With the price of molybdenum mill products at record highs, ODS superalloys are now a viable alternative in vacuum furnace hot-zone construction, offering performance advantages over molybdenum and molybdenum alloys.

Molybdenum is the most widely used refractory metal in vacuum furnaces hot zone construction. Unfortunately, recent dramatic increases in the price of commodity molybdenum oxide (the precursor material to produce pure molybdenum and Mo alloys such as TZM and ML Molys) have forced producers of molybdenum mill products to raise prices markedly over what the material was selling for as little as one year ago. This situation has made oxide dispersion strengthened (ODS) superalloys, such as PM 2000, once too costly for consideration, viable alternative materials. Additionally, in many applications, PM 2000 offers some distinct performance advantages over molybdenum.

Economic background

Molybdenum oxide is the precursor material for the production of molybdenum mill products (sheet, plate, rod, wire, etc.) commonly used in vacuum furnace hot zone fabrication. Over the past year, molybdenum mill products have undergone unprecedented price increases due to the dramatic increase in the cost of molybdenum oxide. Specifically, the price of molybdenum oxide has risen from years of relative stability at $2.50-3.00/lb to a pricing level today approaching $40.00/lb. Only about 5% of molybdenum oxide is used to produce pure Mo and the balance is used for alloying steel. Why the increase? China now consumes 25% of the world's steel production and their demand is growing, with an accompanying increase in the demand for molybdenum oxide. Other metals such as nickel and many other commodity metals have also met this same fate.

Alternatives to molybdenum

One alternative to a Mo-based, all-metal hot zone is a graphite-based, or "insulated," hot zone, but an all-metal hot zone may be the only choice in many applications, such as for process cleanliness purposes (graphite-free environment) and for faster quench times (better heat transfer coefficients). Also, with all parameters being equal (e.g., same chamber size and vacuum pumping system), a metal hot zone pumps-down faster and achieves higher ultimate vacuum levels than an insulated hot zone. So, if graphite is not an option, what hot-zone material alternatives to molybdenum are available? Desirable material properties in vacuum furnace applications include high melting point, low vapor pressure, high sag resistance and a low coefficient of expansion, all of which Mo has, and superalloys do as well.

ODS superalloy Plansee PM 2000 is an iron-base material alloyed with nominally 19%Cr, 5.5%Al, 0.5%Ti and 0.5%Y2O3 (finely dispersed yttrium oxide particulates). It is neither a refractory metal nor a traditional superalloy, but rather is a high heat-resistant ferritic Fe-Cr alloy having excellent physical properties as shown in the following table.

Physical properties of PM 2000 at ambient temperature
  • Melting pt., °C: 1483
  • Density, g/cm3: 7.18
  • Coeff. thermal exp., 10-6/K: 10.7
  • Young's mod. (15-mm bar), GPa: 215
  • Young's mod. (8-mm sheet), GPa: 157 (L), 173 (LT)
  • Specific heat, J/kg K: 480
  • Thermal cond., W/m K: 10.9
  • Temp, cond., 10-6 m2/s: 3.1
  • Sp. electrical resist., μΩm: 1.31
  • Curie Temp., °C: 553
  • Coercive field str., A/m: 290

Unlike typical refractory metals (Mo, W and Ta) used in furnace construction, PM 2000 can be used in vacuum, atmosphere and oxidizing furnace applications up to 1350°C (2460°F). Additionally, PM 2000 has very good creep resistance (to 1300°C, or 2370°F) and excellent hot gas corrosion and oxidation resistance in an oxidizing atmosphere up to its melting point due to the formation of very dense, strongly adherent oxide layers on the surface. Therefore, due to its high strength at temperatures up to 85% of its absolute melting point, PM 2000 is generally referred to as an ODS superalloy (Table 1).

Mechanical alloying

ODS alloys are manufactured using mechanical alloying powder-metallurgy techniques, known as high-energy milling, where pure powders, such as iron, aluminum, chromium and yttrium oxide are processed into extremely homogenous fined-grained structures (Fig. 1). The fine dispersion of these particles (particularly Y2O3) is responsible for boosting the strength and operating temperature of the base material (Fe), allowing the substitution of PM 2000 for Mo in many high temperature furnace applications.

Good oxidation resistance

Molybdenum oxidizes rapidly in air at relatively low temperatures 400°C (750°F) and requires a coating to prevent oxidation. By comparison, PM 2000 forms a very dense, strongly adherent oxide layer of Al2O3 under an oxidizing (air) or hot-gas (variety) conditions due to its high Cr and Al content. The protective layer provides highly effective protection to the base material preventing oxidation (Fig. 2) and/or corrosion up to 1100°C (2010°F), even in high speed gas flows as shown in the following table.

Burner rig test comparison (a)
  • Alloy: Corrosion rate, μm
  • PM 2000: 19
  • Haynes 188(b): 210
  • Inconel 617: 280
  • Hastelloy X(b): 330
  • Haynes 25(b): 1,160
(a) Test condition: 1050°C/300 h (fuel: oil with a 0.4% S and 8 ppm Na) (b) Haynes and Hastelloy are registered trademarks of Haynes International Inc.

Another advantage of PM 2000 is the ability to furnish the material in a preoxidized form to effectively eliminate the damaging effects of high-temperature Ni-braze alloy runoff. Mo has no such resistance and actually forms a lower melting eutectic alloy when it contacts nickel, which frequently results in rapid deterioration of hot zone shielding and components such as the braze fixtures, grids and racks.

Ductility and toughness

Pure Mo mill products begin to recrystallize between 800 and 1200°C (1470 and 2190°F) resulting in a considerable decrease in strength and loss of ductility, which results in furnace components that become highly susceptible to physical damage. By comparison, PM 2000 is very tough and ductile and is ideally suited for hot zones and furnace component construction. These favorable properties are due to its manufacture including mechanical alloying under a high-purity H2atmosphere and careful control of the chemical composition within very narrow limits in combination with a special thermomechanical treatment.

Good processing characteristics

PM 2000 lends itself to a variety of processing techniques including stamping, laser cutting, water jet cutting, turning, milling and wire EDM. While the material has certain peculiarities, such as springback, which must be taken into account when bending, it can be handled satisfactorily by anyone who has worked with refractory metals. The preferred welding methods are laser and electron beam welding. In some cases, GTAW (gas tungsten arc welding) or GMAW (gas metal arc welding) methods can be used, but mechanical fastening (i.e., use of rivets and bolt/nuts) is recommended over this type of welding. PM 2000 can be easily brazed using conventional braze alloys. Essential for good brazing results are clean joint surfaces and brazing under high vacuum. The use of braze foils has also proven advantageous.

Conclusion

Developed in the 1960s, ODS alloys have been used in high temperature furnace applications for many years now. Today, with the price of molybdenum mill products at record highs, ODS super alloys such as PM 2000 are now a viable alternative in vacuum furnace construction and offer performance advantages in addition to their cost savings over molybdenum and molybdenum alloys such as TZM and ML Moly.