Heat & Corrosion Resistant Materials/Composites: Superalloy Materials Now Cost Competitive In Vacuum Furnace Hot-Zone Construction
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 backgroundMolybdenum 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 molybdenumOne 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 alloyingODS 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 resistanceMolybdenum 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
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.