10 Things to Consider When Buying vacuum melting furnace

Acquisition of a vacuum furnace represents a major capital equipment investment and one that creates a long-term relationship with your supplier partner. Thus the choice of what to buy and who to purchase it from requires careful planning and considerable up-front research. You need to know when and how to apply vacuum technology if it will be the most cost-effective solution for what you need to do, what questions to ask and what information to provide.

haoyuefurnace Product Page

The process begins by understanding your specific needs and asking all the right questions. Is it more prudent to upgrade an older piece of equipment, purchase new or purchase used? Is it better to have one large furnace or two smaller ones? Is a batch solution best or is a continuous approach better?

Ask yourself what the equipment must do, what productivity must be achieved (now and in the future) and what type of specifications or compliance requirements (e.g. Nadcap, CQI-9) must be met. The type of material(s) being run, the skill of your workforce, the (internal and external) support available and the type of controls and/or quality records required are additional considerations. All of these will help define how much training and support will be needed from your supplier partner.

The following factors typically influence equipment design and represent information that should be shared with potential supplier partners:

1. Material(s) to be processed
2. Raw material condition (e.g. chemistry, grain size, hardenability, cleanliness, prior microstructure)
3. Manufacturing sequence (e.g. type of operations and potential for induced stress)
4. Part geometry, mass, and design (e.g. thin and thick sections in close proximity to one another, through and blind hole locations, sharp corners, etc.)
5. Dimensional requirements
6. Surface finish requirements
7. Specifications (mechanical, metallurgical, physical)
8. Workload thermocouple requirements
9. Documentation requirements
10. Loading arrangement (size, weight, load density, method of part support)
11. Cleaning (method & degree), level of cleanliness required
12. Temperature requirements (preheat, austenitizing, uniformity)
13. Ramp rates
14. Partial pressure requirements
15. Time at temperature
16. Production rate
17. Process application specific criteria (e.g. brazing, gas or oil quenching)
18. Additional heat treatment steps (e.g. deep freeze, temper, coating)
19. Post heat treatment operations (e.g. plating, stock removal, shot peening)
20. Testing & quality control/quality assurance requirements

Furnace Type & Size

Vacuum furnaces come in a variety of types and sizes including horizontal (Fig. 1) and vertical (Fig. 2) orientations. Vacuum furnaces range from small benchtop laboratory units to large car-bottom style furnaces capable of processing tons of material at a time. The initial capital outlay and operating costs are proportional to size.

The furnace chamber must be physically large enough to handle the largest load or workpiece (including fixturing) to be treated. Ideally, loads should be evenly distributed and spaced to allow for radiant heating and circulation of the cooling media around all surfaces. As a result, the furnace hot zone may need to be slightly larger than first planned. Since it is impractical (and often impossible) to change the internal volume of a vacuum furnace, careful consideration of future needs must be factored into the sizing process. The furnace manufacturer can review the information provided and recommend the most appropriate size and style.

Basic Furnace Features

A critical step in the process after choosing the basic furnace configuration and features necessary to perform the intended application and then select options to support the choices. Four features common to any vacuum furnace are:

• Hot zone (effective work area, hearth design & capacity, maximum & normal operating temperature, temperature uniformity, insulation, heating element insulators, cooling nozzles, cooling system (internal or external heat exchanger) etc.)
• Heating elements (material, type, design, support, current & voltage meters, ground fault indicators)
• Pumps (ultimate & operating vacuum level, wet or dry pumps, blowers, diffusion umps, pump-down time to high vacuum crossover, furnace leak rate)
• Controls (power supply type, control, over temperature and recording instrumentation, furnace & workload thermocouple types).

In addition, ancillary items (vacuum vessel design & materials of construction, partial pressure circuitry, water cooling system, loaders, surge tanks, convective heating, service, spare parts and technical support) are additional considerations that must be evaluated.

Hot Zone Construction.

Choosing the proper hot zone construction (including load support, cooling gas flow orientation and the like) for the materials and processes to be run is one of the most important initial decisions. This choice, in addition to impacting product quality, will influence ease of maintenance, the frequency of repairs, energy usage, and overall furnace life. The most common insulation packages are classified as:

• All graphite (board, fiber, carbon-carbon composite)
• All metallic (radiation shields or shield pack)
• Combination (inner metallic shield(s) separated or backed by ceramic or graphite insulation)
• All ceramic fiber

It is important that the hot zone support structure is designed to prevent distortion of the insulation that would cause warpage, cracking or gaps through which radiant energy can leak. The structure must be simple and allow a fastening system that avoids conductive heat losses while holding the assembly rigid. Hot zone superstructures can be as simple as steel expanded metal mesh or as complex as solid stainless steel enclosures, the latter having the advantage of no rusting and no subsequent outgassing. The critical factor is to help ensure proper temperature uniformity in the workload area and minimize heat loss to the shell.

Another important factor in hot zone design is thermal expansion and contraction, especially important in today’s high-pressure gas quench designs. The expansion rates and temperatures must be taken into careful consideration in the design stage to allow for proper clearances around element supports, nozzles, or restraint systems so that the insulation remains flat with minimal buckling or cracking.

Graphite Hot Zones (Fig. 3)

Today, most vacuum furnaces utilize graphite felt or graphite board hot zones with graphoil or carbon/carbon composite hot faces and solid graphite (circular bands, rods or bars/slats) heating elements. A common option is to utilize metallic heating elements in combination with graphite insulation. For brazing applications, a sacrificial bottom layer of graphite material is common.

Graphite felt (fiber) insulation, especially in the form of fiberboard, has very low adsorption rates ensuring fast pump-down speeds and reduced outgassing compared, for example, to ceramic fiber. The speed at which graphite lined hot zones reach their ultimate vacuum and their life depends strongly on the purity of the graphite. Advantages include ease of installation and extended life.

All-Metal Designs (Fig. 4)

Most all-metallic designs consist of a combination of materials, for example, three molybdenum shields backed by two stainless steel shields would be typical for °C (°F) operation. Radiation shields are made with (relatively) expensive materials and are labor intensive to construct when compared to the purchase of other types of insulation. Their heat losses are high and become higher with loss of emissivity (reflectivity) due to the gradual oxidation and contamination of the shields. Radiation shields can be manufactured from:

• Stainless steel or nickel alloys having a maximum operating temperature of °C (°F) with 980°C (°F) being a common limit.
• Molybdenum having a maximum operating temperature of °C (°F)
• Tungsten or tantalum having a maximum operating temperature of °C (°F).

Metallic shielding is known to have specific properties that coexist well in a vacuum environment including:

• Cleanliness
 – Refractory metals will not flake off particles that could contaminate the work or pumping system.
• Heat Absorption -
 Refractory metals are reflective to the radiant energy assisting in heat transfer.
• Outgassing 
- Refractory metals do not absorb gases as do the other materials and thus avoid prolonged pump down times.
• Low Heat Storage 
- Refractory metals do not hold temperatures as long as other materials and, therefore, allow for faster cooling.

Properly designed, all metallic hot zones have several distinct advantages: (a) surface area is small (relative to fiber insulation) so absorbed and desorbed gases are reduced, facilitating pump-down and (b) heat storage is low, promoting faster cooling.

Combination Hot Zones

Combination (or so-called sandwich insulation packs) designs are composed of one or more radiation shields typically with ceramic wool insulation between or behind them. Combinations of graphite fiber sheets and ceramic insulation wool are also used. These versions are cheaper to buy and maintain but adsorb higher levels of water vapor and gases (due to the very large surface area of the insulation wool). Their heat losses are considerably lower than those of radiation shields. Advantages of this style include low cost, good maintainability, and good insulation value. Disadvantages include a tendency for the blanket to shrink, leaving voids, which allow heat loss; the dusting of the material, particularly after devitrification; and a strong tendency toward absorption of water vapor. The systems must be supported by hangers, which project through the insulation adding to potential heat loss problems. The maximum operating temperature of these designs is typically °C (°F).

Heating Elements

The choice of a heating-element material depends largely on operating temperature. For low-temperature operations such as aluminum brazing or vacuum tempering, inexpensive stainless steel or nickel-chromium alloys can be used for the heating elements. For higher-temperature general heat-treating applications such as hardening, or brazing, molybdenum or graphite are popular choices for element materials. For specialized heat-treating applications above °C (°F), refractory metals such as tantalum or tungsten are popular choices, though graphite is also used. Other processes such as low-pressure vacuum carburizing use graphite or silicon carbide elements.

Almost but not all high-temperature vacuum furnaces are electrically heated. Resistance heating elements are constructed from metal or graphite in a variety of styles. In general, one of the following materials is used:

• Stainless steel alloys – 300 series alloys (e.g. 304L, 316L) can be used for heating elements up to (approximately) 760°C (°F).
• Nickel/Chromium and iron-aluminum based alloys – these typically operate up to temperatures of 900°C (°F) and exhibit good-to-excellent oxidation resistance, making them useful for a number of applications including hot wall type furnaces.
• Inconel® and other nickel alloys – depending on the material and vacuum level can be used up to °C (°F). Above 800°C (°F) there is a risk of evaporation of chromium from these materials.
• Silicon carbide (SiC) – these elements have a maximum operating temperature of °C (°F). There is a risk of evaporation of silicon at high temperatures and low vacuum levels of less than 0.133 mbar (100 microns). Silicon carbide is a glass-like brittle material even at ambient temperature.
• Molybdenum –with a maximum operating temperature of °C (°F). Molybdenum becomes brittle at high temperature and is sensitive to changes in emissivity brought about by exposure to oxygen or water vapor.
• Lanthanated Molybdenum (aka lanthanum doped or MoLa) –has a higher recrystallization temperature than either pure molybdenum or HCT (potassium-silicon doped) molybdenum alloys and typically exhibits improved ductility after recrystallization. Maximum operating temperature remains at °C (°F).
• Graphite – these elements can be used up to °C (°F). Graphite is sensitive to exposure to oxygen or water vapor resulting in the reduction in material thickness. The strength of graphite increases with temperature, however, graphite has limited flexibility and can break if moved.
• Tantalum – Elements made of tantalum have a high duty temperature, typically °C (°F). Tantalum is a strong getter material, becomes brittle brought about by exposure to oxygen or water vapor and is sensitive to changes in emissivity.
• Tungsten – Elements made of tungsten have the highest duty temperature, typically °C (°F). Tungsten becomes brittle brought about by exposure to oxygen or water vapor and is sensitive to changes in emissivity.

Note: The above element ratings are downgraded from their upper operating limits.

Graphite is an excellent choice for heating elements being lightweight, strong at temperature and stronger at higher temperature, has a high melting point and a low vapor pressure. In addition graphite exhibits low contact resistance at internal connections and power feed-throughs has excellent thermal shock properties, is not degraded by constant heating and cooling and has a low heat expansion coefficient. Graphite has the ability to take a very high current density, and therefore very fast ramp-up times can be achieved.

Graphite elements can operate in very corrosive or aggressive atmospheres without significant degradation. The low resistivity of graphite means it requires high current power supplies and correspondingly large feedthrus and cables. Graphite also acts as a getter to oxygen although it is attacked and consumed in the process (forming CO and CO2 gas). Graphite heating elements can be supplied in rod, tube, bar, plate, circular shapes or cloth form.

Lightweight, curved graphite elements are becoming increasingly popular for vacuum furnaces. These elements have lower thermal mass than older-style graphite rod or bar elements. Compared to molybdenum strip elements, the curved graphite element has been reported to be more durable and has better resistance to operating hazards like accidental breakage or braze alloy flashing.

Carbon-carbon-composite (CFC) materials can also be used as heating elements and can be made into very thin sections, typically as thin as 1mm (0.04”) thick, due to their fibrous grain structure. CFC elements have a higher resistance than graphite elements, allowing lower current, higher voltage power supplies to be used. They also have extremely low thermal conductivity, reducing heat loss.

Molybdenum and other refractory materials (W, Ta) are popular choices for heating elements. In sheet form, the watt density of the radiating surface is low compared with cylindrical rod allowing lower operating temperatures. The tradeoff is in mechanical strength and as such good supports and restraining systems are necessary. All refractory metal heating elements undergo changes in electrical resistance so the design of the power system must control current during the onset of heating to avoid damage to the elements. In addition, the heating rate must be limited and controlled. Molybdenum, in particular, goes through a phase change and becomes brittle after initial heating. Metallic elements are typically available in strip, wideband, coil (wire), ribbon or rod form.

Silicon carbide is used in special applications and typically supplied in bar form (for electrically heated units) or tubes (for gas-fired units).

In the following section, we continue our discussion on the many factors that must go into the decision-making process during the acquisition phase of a vacuum furnace.

Earlier in this article, we focused on how one goes about choosing the right furnace for the job and talked about the various choices for hot zones (e.g. insulation, heating elements, etc.). We now continue this discussion by looking at other common vacuum furnace features and options. Recall that the four common elements of any vacuum furnace are:

• Hot zone
• Heating elements
• Pumps
• Controls

Once decisions have been made in these areas, other ancillary items (e.g. partial pressure control, loaders, etc.) must also be considered and will be talked about here as well.

Pumping Systems

Vacuum pumps are the heart of a vacuum system. In order to create a vacuum within a closed container, or vessel, we need to remove the molecules of air and other gases that reside inside by means of a pump. The vacuum vessel and pumps (mechanical, booster, diffusion, holding) together with the associated piping manifolds, valves (mechanical pump, high vacuum isolation, vacuum (break) release, backing), vacuum measurement equipment (molecule counters) and traps comprise a typical vacuum system.

While mechanical pumps have the ability to work against atmospheric backpressure and booster pumps improve the speed and level to which we pump down, these pumps lose efficiency as the system pressure drops. In order to reach extremely low-pressure levels (e.g. 10-5 to 10-6 Torr or lower) the use of diffusion pumps is required. For furnaces using mechanical pumps (wet or dry) an ultimate vacuum in the range of 5 x 10-2 Torr to 1 x 10-1 Torr is typical. When a blower (booster) pump is added, the ultimate pressure will drop into the range of 2 x 10-2 Torr and the speed of pump-down will be significantly increased.

To reach these various vacuum levels, different vacuum pumping systems are required. The foundation of any of these systems is the positive displacement mechanical or roughing pump (Fig 7). The roughing pump, so-called because it is used to produce a “rough” vacuum, is used for the initial pump-down from atmospheric pressure. Mechanical pumps operate under the principle that they take in a large volume of air at the beginning of the cycle, compress it to a small volume and then exhaust it to the outside atmosphere. In a “wet” pump, a thin layer or film of oil creates the actual seal between the moving parts. Gas is exhausted under pressure against a valve disc at the outlet.

The booster pump (Fig. 8), or blower, is a different type of mechanical pump that is placed in series with the roughing pump and designed to “cut in”, or start at anywhere from 20 to 50 Torr. It provides higher speeds from this pressure range to 1 x10-3 Torr. A booster pump is necessary for this intermediate pressure range because the roughing pump is losing efficiency while the diffusion (vapor) pump has yet to start to reach full efficiency. For systems equipped with diffusion pumps, the pump-down time to high vacuum crossover is often important, a target of 10 – 15 minutes being typical.

The diffusion pump (Fig. 9) consists of a boilerplate system in which a high-grade silicon fluid is heated and then subsequently vaporized during boiling typically by means of an external heating element and a stack assembly, or chimney (commonly referred to in the industry as a “Jet Stack” assembly) up through which the vapors pass, exiting the chimney through one or more levels of annular converging/diverging nozzles directed radially outward and downward at an angle of approximately 45 degrees and at speeds well in excess of 400 km/hr (250 miles/hr). The hot vapors are accelerated by the action of the compression stacks within the diffusion pump that serves as a venturi creating supersonic velocities. As they travel outward and downward they collide with molecules of the gases being drawn into the pump inlet by the pressure differential created during the boiling of the oils thus giving them an effective downward velocity toward the exit (foreline) from which they are removed efficiently by the mechanical pumping system.

Diffusion pumps are, therefore, a type of vapor pump (without moving parts) and are used to help achieve lower system pressures that can be achieved by a mechanical pump/blower combination alone. The diffusion pump is capable of pumping gas loads with full efficiency with/at inlet pressures not exceeding 8 x 10-2 Torr and discharge (or foreline) pressures not exceeding 3 x 10-1 Torr. The diffusion pump cannot operate independently, it requires a separate pump to reduce the chamber pressure to or below the maximum intake pressure of the pump before it will operate. Also, while operating, a separate or holding pump is required to maintain the discharge pressure below the maximum tolerable pressure.

Controls technology

Instrumentation and process controls (Fig. 10) used on vacuum furnaces in the heat treatment industry are extremely diverse due in large part to the fact that the life of a vacuum furnace can range from 20 to 50 years! These packages range from simple controllers to sophisticated HMI (Human Machine Interface) systems.

It is important to recognize that furnace instrumentation and control packages are constantly evolving with constant advances in hardware and software. Even basic control packages now often include easy-to-understand touchscreens with graphics displaying operating indicators and alarms. Where more advanced data management applications justify the higher cost, the PC-based control system is a user-friendly and versatile tool. Perhaps its biggest advantage is an access capability that allows the user to analyze, adjust and download operating parameters from remote locations. These systems can be connected to local networks for multiple user access and even to the Internet via secure connections.

Digital data recorders are an advanced data collection system allowing storage and report of process and equipment variables that allows one to monitor conditions over time and observe trends, as well as create a historical record and backup of the data. These types of records help predict when maintenance will be required and avoid unexpected downtime.

Supervisory Control and Data Acquisition (SCADA) systems (Fig. 11) allow furnace users to view data and operational settings for multiple furnaces at the same time on a single display, thus providing more flexibility for collecting and reporting data in one central location.

Network connectivity allows one to review, in real time, critical data about the heat treatment process being run. Features such as notifications of alarms, monitoring of the Programmable Logic Controller (PLC) inputs and outputs and advanced firewall capabilities make this an attractive option. These types of systems allow engineers and field service technicians to remotely view the equipment and recommend adjustments helping to avoid prolonged downtime and maintenance expense.

Finally, with all the available controls technology on the market, it is critical to choose the package that is right for the type of furnace and application being run.

Other Considerations

Grids, Baskets & Fixtures

Component parts that are to be heat-treated must be properly loaded and are usually placed into baskets if relatively small or directly onto grids if larger (Fig. 12). The baskets can either be placed on a grid or in some cases directly onto the hearth rails of the furnace. Today, fixtures are made from either stainless steel alloys (e.g. RA330® or Inconel 600®), carbon/carbon composite materials or graphite.

Work grids can be cast or fabricated, being manufactured from stainless steels or high-temperature alloys such as molybdenum, which is particularly well suited for high-temperature service. Graphite plates are common, but care must be taken to insulate the plate from the metallic workpiece if there is a concern over eutectic melting.

Typically, several sets of baskets or fixtures are required for each vacuum furnace in operation (while one set is in the furnace, the second set is being prepared for the next cycle). Often alloy baskets and fixtures require periodic maintenance such as straightening or weld repair.

Water Recirculation Systems

Water systems (Fig. 13) are an excellent short and long-term way of protecting your vacuum furnace. Most plant systems fall into one of three general categories (or combinations thereof), namely single or multiple pass or recirculation type.

The water source can be city water or well/river water, which can vary in hardness and mineral content, all of which leads to premature issues with hot spots on the vacuum vessel, loss of shell integrity (thickness) due to rusting and/or shorter component life (e.g. power feedthrus). All of which can reduce vacuum chamber life dramatically, in some instances from a normal 30 – 40 year life to as little as 10 – 15 years.

In a single pass system, the cooling/process water is used only once and then discharged from the system. Today, it must often be treated before being discharged into the sewer system and is subject to a number of EPA, federal, state and local codes. Many consider this the least expensive type of system and it is perhaps the most common (although, in this writer’s opinion, not the best). In areas where the water has high hardness levels or large amounts of total dissolved solids, the rate at which rust or scale will form and other problems occur increases exponentially thus necessitating more frequent cleaning or flushing of the cooling system and associated piping.

Recirculation systems are those in which the cooling medium is reused and recycled. Three basic types are: (1) additive systems, (2) open systems and (3) closed systems. An additive system uses make-up water to maintain a specific temperature or temperature range for the cooling system. Higher temperature water is discharged from the system to help maintain temperature control. The advantage over a single pass system is that the amount of water used is minimized. A typical system requires a recirculation pump, sump tank, and automatic water temperature control system.

Open recirculation systems utilize evaporative cooling in a cooling tower, evaporative condenser, sump tank or spray pond to remove large amounts of heat with small amounts of water loss. One concern with this type of system is the tendency to concentrate contaminants and must be dealt with by regulating the amount of water that is bled off the system and by monitoring the condition of the water present in the system. The issue of dissolved oxygen in the water is also a great concern.

Closed recirculation systems with water/water, water/liquid or water/air heat exchangers have become popular methods of controlled cooling especially when water treatment costs are to be minimized or “zero” discharge situations are required.

Ancillary Items

There are a variety of options available on most vacuum furnaces. The list varies by manufacturer and personal preference but some of the more common include:

• Furnace loaders can be manual, electric motor, pneumatic or hydraulically operated and usually include a feature which allows someone to load parts with precision and avoid breaking or damaging the furnace hot zone.
• Accumulator (surge) tanks allow an adequate gas supply, at the right pressure and volume to be delivered to the furnace on-demand.
• Convection heating is a convenient option for those who process dense loads and depending on the part geometry and load configuration, this feature may help achieve better uniformity in heating parts and help to decrease cycle times.
• Spare parts kits can help minimize downtime by keeping critical or long-lead spare parts and maintenance items in inventory.

The first part of this article focused on how one goes about choosing the right furnace for the job and talked about the various choices for hot zones (e.g. insulation, heating elements, etc.). In the second part of this article, we discussed pumping systems, controls and ancillary support items (e.g., grids/baskets/fixtures, water systems, features & options).

In the third part of this article, we continue our discussion on the factors that must go into the decision-making process during the acquisition phase of a vacuum furnace. It is now time to understand how your supplier partner will handle the project once an order is received, including project management, codes & standards, approvals, installation, commissioning and long-term support.

Order Processing

Okay, you have selected a vendor partner to supply your vacuum furnace, so what type of support should you now expect? To begin with, it is important to understand how your order will be handled internally by the company you have selected. The first task they face is to transfer the order from their sales team to their engineering team and, ultimately, to the manufacturing and service/support teams.

After receipt of an order, your supplier partner will typically schedule an engineering “kickoff” meeting where the project is given to engineering and a project manager (or project engineer) is assigned. Out of that meeting will be generated the final equipment specification and this should be provided to the purchaser shortly (e.g., 1 – 2 weeks) after receipt of the purchase order for approval.

Engineering Phase

Whether the unit is a standard model or a completely custom design, the role of engineering is to ensure that the unit purchased is fit for its intended purpose. Since you as a purchaser may have never met or interfaced with the individual(s) assigned to your project, it is critically important that you make sure they are aware of your specific application needs and production requirements. This can be done through your sales contact or by directly interfacing with the engineering department. One would be amazed by how many engineers work on projects without ever having a complete understanding of what the end user intends to use the equipment for! Do not let this happen. This same engineer may or may not be the project manager who will be assigned to oversee all aspects of the internal build as well as interfacing with the purchaser for problem-solving after the unit has shipped. Understand how your supplier partner’s internal and external project management works.

The engineering department will then decide on and finalize the necessary design features and issue drawings, bills of material and other documentation or instructions that will allow manufacturing to fabricate the unit.

Codes & Standards

Today, all vacuum furnaces sold in North America should, at a minimum, meet the most recent revisions of the following standards. If you have other requirements, be sure to specify them.

• OSHA (Occupational Safety & Health standards, www.osha.gov)
• NFPA (National Fire Protection Agency, www.nfpa.org).
  • Standards 86 (Standard for Ovens and Furnaces) – Note: Class D applies to vacuum furnaces;
  • Standard 70 or 79 (National Electric Code)
• ASME International (www.asme.org). (If applicable) Boiler & Pressure Vessel Code Section VII, Division 1.
• UL (Underwriters Laboratory, www.ul.org)
  • Individual components (as opposed to the entire system)

In addition, many vacuum furnaces must meet the requirements of the following specifications or organizational compliances for accreditation:

• AMS International (www.ams.org)
  • AMS (Pyrometry)
  • AMS (Heat Treatment of Parts in a Vacuum)
  • AMS (Heat Treatment of Steel Parts, General Requirements) and related slash numbers
• Nadcap (www.pri.org)
• CQI-9 (www.aiag.org)

Finally, electrical components used in the fabrication of the furnace system for international service must be CE (Conformité Européene or European Conformity) approved and/or in some cases CSA International (Canadian Standards Association) approved.

Manufacturing Phase

Once materials have been ordered and drawings issued by engineering, the manufacturing phase of the process begins. Once again, it is important to be sure that as the purchaser you have reviewed the manufacturing capabilities of your supplier partner. Knowing what components are fabricated in-house, which are purchased, and which are fabricated or assembled by third parties is important should problems arise once the equipment is in the field. For example, is your supplier partner an ASME coded shop or will the pressure vessel (if required) be fabricated by an outside company – and if so, what are their capabilities and competency?

Many purchased components have limited warranties, which may have to be extended by agreement with these suppliers. Other components, such as instrumentation or programmable logic controllers may have been chosen from numerous model numbers – check to make sure that they have the right features for your needs (e.g. communications capability, etc.).

Installation Phase

Preparation is the key to a successful installation. Some of the factors to consider include:

Want more information on vacuum heat treatment furnace price? Feel free to contact us.

Instruction Manuals / Documentation & Drawings

An equipment operation manual (either in hardcopy or electronic format) and an overall layout drawing along with piping, wiring and electrical drawings are typically provided with a vacuum furnace purchase. You want copies of these drawings as early in the process as possible to aid in installation planning. For example, the equipment layout drawing normally contains the required utility locations and supply requirements. It is normally provided at the completion of engineering and must be available in time to prepare the site. Finally, if detailed assembly drawings of any or all systems are desired, it is often necessary to negotiate this up front with your supplier partner.

Utilities

The most common utility requirements involve the following:

• Electrical Supply. The vacuum furnace electrical requirements are based on the customer’s plant voltage. Incoming power fluctuations of ±10% are typically anticipated with the total power required based on the furnace size, type, and features provided.
  • Power Supply. In many cases, the heating element power is supplied by (single phase) variable reactance transformer (VRT) assemblies. Most VRT’s have a 0 – 100% trim control for optimum temperature uniformity in the hot zone.
• Water Supply. The end-user is responsible for ensuring that the water supply is suitably treated to prevent corrosion, mineral build-up and cooling system blockage due to particulate matter. Furnace systems typically require the water to be at a given temperature, pressure and flow rate depending on the specifications of the specific furnace involved.
  • Water systems are often separated into zones (e.g. front door, shell, heat exchanger, etc.) with individual inlets, outlets, and drains.
  • Both open and closed water systems are common, depending on the type and design of the vacuum system.
• Air (or nitrogen) Supply. Clean, dry, filtered air is required at a specified (minimum) pressure. Typical values would be 80 – 100 psig.
  • The supply pressure must be constant and often available even when the furnace is shut down.
• Gas Supply (e.g. nitrogen, argon, helium, hydrogen). An inert gas species is normally supplied for end of cycle backfill to atmospheric pressure.
  • If partial pressure or (gas) quenching is required, the gas species is often dictated by application requirement. Pressure, flow and volume depend on the specifications of each individual furnace (e.g. backfill cycle).
  • The use of an accumulator (surge) tank is often necessary, especially when performing a high-pressure gas quenching operation, to provide adequate capacity to the system to meet the demand.

Installation & Startup Assistance

Many vacuum furnace systems are shipped as complete units, factory tested and ready for installation. Various suppliers, however, have different interpretations of factory testing so this should be clearly understood by all parties prior to equipment purchase.

Testing may involve functionality testing of components, cold or hot cycling, conducting temperature uniformity surveys on the shop floor or even (in rare instances) running sample loads. All vacuum furnaces should be leak tested prior to leaving the factory and most vacuum furnaces will be cold cycled to check mechanical functionality of the entire system. Computer controls, software, instrumentation (temperature & vacuum) and electrical systems are normally fully debugged prior to shipment.

It is also important for the purchaser to understand how a particular vacuum furnace needs to be disassembled (and, of course, reassembled) for shipment. In particular, how the component parts are to be dismantled and protected from damage. The choice of shipping method (e.g., dedicated trucks, lowboys, special permitting, etc.) must be taken into consideration as well as necessary insurance coverage.

To facilitate and coordinate these operations, the purchaser is often invited to send personnel to witness these tests and/or for operational and maintenance training and pre-installation instruction. This type of training is commonly provided at no charge or for a small fee (the purchaser being responsible for travel and living costs). Some supplier’s offer training classes as well.

Since the purchaser is normally responsible for making all necessary site preparations and connecting power, water, inert gas and compressed air supplies, a great deal of pre-shipment coordination and planning is required. This is often started during the engineering phase of the project and continues literally up until the arrival of the equipment at its final destination. Movement of the vacuum furnace components from the trucks to the final site is an important consideration as well.

Most suppliers offer installation supervision and/or complete installation services. After installation, a qualified technician travels to the site to commission the equipment and provide instruction in furnace operation.

After the equipment has been installed and connected to its service utilities either by the purchaser or the supplier partner, a qualified field service engineer is sent out for startup (commissioning) and final onsite training. These activities normally include instrument configuration, verification of electrical, mechanical and thermal functions and performance testing consisting of a temperature uniformity survey and measuring of ultimate vacuum, leak up rate and pumping speed. The cost for these services is either established during the equipment acquisition phase or when needed.

One of the most frustrating aspects of any furnace installation is understanding how long startup will take (delays may be induced by either party) and the time/cost impact of warranty work. Warranty work is often poorly documented and misunderstood (by both parties) and the procedures to handle this circumstance if/when it arises are almost never negotiated by the purchaser up front. It is also compounded by trying to differentiate between the hours spent in getting a piece of equipment operational and in troubleshooting problems. A clear understanding of the issues and good documentation are key.

Warranty

Today, most vacuum furnace suppliers warrant their equipment for either one or two years, either from date of shipment, date of acceptance, or a specified number of days after shipment. The supplier has general terms and conditions. It is as important for the purchaser to review and negotiate warranty (and other terms) as it is to negotiate the final package price for the system.

Maintenance & Ongoing Support

What is important for a potential vacuum furnace purchaser to understand is that routine maintenance of a vacuum furnace is usually his/her responsibility and that the supplier is ready, willing and able to provide ongoing engineering, technical (and in some cases application), service and spare parts support. For their part, the purchaser has to understand the type of maintenance required and have the maintenance personnel with the necessary skill sets and tools to do the job per the manufacturer’s instructions.

Sample Preventative Maintenance Schedule (Single Chamber Vacuum Furnace)

Weekly (or more frequently)

• Check lubrication on roughing, booster and holding pumps.
• Drain roughing pump drip leg and air line filter.
• Check airline lubrication.
• Inspect furnace hot zone for defects and clean furnace doors and walls (where accessible).
• Inspect flow through cooling water return lines at drain manifold in water reservoir.
• Bake-out furnace and measure leak up rate. Note: Leak up rate should be 10 – 20 microns/hr. or less.

Monthly

• Check grease caps on valves and lubricate valve stems.
• Check diffusion pump oil level.
• Inspect drive belts on all pumps and water system roof evaporator unit.
• Check water and air pressure alarm systems.
• Check water treatment system (if applicable).
• Inspect roof evaporator unit for sediment build-up in water sumps and for plugged or incorrectly adjusted nozzles.
• Check make-up water float switches and solenoid.
• Inspect all hoses and connections.
• Check pick-up placement, alignment and travel limits of load dolly (if dolly is present).
• Perform general visual inspection of the entire furnace system.

Quarterly

• Check over temperature safety alarm.
• Check Y-strainer on water system (if fitted).

Semi-Annually

• Check all electrical connections, including element power terminals.
• Check all safety interlocks for correct operation.
• Drain and replace oil in roughing booster and holding pumps (customer supplies oil).
• Change control and over temperature thermocouples (customer supplies thermocouples).
• Check conductivity on ceramics insulators.
• Change holding pump oil (customer supplies oil).
• Check the operation of water and air pressure alarms.
• Check the operation of inert gas supply pressure alarm.

Annually

• Inspect and clean all vacuum valves and replace O-rings (if required).
• Inspect and clean roughing pump and replace clappers and springs (if required).
• Inspect and clean diffusion pump (if applicable).
• Inspect and clean quench motor housing and quench pipework.

Summing Up

The purchase of a vacuum furnace represents both a long-term investment and establishes a long-term partnership between the purchaser and the supplier of that equipment. For this if no other reason, taking the time to differentiate between needs and wants and carefully understanding both the production requirements and the capabilities and limitations of the potential supplier partners is of critical importance. Be sure to evaluate all aspects of your process needs, both now and into the foreseeable future and consider all relevant factors – from material type to the geometry and mass of the component parts that must be processed; from the production throughput to the property (mechanical, physical, metallurgical) requirements that must be met. Finally, understand the equipment choices and options available to you to determine which supplier partner is the best choice for you.

References

Can An Ordinary Open Melting Furnace Really Satisfy all Metal Smelting?

Smelting involves obtaining metal from a mineral-bearing ore. This process involves the reduction of metal oxides or ore into metal and the formation of non-metal oxide waste known as slag. Metals were smelted traditionally using a bonfire long before now.

A furnace (open or closed) is required to smelt metal ores because these ores have to be heated at a very high temperature to produce pure metals, much higher than what an open fire can produce. Using a furnace helps to maintain certain chemical conditions during smelting.

In an open melting furnace, there is a very high possibility of air getting into the molten metal because of the absence of vacuum that seals off air when melting is ongoing. This furnace is not advisable for smelting metals because the quality of metals are compromised when air gets into molten metal before casting, leading to oxidation and brittleness of such metals.

A closed furnace which is also a vacuum melting smelts and melts various types of metals and super alloys under a vacuum condition. This simply means that no gas or air gets into the melting chamber during the melting process. Air is known to cause fast oxidation within metal, therefore making the metal deteriorate very fast. The closed furnace is highly recommended for efficient and high quality smelting and melting.

1.1, Easily Oxidized Metals are Easily Contaminated

Oxidation in metals usually occurs when the iron molecules in a metal react with oxygen in the presence of water to produce iron oxide. Oxidation usually appears on metals as red formed, scaly, loose, and easily falls away exposing more basis material to the environment. Oxidation on certain types of metals serves to protect them. Aluminium oxide, copper carbonate, and chromium oxide acts as protective coatings for the underlying metals.

Not all metals contain iron but they can corrode or tarnish in other oxidizing reactions.
Noble metals such as platinum, palladium, silver, gold, etc resists oxidation in their natural state. Many corrosion resistant metals have been invented by humans such as stainless steel and brass.

One would however think that all oxidation-resistant metals are classified as noble metals, but that is not the case. Titanium, niobium and tantalum all resist oxidation, but they are not classified as noble metals. Metals that resist oxidation are opposite of metals that are prone to it, they are called base metals. Base metals are aluminium, nickel, zinc, iron, steel, tin, lead, copper, brass, bronze and the alloys of these metals. All these metals can oxidize easily.

Below is a series of oxidation activities for some common metals. The table below shows the ability of metal that oxidizes very fast to the metals that least oxidizes.

1.2, The Casting Process after Melting Is Exposed to Air

It is generally recommended that metal should be melted above the melting point of the metal. It is important that the metal is protected from oxidizing and absorbing unwanted gases. Protecting the metal from coming in contact with air can be achieved in the following ways: using a gas flame, a protective inert gas such as Argon, Nitrogen, or by using flux.

Air can get entrapped in molten metal during melting and during casting, thereby causing porosity, oxidation and incomplete casting. This is why an induction furnace is often used for casting in order to prevent air from getting into molten metal.

If air then gets into metal (this cannot be avoided especially with the traditional methods of casting, the solution is to melt the metal all over again in an induction melting furnace or a vacuum induction melting furnace. An induction furnace melts the metal and no air gets into the melting chamber. The molten metal is then removed and poured in a mold.

Types and Comparison of Vacuum Melting Furnaces

2.1, SuperbMelt Vacuum Induction Melting Furnace

The vacuum induction melting furnace is used to melt metals via electromagnetic induction under vacuum. An induction furnace contains a refractory lined crucible surrounded by an induction coil located within a vacuum chamber. Metals and alloys that have a high affinity for oxygen and nitrogen are usually melted in a vacuum induction furnace to prevent contamination with these gases.

The breakthrough for the vacuum induction casting machine was in the early 20th century and it has continued to advance since then. Vacuum induction is indispensable in the manufacture of metals and alloys because the vacuum induction melting furnace has the following features:

• Flexible for melting due to small batch sizes
• Has a low level of environmental pollution
• Temperature of melting can be easily controlled
• Easy to Operate
• Able to melt high temperature metals
• Able to reduce the loss of metals and alloys
• Capable of casting high quality metals
• Able to make use of all the heat to melt within the vacuum chamber
• Able to eliminate gas in metals
• Has a chemical composition control and process control

We can describe the vacuum induction furnace as a melting crucible inside a steel shell that is connected to a high speed vacuum system system. The heart of the furnace is the crucible, with heating and cooling coils and refractory lining. Heating of the furnace is done by electric current that passes through a set of induction coils. The coils are made of copper tubing that is cooled by water flowing through the tubing.

The passage of current through the coils creates a magnetic field that induces a current in the charge inside the refectory. When the heating of the charge material is sufficient that the charge has become all molten, these magnetic fields cause the stirring of the liquid charge.

Features found in most vacuum induction melting furnaces are: casting chambers, control panels, tilt and pour mechanisms, mold holding facilities for automated and semi-automated processing, crucible, etc.

Apart from melting a wide range of metals, a vacuum induction melting furnace can also be used for:

• Refining of high purity metals and alloys
• Electrodes for remelting
• Master alloy stick for processes such as investment casting
• Casting of automotive, construction, military, aerospace components.

Vacuum induction melting was initially developed as a method to refine alloys like nickel and cobalt. Right now, there the furnace is more widely used for other metals. Many of these metals offer a high level of cleanliness and a variety of properties that allow them to be used in numerous manufacturing processes, such as melting metals for aerospace and nuclear industries. Vacuum induction melting furnaces might have been developed to create superalloys, but it can also be used for stainless steel and a host of other metals.

The process of melting with a vacuum casting machine: It is important to note that a vacuum induction melting furnace provides a non-contact melting process, i.e, the molten metal does not have a direct contact with the heating coil, therefore, making the molten metal to have no contamination. Whether you need to melt a few grams of metal in small crucibles or several kilograms of metals in large furnaces, the process of melting is the same.

The melting process is carried out using a vacuum induction furnace.

In this metallurgical process, metal is melted via electromagnetic induction under vacuum. Electrical eddy currents are used to make the melting process possible, which is not possible through other melting processes. This is because certain metals and alloys are highly combined with hydrogen and nitrogen, therefore they cannot be melted in air.

Inside the vacuum chamber, there is an induction furnace that contains a refractory lined crucible enclosed by an induction coil. The furnace is air tight and has the ability to withstand the required vacuum for processing. The metals used in vacuum induction melting have melting points up to degree celsius.

2.2, Vacuum Arc Furnace

The vacuum arc furnace is an electric furnace that directly heats the smelting metal in a vacuum furnace. In other words, a vacuum arc furnace is a furnace that uses consumable electrodes to melt under vacuum at a carefully controlled rate using heat generated by an electric arc struck between the electrode and the ingot. Exposure of the molten metal to the low near vacuum pressure reduces the amount of dissolved gases such as oxygen, nitrogen, and hydrogen in the ingot.

Heating of metal ore or scrap metal is done through an electric arc. Metallurgical furnaces can be heated with different heat sources but the vacuum arc furnace unlike the induction steel furnace, the charged metal is directly heated by the electrical arc and with the electric current running from the furnace’s terminals through the charged material.

The gas in the furnace is thin by the molten metal vapor arc that makes the arc stable, mainly for direct current. The electric furnace arc is divided according to whether the electrode is consumed in the process of melting, it is divided into a self-consuming furnace and non-self-consuming melting furnace. Most industrial applications of the vacuum arc furnace are self-consuming furnaces. Vacuum arc furnaces are used to smelt special steel, active and refractory metals like titanium.

Arc electric heat can be considered as arc resistance. The stability of arc (arc resistance) is a necessary condition for the normal production of furnace. Arc melting is used for melting metals to form alloys.

The melting process of a vacuum arc furnace: The melting process starts at a low voltage (short arc) between the electrodes and the scrap. The scrap is loaded into baskets, the scarp basket is then taken to the melt shop. The roof of the furnace swung off the furnace and the furnace is charged with scrap from the basket. Melting begins after the roof is swung back over the furnace.

The electrodes are lowered onto the scrap, an arc is struck and the electrodes are then set to bore into the layer of the shred at the top of the furnace. Once heated, the meltdown process begins. Electrodes are lowered down into the scrap to produce the arc in a low voltage condition. Once the arc is formed, the voltage is increased to speed up the melting process.

A vacuum arc furnace is applicable in: Super alloys for aerospace, melting of reactive metals for aerospace, chemical, oil and electronic industries. Copper and copper alloys for high voltage circuit breakers, die steels, tool steel for milling cutters, drill bits, etc

2.3, Electron Beam Melting Furnace

An electron beam melting furnace is often distinguished by its superior refining capacity and offers a high degree of flexibility of the heat source and the distribution of power. That is, the electron beam melting furnace uses a high-energy electron beam inside a vacuum as a means of transferring heat to the metal being melted.

Thereby making it ideal for remelting and refining of metals and alloys under high vacuum in water cooled, ceramic free copper molds. The electron beam process is employed for production of refractory and reactive metals such as tantalum, titanium, niobium, tungsten and their alloys. The electron beam furnace uses a hot cathode for the production of electrons and high voltage towards melting metals at a fast rate. The electron beam furnace performs the same function as the electric arc furnace.

The electron beam melting furnace plays an important role in the manufacturing of ultra-pure sputtering material and alloys for the electronic industry and the recycling titanium scrap.

How the electron beam melting process occurs: Electron beam guns represent high power heat sources that are able to exceed at their beam spot the melting and the evaporation temperatures of all metals at their beam spot.

By magnetic deflection and rapid scanning at high frequencies, the electron beam can be effectively directed at targets of multiple shapes. Therefore making it the most flexible heat source in remelting technology.

The electron beam strikes the target with typical power densities of 100W/cm2. Depending on the properties of the metal being melted, the power transfer efficiency ranges from approximately 50-80%.

Since electron beam melting is a surface heating method, it produces only a shallow pool at acceptable melt rates which positively affects the ingot structure as regards to porosity, segregation, etc.

The exposure of the superheated metal pool surface to the high vacuum environment at levels of 1-0.Pa results in an excellent degassing of molten metal.

Metallic and non-metallic constituents with vapor pressures higher than the base material are selectively evaporated, thus generating the desired high purity of the ingot material.

Rapid scanning of the beam spot along the melt surface avoids local overheating and allows a consistent production of alloys.

The Electron beam melting has four process variations:

• Drip melting- Classical method for processing refractory metals such as Tantalum and Niobium among others. Raw material in the form of bars is usually fed horizontally and drip-melted directly into the withdrawal mold. The liquid pool level is maintained by withdrawing the bottom of the growing ingot. Refining is based on degassing and selective evaporation. For repeated remelting, vertical feeding is applied.
• Cold hearth refining- Electron Beam Cold Heart Refining is of great importance for the processing and recycling of reactive metals. The feedstock is drip-melted in the rear part of a water-cooled copper hearth from where it overflows into the withdrawal mold. During the dwell time of the molten material in the hearth system gravity separation of high- and low-density inclusions are achieved in addition to the refining mechanisms described above. The hearth must be properly sized to provide sufficient dwell time of the molten metal in order to permit efficient gravity separation of high and low density inclusions.
• Button melting- Button Melting is used for a clean evaluation of superalloy samples regarding type and quantity of low-density, non-metallic inclusions. The equipment features programmed automatic sample melting and controlled directional solidification. Low-density inclusions (normally oxides) float to the surface of the pool and are concentrated in the center, on top of the solidifying button.
• Floating zone melting- Floating zone melting is one of the oldest techniques for the production of metals with highest purity.

Process control

Electron beam furnaces operate in a semi-automatic control mode. Process automation includes:

• Vacuum pump system
• Vacuum pressure control
• Cooling water system
• Material feed rate and ingot withdrawal rate
• Processor-based high voltage and emission current control
• PC-based automatic beam power distribution, data acquisition and archiving.

2.4, Comparison of Various Vacuum Furnaces: Why Choose Superbmelt Vacuum Induction Furnace

From the comparison of all vacuum furnaces for melting metals and alloys, the best and most convenient of all is the Superbmelt vacuum melting furnace. Here are the benefits you derive from using the vacuum melting furnace:

• Melting requires a close temperature control, the vacuum melting furnace gives you that opportunity of monitoring the melting temperature during the melting process. This therefore means that high quality melting is guaranteed.
• With our vacuum melting furnace, affordability and proper maintenance of the melting furnace is certain. Vacuum arc furnaces and electron beam furnaces have a higher maintenance cost.
• The energy required to power the vacuum melting furnace is quite low compared to the vacuum arc furnace and electron beam furnace. Therefore, power charges are not exuberant on your cost of production.
• Since all heating and melting is in a vacuum chamber of the vacuum melting furnace, there is absolutely no gas, heat or other uncomfortable elements that make melting harmful for you or your work environment.
• Metals and alloys melted with the vacuum melting furnace do not oxidize easily, therefore, metals cast lasts longer than metals cast with other furnaces.

Conclusion: Superbmelt Vacuum Induction Furnace Is Your Best Choice

Superbmelt vacuum induction furnace is your best choice considering the fact that there are a number of features and benefits you enjoy when you use our vacuum induction furnace.

Safety: Our vacuum induction melting furnaces are totally safe for all your melting operations. The vacuum induction melting furnace is equipped with a primary safety detector, which is designed to protect you against electrical shock and warning of metal to coil penetration, a highly dangerous condition that could lead to furnace explosion.

Flexibility design: Whatever the size, whether large tons or small size melting and casting operations, our vacuum induction furnaces can handle it.

Temperature control: You have total control over the melting temperature of your vacuum furnace, thereby giving you a hitch free melting and casting processes. Hence, you don’t have to worry about your metal or alloy not reaching its melting temperature or going above its melting temperature.

Improved productivity: Melting and casting with our vacuum induction melting furnace is fast, this means that you don’t have to spend a long time during production. Therefore, you are able to meet up with production demands.

Affordable vacuum induction furnace: You get the best price of a vacuum induction furnace when you buy from us. The affordability of our vacuum induction furnace is a great investment.

High quality products: Melting and casting with our vacuum induction furnace gives you a high quality on all your products. This is so because of the vacuum machine’s ability to remove dissolved gasses like hydrogen from your metal. The presence of hydrogen in your metal makes it very easy and fast for the oxidation process to begin rather quickly on your metal.

Zero pollution: Not only is your metal free from impurities brought in by raw materials that will later cause your casting to get damaged, also the melting and casting environment is not polluted with chemicals and harmful gases. Thereby making our vacuum induction furnace safe for humans and the environment at the same time giving you a quality product.

Recycling: It is totally possible to recycle your metals and alloys using our vacuum induction furnace, therefore, there is no wastage of metals.

Warranty: We have a 12 months warranty coverage over your vacuum induction furnace when you purchase from us. Through this, we are confident of giving you the best of our vacuum induction furnace.

Multiple usage: Our vacuum induction furnace can be used in all industries including industries that make use of heavy metals and alloys such as aerospace engineering.