Many decisions must be made when a business assesses the viability of establishing a new vacuum heat treating capability. Innovations in furnace design such as hot zone materials, control packages and quenching systems can offer many operating and performance improvements, but will also affect the capital investment. There can be significant costs beyond the price of the furnace equipment that also need to be understood. Following are some guidelines to assist in this process.
Selecting a furnace
Factors to consider in selecting the best furnace equipment for a new vacuum heat treating operation include furnace size, budget and throughput.
Vacuum furnaces are available in many sizes, and initial capital outlay and operating costs are roughly proportional to size. The size should be based not only on current heat treating requirements, but also to expected future processing requirements, because it is usually impractical (and sometimes impossible) to enlarge a vacuum furnace chamber to handle larger heat treating applications.
The basic equipment price for most popular smaller and medium sized vacuum furnaces (Fig. 1) ranges from $300-500,000. Additional costs are likely for site preparation and installation, auxiliary systems, tooling and other incidentals, which can range from $50-100,000.
Furnace size dictates the amount of work that can be processed in a single load. Typical vacuum heat treating cycle times range from about five hours minimum to as long as 12+ hours. Thus, allowing for loading and unloading time, only three or four cycles/day typically are possible.
Site preparation and utilities
Vacuum furnaces usually do not require special foundations. Floor loading rarely exceeds 40 psi, so furnaces can be installed on any industrial grade concrete floor. The furnace should be located near a wall if possible for easiest routing of utilities. Floor space requirements varies with furnace size, but most medium size horizontal furnace installations usually require a minimum ceiling height of 15 ft and a minimum floor area of 400 ft2 (approx. 20 ft sq).
Vacuum furnaces require connections to supplies of electricity, water (for cooling), quench gas and compressed air. The full load electrical power requirement for smaller and medium size furnaces can range from 300 to 600 A, or more for auxiliary systems. Operating electricity costs vary with furnace size and type of heat treating. The furnace manufacturer can assist in estimating these costs.
If a central cooling water recirculation system is not already in place, installation of an auxiliary cooling system is recommended consisting of a reservoir/heat exchanger/pumping package located inside the building (Fig. 2) and a cooling tower outside. Cooling towers with gravity-feed returns must be positioned above the reservoir. Ideally, the cooling tower is positioned on the roof of the building directly above the support wall closest to the reservoir, but can be mounted on a stand on the ground if necessary. Water consumption in these systems occurs only through evaporation and is relatively low.
All vacuum heat treating processes use inert gases for quenching at the end of the heating portion of the cycle. Nitrogen, the most popular quench gas, is inexpensive and has good cooling properties. Because nitrogen can be slightly reactive under certain conditions, some heat treating applications specify argon for quenching. Argon is much more expensive than nitrogen and has less effective cooling properties, but is totally non-reactive with metals. Helium is gaining popularity as a quench gas but is very expensive. The quench blower in the furnace must be designed to suit the properties of the gas used for quenching.
The amount of quench gas used in each heat treating cycle is directly proportional to the size of the furnace chamber and the quenching pressure required. For all but very small furnaces, it is recommended that a permanent liquid bulk supply of quench gas be installed. Charges from an industrial bulk gas supplier include the cost of a liquid gas reservoir tank installation, monthly rental fee for the reservoir and the cost of gas. The required optimum size for the bulk supply reservoir can be determined by the furnace manufacturer and gas supplier.
A supply of compressed air is required to operate valves and other pneumatic devices on the furnace. The air supply pressure is regulated to between 80 and 100 psig. While this supply should be continuous (even when the furnace is not operating), consumption is very low.
Noise and environmental issues
Modern, well-maintained vacuum furnaces do not radiate excessive heat to the surrounding environment or release harmful emissions. However, many mechanical vacuum pumps operate with a continuous noise level approaching 82 dbA, and there may be occasional noise spikes slightly above this level when the furnace backfills or the quench blower activates. These spikes are short (<15 seconds) and would typically occur only a few times in each 24 hour period. The system can be soundproofed if desired.
Oil-lubricated vacuum pumps are the most common and emit a small volume of oil mist during the initial stages of evacuating the chamber. The pump exhausts are vented to the outside of the building. If venting to the outside is a concern, the pump exhaust can be equipped with a filter at a cost of around $2,000.
Quench gases must be vented from the furnace at the end of each heat treat cycle through an exhaust from the furnace chamber to the outside of the building. The gas is non-polluting and can be vented outside without requiring treatment.
It is usually the customer's responsibility to prepare all openings in the roof and walls of the building to accommodate vent lines and connections to the cooling tower and bulk gas supply. Roof openings require a weather-proof cap.
Heat Treating operations are usually run 24 h/day, often 7 days/week, due to the length of typical heat treating cycles and to generate the best return on the investment in the equipment. Fortunately, after modern vacuum furnaces are loaded, they can be operated in a fully automated mode without an attending technician. There are numerous interlocks built into the control system that will not allow the furnace to operate in an unsafe condition. A datalogger records all relevant information on operating parameters during each furnace cycle. Control systems (Fig. 3) also can include a remote access capability, which allows monitoring and regulating the furnace via secure connections. These systems can also be used to activate pagers or dial phone numbers if an alarm condition develops.
However, a technician is usually required to manually prepare loads for processing, attach thermocouples and load/unload the furnace. While one properly trained technician can effectively operate and maintain one or two furnaces, safety regulations may require at least two persons per shift. A quality control inspector is usually required at some point in the operation, but may not be needed 24 hours per day.
Vacuum brazing applications may involve additional staff. Considerable labor is required to prepare work pieces for some brazing processes. It is generally recommended that a separate, clean and enclosed area be designated for brazing preparation. Other preparation operations could include degreasing or grit blasting, which may not only involve more labor, but also investment in additional equipment.
Fixtures and tooling
Parts for heat treating must be properly loaded onto trays or into baskets (Fig. 4), which are usually placed on a grid that rests on the hearth rails of the furnace. Most heat treating tooling is made from Ni-Cr heat resistant alloys. Some grids are manufactured from molybdenum, which is very expensive but particularly well-suited for high temperature service. Carbon plates are gaining in popularity as a durable, less expensive approach to heat treating tooling, but care must be taken to insulate the carbon plate from the metallic workpiece to prevent eutectic melting.
Usually, two sets of tooling are assigned to each vacuum furnace; one in the furnace while the other is being loaded for the next cycle. The initial capital outlay for sufficient tooling to support a medium size furnace can easily reach the range of $10-20,000. Alloy heat treating tooling also requires periodic maintenance such as straightening or weld repair, which generates additional cost.
Maintenance and miscellaneous operating expenses
With a reasonable preventative maintenance program and good operating practices, a vacuum furnace can provide reliable operation for 20 years or more. preventative maintenance involves checking oil levels in pumps, inspecting for proper cooling system flow, checking belts and O-ring seals for damage or wear and ensuring the vacuum chamber is free from debris and contamination. It may also involve treating the water used in the cooling system to inhibit corrosion or mineral deposits. The cost for consumables associated with preventative maintenance is relatively low, usually less than $5,000/year/furnace. Furnace suppliers usually have recommended preventative procedures. For example, Vac Aero provides its furnace customers comprehensive training in preventative maintenance and good operating practices. This training should allow the customer to use in-house resources for many maintenance and repair activities.
The heat treater could incur some major maintenance expenses after each five to seven years of operation. For example, mechanical vacuum pumps may require overhaul at a cost of $2-4,000 (depending on the type of pump). Hot zone repairs may also be necessary. For a smaller or medium size furnace, these repairs can range from $10,000 for a relatively minor restoration to more than $50,000 for a complete rebuild. Furnaces used extensively for brazing tend to require more frequent hot zone repairs (Fig. 5).
Thermocouples are used to control the temperature in the furnace and monitor the temperature of the workload. The average annual expense for thermocouples would typically be in the range of $2-4,000 per furnace. Many heat treating specifications require that furnace control and monitoring instruments be calibrated on a regular basis, usually once or twice a year. Most heat treaters use approved subcontractors for calibration services at a cost $2-3,000 per visit. IH