An estimated 70% of the hot zones for large vacuum furnaces are of an insulated design, with the balance being the all-metal type. Common insulation packages consist of rigid graphite board with graphite felt backing (with or without graphite-foil bonded, or layered, on top of the hot face), a nonfiber foam board with graphite felt backing, graphite-foil hot face with graphite felt backing and a molybdenum hot face with graphite felt or high-purity alumina backing. Table 1 lists some hot-zone types.
Insulated hot zones were once selected for their relatively lower capital cost, on-site repairability and improved energy operating efficiencies as compared to all-metal designs. On the other hand, all metal hot zones typically are selected for their rapid quench rates, faster pump down times, cleaner (carbon free) work environments and higher achievable vacuum levels. The all-metal Enerzone with Plansee blackbody-coated radiation heating elements have nearly the same energy consumption as their insulated counterparts. Thus, energy savings is no longer one of the key criteria for choosing an insulated hot zone. However, the higher capital cost of the technically advanced, energy-saving all-metal hot zone still is a factor. (Note: standard all metal hot zones are priced comparable to a quality insulated hot zone.) Further evolution of the insulated hot zone has occurred because many heat treaters cannot justify the expense of the energy-efficient all-metal hot zone or do not require the operating characteristics of an all-metal hot zone.
Improvements to an insulated hot zone start with the plenum/retort/jail assembly. This component has several purposes including:
- A support structure to hold the insulation or radiation shield packs, elements and element supports
- A mechanism to distribute the cooling gas (in a gas-quenching furnace) uniformly throughout the hot zone via a double-wall plenum and a series of gas nozzles that are distributed uniformly around the main hot zone body. Gas nozzles sometimes are located on the rear wall of the hot zone as well. With a gas-quenching retort design, the equalization of the cooling gas is accomplished by partitioning a portion of the inner vacuum chamber to construct a sort of plenum.
- To serve as a further radiation-shielding barrier in addition to the normal hot-zone insulation shield package
Upgrading from traditional carbon steel to gas tungsten-arc welded AISI Type 316 stainless steel provides a stronger plenum/retort/jail assembly with greater thermal efficiency. Stainless steel offers improved high temperature strength of the hot zone. Although stainless steel has a higher specific heat ratio than carbon steel, it is significantly stronger at elevated temperatures, and, therefore, less material is required in construction. Less mass in a hot zone means faster heat up and cool down. Stainless steel also has a greater emissivity than carbon steel, so it radiates more energy to the center of the hot zone.
Radiation shields/insulation packages
All vacuum furnace hot zones (insulated type, all-metal designs and hybrid combination designs) eventually require replacement due to degradation at high temperatures, vacuum level vaporization effects and the sometimes corrosive nature involved with many vacuum heat treating processes.
Insulated hot zones are available in a wide variety of materials and combinations, each with their own set of advantages and disadvantages. One popular material is a rigid, coated and uncoated carbon-fiber insulation (graphite board). Typically, a series of boards are combined to form a series of interlocking panels for radiation shielding purposes. This material is relatively easy to work, install, maintain and repair. The weakness of this material is gas erosion along the exposed seams, machined joints and penetrations of the individual boards, particularly in 2 to 20 bar rapid gas-quench applications.
Erosion often causes failure of a graphite board hot zone. In addition, some free carbon in the furnace results from gas erosion of the board due to the binder vaporizing out of the board, leaving the fine graphite powder that makes up much of the mass of the board. Free graphite powder creates an unclean working environment that can contribute to contaminating the work load, possibly fouling the vacuum pumping system and can deposit on the cooling fins and tube heat exchanger, decreasing its efficiency through the loss of heat transfer ability.
There are several design improvements that can extend the life expectancy of the graphite board insulated zone and prevent the detrimental affects of free graphite. One alternative is to switch to nonfibrous foam board made of the material used in the construction of a typical foil. This material is graphite-free, and has low thermal mass and high resistance to gas erosion. Limitations of nonfibrous foam board compared with conventional graphite board are that it has very little mechanical strength (extremely fragile to handle), breaks very easily and has very poor insulating capabilities (e.g., only 25% the insulating properties of graphite felt).
Another alternative to graphite board is a ML Molybdenum hot face backed by graphite felt insulation. These combination hot zones (hybrids) provide high emissivity due to the metallic hot face and good insulation properties for energy efficiency. They don't quench as well as an all-metal hot zone, but quench superior than an all-composite hot zone because the molybdenum (pure Mo or ML) in the quench mode readily gives up its heat. Table 2 compares the high-temperature performance properties, mill-product forms and hot-zone components for selected high-temperature material. Figure 1 shows typical service temperatures of selected high-temperature materials.
Plansee ML Molybdenum has increased ductility, resistance to breakage and high sag and creap resistance over pure Mo, and is used when brittle fracture after recrystallization must be avoided and where dimensional stability at high temperatures are required, such as in vacuum furnace shield construction and heating elements. Doping molybdenum with small amounts of lanthanum oxide (La2O2) increases the recrystallization temperature to 2550 F (1400 C), and ductility is partially retained even after recrystallization. The creep resistance of ML molybdenum is superior to that of pure molybdenum. These properties are extremely desirable for vacuum furnace radiation shielding.
Graphite felt has significantly improved thermal properties over high purity alumina fiber insulation for vacuum furnace applications. Graphite felt operates at higher temperatures, is less susceptible to shrinkage, has superior insulation properties, and because it does not contain silica, it is not considered a carcinogen.
PM 2000 or ML molybdenum end caps
Insulated graphite hot zones (rigid board or felt) typically are painted with a graphite coating to the graphite surface to help control dusting and to provide some measure of a reflective surface for radiation (emissivity) purposes. However, graphite paint is not particularly durable. Substituting metal end caps (such as Plansee PM 2000 ODS superalloy) on the front of the opening of a large vacuum furnace hot zone and on selected penetrations where gas corrosion could potentially lead to premature failure of the insulation shield pack can improve performance.
PM 2000 is an iron-base oxide-dispersion-strengthened (ODS) superalloy. It has high creep strength, excellent resistance to hot-gas corrosion, oxidation resistance to the nickel-braze alloy run off in high temperature vacuum braze applications, is extremely tough and resilient, and has good resistance to impact damage strength. A limitation is a maximum use temperature of 2375 F (1300 C). Fortunately, the majority of typical heat-treating and many braze processes are at or below this operating temperature.
PM 2000 is an excellent replacement for CFC (carbon-fiber carbon) on the bottom third of a horizontal vacuum furnace hot zone and on the bottom head of a vertical furnace. Additional benefits of the material over CFC are that it offers greater emissivity (reflectivity), is not porous (pumps down faster because it is not in anyway hygroscopic i.e. water absorbing) and contributes no free graphite/carbon to the hot zone for a cleaner process environment.
A typical vacuum hearth assembly consists of posts (pins) caps and support rails. These hearth assemblies are simple in design with relatively uncomplicated components. Use of high-performance materials such as TZM can help improve even the most simple vacuum furnace component by significantly improving strength, reducing distortion and decreasing the mass of the hearth assembly.
TZM is a molybdenum-based alloy containing small additions of finely dispersed particles of titanium (0.5%), zirconium (0.08%) and carbon (0.01-0.04%) to inhibit the grain growth of pure molybdenum at elevated temperatures. For vacuum furnace hot zone hearth assemblies, TZM exhibits significantly higher strength and ductility than pure molybdenum at typical vacuum furnace operating temperatures. In addition, less TZM material is required for a given application compared with its pure molybdenum refractory metal counterpart for the same application. Less material equates to lower costs and enables higher efficiencies due to the less mass to heat up and cool down.