The term “refractory metals” is generally applied to metals that have a melting point greater than 3630°F (2000°C). Renowned for their high melting points, refractory metals are widely used in the construction and design of components for high-temperature furnaces.

Principal refractory metals are tungsten (W); molybdenum (Mo); molybdenum-lanthanum (MoLa) alloys with La₂O₃ of 0.3 wt.%, 0.6 wt.% or 1.1 wt.%; molybdenum alloy (TZM) with 0.5% Ti and 0.08% Zr; tantalum; and niobium. One of the most important components of high-temperature furnaces for heat treatment is the heating element (Fig. 1). When choosing a suitable material for heating elements and their design form, engineers need to follow some standard criteria, including:

A.  The process type – meaning temperature vs. atmosphere vs. heat-treat product

B.  The heating capacity considering all heat loss as well as the heating rate

C. Geometric parameters of the heating elements, such as width x thickness x length for metal strip heaters; inner diameter for elements such as rod; inner and outer diameter in the case of tubular elements; and wire diameter, mandrel diameter, number of parallel coils, coil pitch and the ratio of the total surface in the frontal area for mesh elements

D. Type of electrical connection (e.g., star, delta or Scott transformer)

E.  Specific electrical, physical and strength parameters, which belong to the following:

i.   Radiation from the surface (W/cm²) in operating temperature

ii.  Emissivity in operating temperature

iii. Resistivity (Ohm mm²/m) in operating temperature –
for calculations substitute resistance (Ohm) for complete heating control zone, size and secondary voltage of the transformer (voltage, kW) and finally current in power feedthrough amps (amperage)

iv. Mechanical properties like module of elasticity (Fig. 2),
linear expansion coefficient (Fig. 3) and ultimate tensile strength

H.C. Starck’s Refractory Metals

When choosing a refractory metal for a furnace application, operating conditions and the properties of the product being processed should be considered. Table 1 shows the compatibility of the most-frequently used refractory metals with furnace atmospheres, and Table 2 shows compatibility with common refractories.

Refractory metals like tungsten (W), molybdenum (Mo), molybdenum-lanthanum (MoLa) alloys with La₂O₃ (0.3, 0.6 or 1.1 wt.%) and titanium-zirconium-molybdenum (TZM) alloy with 0.5 wt.% Ti and 0.08 wt.% Zr are the “best choice” for metallurgy processes and for producing parts that come out bright and clean. These sensitive processes include diffusion bonding, aluminum brazing, stress-free annealing, degassing and cleaning, vacuum and protective-gas brazing, and sintering and the MIM processes in both batch and continuous furnaces.

Refractory metals specifically engineered for heat treatment are materials such as superalloys (e.g., titanium, Rene-80 nickel-based superalloy, Hastelloy and tungsten). Heating elements made of refractory metals must meet Class 1 AMS 2750E standards with temperature-uniformity range between +/-5°F compared to the graphite heating elements. Graphite heating elements are not recommended for use above 2400-2700°F (1315-1482°C). The maximum ramp rate for temperatures lower than 2400°F for graphite heaters is 45°F per min. Furnaces with graphite heaters are defined as Class 2 AMS 2750E.

Molybdenum-Lanthanum (MoLa) Alloys

MoLa alloys with 0.3 wt.%, 0.6 wt.% or 1.1 wt.% La₂O₃ are one type of oxide-dispersion-strengthened (ODS) molybdenum containing molybdenum with a very fine array of lanthanum trioxide particles. This combination creates extraordinary characteristics of the MoLa material, which demonstrates resistance to recrystallization and high-temperature warpage.

MoLa alloys have a microstructure stable at up to 3632°F (2000°C). For furnace components like heating elements, these alloys are the best choice for a furnace setpoint range of 2462°F up to 2912°F (1350-1600°C). Temperature of the heaters has a much higher furnace setpoint. The approximate temperature of the heating element can be calculated using formula in equation (1).

where:

Te is heating-element temperature °F

Tf is furnace setpoint temperature °F

Rfs is radiation from the surface W/square inches

Em is emissivity

Radiation from the surface (W/cm²) for refractory metals linearly increases (e.g., 3.86-116.7 W/cm² from 2060-4580°F). For engineering calculations, radiation (Rfs) values most commonly assumed are 3.86 W/cm² at 2060°F, 8.7 W/cm² at 2600°F, 18.6 W/cm² at 2960°F and 30.3 W/cm² at 3320°F. Average emissivity and resistivity parameters for molybdenum, molybdenum alloys and tungsten (unoxidized) are summarized in Table 3 and Fig. 4.

The “best value” MoLa alloy is the one containing 0.6 wt.% lanthanum. It exhibits the best overall combination of properties. Low-lanthanum MoLa alloy (0.3 wt.%) is an equivalent substitute for pure Mo in the temperature range of 2012-3452°F (1100-1900°C). The advantages of high-lanthanum MoLa, like superior creep resistance, are only realized if the material is recrystallized prior to use at high temperatures. From 2012°F, the creep stress in 1,000 hours decreases from approximately 26 (Ksi) to approximately 1.45 (Ksi) at 3272°F.

Titanium-Zirconium-Molybdenum (TZM) Alloy

TZM is an established molybdenum alloy (0.50 wt.% Ti, 0.08 wt.% Zr, balance Mo), which is consolidated by either the powder-metallurgy or vacuum arc-casting processes. It gives excellent service in applications that require high strength and creep resistance at elevated temperatures.

TZM also permits higher service temperatures without softening or weakening. For furnace applications like heating elements, TZM is the best choice for a furnace setpoint range of 2102°F up to 2462°F if the process can accept Ti and Zr components. From 2462°F, the creep stress in 1,000 hours decreases from approximately 3 Ksi to approximately 0.5 Ksi at 2642°F. For 2102°F, the creep stress at 1,000 hours is approximately 4.4 Ksi.

Conclusion

The available data on the mechanical behavior of molybdenum and tungsten is critical for the right choice of material and type of heating element. Several important conclusions have been reached as a result of this short study. The most important one is that there is sufficient engineering data available for the design of complex heating systems using refractory metals.

Engineers will find that each material reviewed has certain unique properties that make it desirable for specific applications. Molybdenum and tungsten have low coefficients of thermal expansion, excellent thermal conductivity (Fig. 5), chemical compatibility with a variety of environments and a high modulus of elasticity. This means that refractory metals can be used in a broad spectrum of metallurgical furnace applications and are ideal for heating elements for precise processing.

Heating elements may be constructed from nonrefractory metals in a variety of styles. The geometry of heating elements is very often due to the patented solutions from the past – in particular, ways to connect the heating elements to themselves and not from technology limitations.

Engineers always have to be very careful in designing heating elements from nonrefractory metals. Refractory metals do not have the same limitations, which is one of the most important advantages. To create a quasicomparative discussion platform, it is necessary to compare characteristics of the most important parameters of refractory metals versus nonrefractory metals used for heating elements in heat-treatment furnaces.

For more information: Contact Greg Matula, Msc. Dipl-Ing., application engineer, H.C. Starck Fabricated Products Division, 21801 Tungsten Road, Euclid, OH USA; tel: +49 151 / 14 79 46 04; fax: +1 216 692 0031; e-mail: grzegorz.matula@hcstarck.com; web: www.hcstarck.com

References:

1. Herring, Daniel H., Vacuum Heat Treatment, BNP Media, 2012
2. Sandvik Materials Technology, Kanthal Super – Electric heating elements – control, power supply and wiring - S-KA060-B-ENG.2-A-3-3.01.2012
3. David Rowe, “Refractory Metals,” Heat Treating Progress, H.C. Starck Ltd., U.K., Nov./Dec. 2003
4. The Ipsen Harold, A look inside the furnace: Choosing the best hot zone for your needs, April 2015
5. William F. Mattson, William L. Maag, Analytical investigation of electrically heated wire mesh for flowing gas heaters, Lewis Research Center, Cleveland, Ohio, June 1968
6. Jeff Pritchard, “Hot-Zone Design for Vacuum Furnaces,” Vac Aero International Inc., Oakville, Canada, Industrial Heating, Sept. 2007