR/MLS)

PLANSEE developed the original MLR molybdenum alloy, which is delivered in a recrystallized condition. Lanthanum-oxide particles and a sophisticated thermo-mechanical dispersion treatment during production lead to an extremely stable microstructure with elongated grain structure and jagged grain boundaries. MLR offers excellent creep resistance up to temperatures of 3400°F (1871°C) (Fig. 1) and is able to retain its ductility after exceeding these operating temperatures (Fig. 2).

MLR is utilized whenever temperatures exceed 2282°F (1250°C) and when dimensional stability at high temperatures is required or brittle fracture after recrystallization must be avoided. It is the ideal material for mechanically loaded furnace parts such as plates, boats, rails, grids and racks (Fig. 3).

Fig. 4. Sagging behavior of lanthanated molybdenum sheet materials (load for 1.5 hours at 1600°C in a high-vacuum furnace)

MLR Qualities

The MLR material has been a real “success story” in high-temperature furnace applications since its introduction. Due to this fact, other suppliers introduced alternative materials with nominally similar compositions. Since the properties of the original MLR are the result of an advanced and proprietary production route and not just composition alone, the qualities and high-temperature properties of such alternative materials can show considerable differences (Fig. 4).

In order to further optimize the properties of MLR for applications that require the highest creep resistance, an R&D project is taking place focusing on material composition and the thermo-mechanical production route. The project leverages the latest in equipment technology for the production of refractory-metal sheet material, including the world’s largest hot rolling mill. The MLR NEW material is already available for specific applications and products such as sintering plates and high-temperature boats as well as for applications that require the highest creep resistance.

According to Figure 4, the NEW PLANSEE MLR material shows only half of the sagging deformation of the original MLR material. The sagging resistance of the alternative material from another major supplier, however, is considerably lower. In applications where product deformation and sagging behavior determine the lifetime of the parts (e.g., boats, sintering plates, furnace hearths, racks, grids, etc.), the impact on the total costs of the operation can be significant.

The differences in sagging behavior are mainly caused by microstructure effects. The plain and large grains in the PLANSEE MLR material, in general, lead to the excellent creep resistance and, thus, sagging resistance compared to finer grain structures. Another important impact on the high-temperature strength is the size and distribution of the lanthanum-oxide particles in the microstructure (nano-scale effect).

Fig. 5. Micrograph of SIBOR® on molybdenum after three months use in air at 2642°F (1450°C)

SIBOR - Advanced Coating for Protection of Molybdenum

Molybdenum starts to react with oxygen at approximately 750°F (399°C) and forms a yellowish, sublimating oxide at higher temperatures. This reaction excludes the use of unprotected molybdenum above 950°F (510°C) in atmospheres containing oxygen. With a SIBOR® coating on molybdenum this problem has been conquered, and for the first time it allows the utilization of molybdenum in oxidizing environments up to 3100°F (1704°C).

SIBOR is a patented coating based on silicon and boron that is applied by Atmospheric Plasma Spraying (APS). During the subsequent heat treatment, SIBOR reacts with the base material to form a dense and stable layer that, depending on the application, is 150-300 µm thick and consists of molybdenum-silicide compounds with a SiO2surface sealing. The structure of the coating is shown in Figure 5. This coating design protects molybdenum safely for several thousand hours of high-temperature operation. SIBOR is the first and only coating in the market that offers 100% guaranteed resistance when operated in air. It is self-healing and does not flake off like ceramic layers.

Due to the outstanding mechanical properties of molybdenum at high temperatures in combination with the new type of protection against oxidation and corrosive media, SIBOR on molybdenum has unique characteristics, resulting in numerous new applications. In the high-temperature processing and furnace industry, SIBOR allows the use of molybdenum in oxidizing or extremely wet hydrogen atmospheres.

Typical components are thermocouple protection tubes, gas lances, burner components and protection shielding. Even complex and joined components such as annealing boxes and rotary furnaces can be protected with SIBOR (Fig. 6).

Fig. 6. SIBOR®-coated molybdenum annealing box to be used in air at 2500°F (1371°C)

ENERZONE with Low-Mass Concept

In general, it is a key goal for engineers to reduce the mass of advanced products. Lightweight automobiles accelerate and brake faster, and they consume less gas. Lower mass in the hot zone equates to less material to heat up and cool down for improved thermal efficiency and overall performance:
  • Faster ramp-up and change of the furnace condition (temperature, atmosphere)
  • Improved quenching capabilities
  • Less stress on hot-zone components due to less temperature gradient in materials
  • Shorter cycle time due to less energy demand during heating and cooling
  • Less total energy consumption (shorter cycle time and less mass to be heated and cooled)
Measures to reduce the weight go hand-in-hand with technological advances (Fig. 7):
  • The usage of doped molybdenum allows reduction of material thickness, e.g., for hearth components such as rails and posts
  • Improved design and fabrication methods for hearth rails (U-shaped rails), element supports (one-piece bending) and power feed-throughs
  • Reduction of the sheet thickness in the shield pack due to the development of circumferential spacer system
  • Replacing of stainless steel in the shield pack (efficiency of one molybdenum layer is equivalent to five stainless steel layers)
  • Usage of high-strength stainless steel for support frame


Fig. 7. Measures of weight reduction for hot zones (*all-metal hot zone for round horizontal vacuum furnace, useful working area 24”x24”x36”, maximum temperature 2400°F (1316°C), shield pack consists of three molybdenum plus two stainless steel layers)

The combining of the low-mass concept with additional measures to reduce the heat loss – adding more molybdenum shield layers, covering feed throughs in the shielding, and reducing heat conduction by smaller cross-section of posts, pins and feed throughs – results in ENERZONE™, a premium hot-zone product.

The benefits of converting a standard 24-inch x 24-inch x 36-inch all-metal hot zone into a low-mass hot zone according to the measures listed in Figure 7 are:
  • Reduction of total hot-zone weight by 15-20%
  • Decreased heat loss by approximately 15 kW (one molybdenum layer replaces two SS layers)
  • Lower energy demand for heating up and cooling down per cycle by 25-30 kWh
  • Quench performance is improved by a minimum of 15%
  • Faster ramp-up by 20% (empty furnace)
  • Enhanced performance and reduced operational costs
By teaming with a partner who has a deep understanding of material properties with fabrication capabilities and the necessary design expertise, the listed benefits can be achieved while maintaining operational efficiency.IH

For more information:Contact PLANSEE USA LLC, 115 Constitution Blvd., Franklin, MA 02038; tel: 508-553-3800; e-mail: usa@plansee-usa.com; web: www.plansee-usa.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: hot zone, refractory metals, creep resistance, sagging, plasma spraying, hot isostatic press

Fabrication of the molybdenum cylinder for world’s largest HIP unit

SIDEBAR: Molybdenum Cylinder for World's Largest HIP Unit

At the end of 2008, PLANSEE delivered the molybdenum cylinder for the world’s largest hot isostatic press (HIP) for its customer Avure Technologies AB in Sweden. The HIP press is due to start operation by 2010.

The cylinder has been made entirely of a doped molybdenum material, which enables the furnace of this press to operate at temperatures of 1350°C and a pressure of 1,180 bar. It is approximately 16.5 feet long and measures more than 7 feet in diameter. It required about 40,000 rivets to assemble. The molybdenum cylinder, which is the innermost part of a complex heater/thermal barrier assembly, is a key element of the press.

This project required drawing on all of the available experience of our engineering and fabrication departments, as well as the expertise in our manufacturing and refractory-metals processing facilities. A major factor enabling the production of such a high-quality, complex product is that the entire manufacturing process – from the powder to the finished molybdenum cylinder – is handled and controlled within one vertically integrated operation inclusive of high quality-assurance standards.

Manufacturing this cylinder required the use of PLANSEE’s full range of refractory-metals processing technologies: laser and water-jet cutting; hot working such as spinning, bending and rolling; joining using doped molybdenum rivets; and flame spraying. In addition, PLANSEE manufactured the required tooling and fixtures to make the fabrication possible.