Selecting the Right Vacuum Furnace for the Job - VAC Aero

Applications involving vacuum heat-treating are typically performed for one of the following reasons:

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All of the common (and several uncommon) heat treatment processes can be run in vacuum, from annealing and brazing to sintering and tempering. Many companies that currently outsource vacuum heat-treating ask themselves if they would be better served by setting up this capability in-house. Others who already have an in-house heat treat department wonder if switching to vacuum processing will offer them a competitive edge. This article will help address these questions.

Vacuum furnaces are typically characterized by their method of loading, horizontal or vertical, as well as if there is internal load movement, being classified as either batch or continuous (i.e. multi-chamber) types (Fig. 1 – 2). The various furnaces sizes, production capabilities, and feature configurations are almost endless and detailed extensively elsewhere [1], [2]. Since most vacuum furnaces have a life expectancy of 40 – 50 years, decisions as to what to purchase, and from whom, become very important.

Features

The features necessary for a particular vacuum furnace to run a specific heat treatment process are often determined by results one wishes to achieve, the type and geometry of the component parts to be run (Fig. 3 – 4), the productivity requirements, the physical size of the load, the pressure and temperature to be attained, and the medium (gas or oil) to be used for cooling the load. It is important to recognize that a furnace designed for one application use will in all likelihood be used for many other applications over the span of its lifetime. As such, having the greatest possible flexibility in features and options is highly desirable.

Furnace Design & Construction

The main parts of a vacuum furnace are the:

• Vacuum vessel;
• Pumping system;
• Heating chamber/Hot zone;
• Quench/Cooling system;
• Vacuum and Temperature Controls.

Vacuum furnace vessels can be grouped into so-called hot wall and cold wall designs, the latter being far and away the most common. A typical hot wall furnace has a retort that is commonly metallic or ceramic, dependent on the temperature. The heating system is usually located outside of the retort and consists of resistive heating elements or induction coils. Limitations of this retort-type furnace are the restricted dimensions of the heating zones and the restricted temperature range of the metallic retort, usually limited to °C (°F) maximum. With cold wall furnaces, the vacuum vessel is cooled with a cooling medium (usually treated water) and is kept near ambient temperature during high-temperature operations.

Pumping systems are the heart of a vacuum system. Common to most vacuum systems are mechanical pumps, which have the ability to work against atmospheric backpressure and booster pumps used to improve the speed of pump down as well as the level of vacuum that can be reached. Diffusion pumps are a popular option to help reach extremely low vacuum levels while other types of pumps (turbomolecular pumps, cryo pumps) are used to reach ultra-low vacuum ranges.

Hot zone designs vary by type of insulation and materials of construction. In general, they can be classified as:

Thousands of hours of operating service have confirmed that graphite insulation (felt or board) is suitable for almost all high vacuum applications, including brazing of advanced superalloys. When combined with a hot face of carbon-carbon composite, maximum hot zone life can be achieved, especially in high-pressure gas quenching and brazing furnaces.

It is important that the hot zone support structure be designed to prevent warpage of the internals since, for example,

insulation can crack or gaps can be created through which radiant energy can leak. The structure must be simple and allow a fastening system that avoids undo 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.

Most vacuum furnaces are electrically heated. Resistance heating elements are constructed from metal or graphite in a variety of styles. The following elements materials are commonplace: stainless steel alloys (300 series) for temperatures to 760°C (°F); nickel/chromium and iron-aluminum based alloys to temperatures of 900°C (°F); Inconel® and other nickel alloys for use to °C (°F); silicon carbide (SiC) for operating temperatures to °C (°F); molybdenum to achieve °C (°F); graphite for use up to °C (°F); tantalum, typically to °C (°F) and tungsten to reach °C (°F).

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, graphite or molybdenum are popular choices for element materials. Lightweight, curved graphite elements are becoming increasingly popular for vacuum furnaces. These elements have the advantage of lower thermal mass and have excellent structural integrity. For specialized heat-treating applications above °C (°F), graphite or refractory metals (tantalum, tungsten) are popular choices. Still other processes such as low-pressure vacuum carburizing use graphite or silicon carbide elements.

Quench (cooling) systems are typically either oil or (high pressure) gas. Many companies use oil quenching to achieve consistent and repeatable mechanical and metallurgical properties and predictable distortion patterns. The reason oil quenching is so popular is due to its excellent performance results and stability over a broad range of operating conditions. Oil quenching facilitates hardening of steel by controlling heat transfer during quenching, and it enhances wetting of steel during quenching to minimize the formation of undesirable thermal and transformational gradients which may lead to increased distortion and cracking.

Gas pressure quenching has seen a tremendous rise in popularity in recent years and its success is highly dependent on a number of factors including material, component geometry, loading, net to gross load ratio, gas parameters, and equipment design. Pressure ranges span sub-atmospheric, low, medium, high and ultra-high pressure (up to 25 bar in commercial practice) irrespective of the type of gas used.

There are two popular quench cooling loop designs available for vacuum furnaces, internal and external. While cooling performance criteria are very similar for both designs, each has some advantages and drawbacks. Furnaces with internal heat exchangers have the advantage of a compact design. The furnaces occupy slightly less floor space than those with a similarly sized external cooling loop. Its chief drawback is the close proximity of the quench blower motor (including drive shaft and bearings) to heat emanating from the furnace hot zone. If a failure of a motor component or heat exchanger does occur, it is often necessary to remove the entire hot zone to gain access for repairs. There is also the risk of damage to the furnace internals, and possibly the load if the heat exchanger should ever develop a water leak.

In the external quench loop design, the blower housing, heat exchanger housing and quench piping are located outside the vacuum chamber. The external quench loop design permits easier maintenance access to the blower and heat exchanger. The external loop design also minimizes the occurrence of quench blower failures by isolating sensitive components from exposure to heat from the vacuum chamber. Furthermore, any water leaks that might occur in the heat exchanger are confined within the separate heat exchanger housing.

Vacuum furnace process control systems (Fig. 5) are somewhat similar to atmosphere furnace control systems; however, they tend to be somewhat more sophisticated, especially from a temperature control standpoint. As they continue to evolve, even the most basic control systems often include easy-to-read digital displays and touchscreens with graphics to display operating parameters 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 their biggest advantage is the ability to access information 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.

Most of today’s temperature control systems involve “adaptive” process control. Depending on the machine or process, different variables exist that must be monitored, controlled and/or changed during the cycle. Sensors monitor a selected process or equipment parameter; send the gathered data back to a controller, which then compares it to a predetermined value or set point. Through calculations, a controller sends a signal back to the device to make the proper adjustments to obtain a controlled process. Programmable Logic Controllers (PLC’s), sensors, and computers make this all possible. In turn, data trending, real-time process monitoring, and data collection for permanent retention are commonplace. By analyzing this data, new cycles, containing modified variables, can give better results in less time.

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Vacuum controls depend to a great extent on the type of vacuum system and the required operating vacuum level. For each operating range, different vacuum gauges are required, often in combination with one another, to accurately determine and/or control the vacuum level of the chamber at any given moment in time. The criteria for selecting a vacuum gauge are dependent on various conditions such as:

Vacuum gauges are divided into three basic categories based on their working pressure. These include absolute pressure gauges; vacuum gauges useful down to around 0.001 mbar (1 micron); and vacuum gauges for use below 0.001 mbar (1 micron).

Used Equipment

The purchase of a vacuum furnace involves a considerable capital investment. As a result, the question of buying a used furnace often at a lower cost than a new furnace is a fairly common one. Most used furnaces are sold on an as-is, where-is basis with no manufacturer’s warranty. One of the most important issues to assess is the condition of the vacuum chamber water jacket. In situations where the furnace cooling system has been connected to an untreated water supply, water jacket blockages from mineral build-up are all too common, and moving the furnace triggers untold problems. Ultrasonic testing of the shell is a good first step, as is looking for discolored or blistered paint, a sure sign of overheating.

The condition of the hot zone, heating elements, pumping system, and vacuum controls must also be considered. A thorough visual inspection is usually the first step in the process as is consultation from various suppliers of the component on the system. For example, when looking at the condition of the hot zone, distorted, discolored or crumbling insulation, warped or broken elements or hearth rails, missing element insulators, and other damaged hardware are all signs that restoration work is required. All-metal hot zones are often more expensive to repair.

The control system is another important consideration when evaluating used furnaces. Having State-of-the-Art controls is highly desired and with technology obsoleting itself at an extremely rapid pace, even 10-year-old control systems can be problematic.

In Conclusion

The right vacuum furnace is the one that performs the intended application with the highest degree of uptime productivity and uncompromised quality. While an array of business decisions must be made to justify the expense, it is comforting to know that today’s vacuum furnaces offer the latest technological advances in furnace design (hot zone materials, control packages, and quenching systems) and offer many operating and performance improvements. It is often advisable to look to companies who have proven experience both as a heat treater and furnace manufacturer. This type of knowledge will shorten the learning curve.

When the time comes to purchase a new lab or industrial furnace, there are several points that need to be considered before going out for bids. One of the first considerations should be the desired operating temperature that will be used most often. “Most often” because many new lab and industrial furnace purchasers often assume that one furnace can operate through a broad temperature range.

For instance, a common inquiry would be, “I need a furnace that can be used from 1,000°C to 1,800°C.” While this request is certainly possible, the requested temperature range crosses all three temperature boundaries and price ranges that will be covered next. This is a common mistake that leads to sticker shock that can be discouraging to new furnace purchasers. Upon further discussion, users may elude to the fact that 95 percent of their processes only require a maximum temperature of 1,100°C, which would be more realistic and certainly more cost-effective at purchase time.

Temperature Ranges

Lab and industrial furnaces can be divided into three temperature ranges, based on their heater technology. The first is based on wire heating element technology, which extends to a maximum of 1,300°C, although some special-use applications claim up to 1,400°C. The second group is based on silicon carbide (SiC) heating elements and generally has a useful upper range of 1,550°C. The third group uses molybdenum disilicide (MoSi2) heating elements that can easily reach 1,750°C and with care, can be used up to 1,800°C. Of course, with increasing temperatures, the pricing also increases. A rough rule of thumb indicates that if a furnace with a maximum temperature of 1,300°C costs one unit, a furnace with a maximum temperature of 1,550°C will cost two to three units, and a furnace with a maximum temperature of 1,750-1,800°C will be three to four units. This rough order of costing assumes the same general heated chamber geometry for each unit.

As a side note, the terms “furnace” and “kiln” should be explained. Basically, the terms are industry-specific jargon. The term “kiln” is most often used in ceramics processing and the cement industry, while the term “furnace” is used most often in metallurgical processing, general heating, and material characterization applications. Depending on the industry, the same unit may either be referred to as a kiln or a furnace. Generally, the terms can be used interchangeably.

Future Considerations

Another point to consider may be future expansion. It may be required that a furnace is ordered with specific working dimensions for existing projects, but plans may indicate that future projects will require a larger unit. Depending on time frames and cost restrictions, it might be wise to seriously consider a larger unit for the initial purchase. Unfortunately, due to a wide range of variables, there is no easy way to estimate how much more the larger unit will cost without getting an actual price quote. In some instances, doubling the working volume will add less than twice the cost and delivery time. In other instances, the cost could more than double, and the delivery could be significantly extended. If future projects dictate a higher temperature unit will be required, several factors must be evaluated.

There may be a higher cost for the increasing temperature range; the external size of the unit can increase with temperature because of the need for more insulation. You must also consider the current existing normal operating temperature parameters with respect to issues of maintenance, reliability, and temperature uniformity as compared to a higher temperature unit. Furnaces are designed to operate most efficiently and give the best uniformity at their specified operating temperature. Purchasing a 1,600°C rated furnace based on possible future applications and then using it for day-to-day operation at 600°C only creates operational issues with process control and temperature uniformity. This would be the equivalent to buying an F1 racecar to drive to town to buy groceries with the remote possibility that you may have a chance to drive in the Monte Carlo Grand Prix some day. Capital costs, reliability, low speed performance, and maintenance issues would be such that perhaps a used VW would have been a wiser purchase, not to mention advances in technology that may occur, rendering the unit somewhat obsolete and out-of-date.

Furnace Geometry

The next point to consider should be furnace geometry. Should you purchase a box unit or a tube unit? The box unit is great for loading samples in a batch operation, while a tube unit is generally better for a continuous application such as gas conditioning or material characterization testing that can take place inside of a process tube. Each style also has additional options to consider. With a box unit, there are several types of doors such as a simple front door (either vertical or hinged side swing) or perhaps due to special process applications, a bottom loading or elevator-type unit may be required where the samples are loaded on a base that is then raised into the bottom of the furnace. The elevator unit typically can be made more uniform and can be loaded “hot,” and if designed correctly, has a faster recovery time and can be more efficient, but at the drawback of a significantly higher purchase price and possible higher maintenance costs due to moving parts.

With tube furnaces, you have the option of either a solid tube or a split tube type. If the application requires repeated access to the internal heated chamber, then a split tube is the better choice. The solid tube unit will offer a generally flatter section of radial uniformity and will cost typically 20 percent less. In some cases, the process requires that samples be shielded from direct radiation from the heaters. In this case, the solid tube unit can be designed with a thermal diffuser built in as an integral part of the heating structure, while the split tube will need a separate unit installed and supported. This thermal diffuser will also somewhat negate the advantages of quick access to the samples being processed, unless the thermal diffuser is also used as a carrier that is loaded ahead of time and then placed in the furnace for processing. In some cases, rather than having a flat uniform temperature in the working area, it is required to have a known temperature gradient across the work area. In this case, a tube furnace would be the most beneficial, simply adjusted solution.

Uniformity

Uniformity is another issue that must be addressed. A general rule of thumb states that the center 80 percent of the working dimensions of a furnace will exhibit a +/-5°C temperature variation. Should a greater uniformity be required, several options exist. For lower temperature units (approximately 700°C or lower), stirring fans or recirculating air heating systems would be necessary. For higher temperatures, the furnace may need to be larger in order to achieve the required temperature uniformity or “flat zone.” Perhaps a different heater configuration may be recommended, or the addition of multiple heat zones may provide the solution. Unfortunately, no hard or fast rule covers all situations. It is often the case that the design requirements are specific to the user’s uniformity and operating requirements.

It may be required to add some sort of forced or controlled cooling due to process requirements. Many options are available ranging from the introduction of cooled gases, to vents, fans, or a combination of all the above including special programming of the temperature controllers to achieve a controlled cool-down cycle.

Controls

Another important consideration is the type of controls desired. Will a standard single set point temperature controller work (unit ramps at an uncontrolled rate to a set process temperature and stays there until manually shut down), or will a programmable unit be required (adjustable ramp rates with hold times, soak times, and shut down after completion of the process)? In addition, some sort of data logging and/or computer interface may be desired or an over-temperature control to ensure that the unit does not self-destruct. Of course, with increasing degrees of sophistication and technology, the price will also increase. Thanks to advances in technology and electronics, many of these options are significantly less expensive than they were just a few years ago, and in some cases, one controller can perform many different functions.

Atmosphere

Another consideration should be the atmosphere that will be used in the unit, as this can have a significant bearing on purchase and maintenance costs. Normally, if the unit is going to be operating in an air atmosphere, no special considerations are required. Should the process generate off-gassing of volatile compounds, then provisions must be made for venting and perhaps protecting the inside of the furnace from chemical attack, depending on the types of gases released. If an atmosphere is required and it is a simple “blanketing gas” such as nitrogen or argon, then all that may be required is to provide a gas inlet and exhaust port in an otherwise standard furnace with the user providing a means to safely exhaust the spent gas from the work area either via an exhaust hood or exhaust manifold piping. It should be noted that when using nitrogen with the higher temperature classes, special care must be taken to prevent damage to the elements due to interaction of the nitrogen and compounds used in the silicon carbide and molybdenum disilicide heaters. Should the atmosphere be some sort of “forming gas” or explosive in nature such as hydrogen, then various safety features will be required and the use of a retort may be dictated. A retort is simply a sealed containment vessel that serves to protect the furnace from attack as well as containing hazardous compounds. A retort can significantly add to the purchase price of the unit, not to mention, operational, safety, and maintenance issues.

Customization

A final item to be considered, although it might seem trivial, would be cosmetic issues. Will the vendor’s standard color scheme work, or must the unit be painted a special color to match existing equipment or standards? Bear in mind that as soon as the word “custom” enters the equation, purchase prices start to increase along with lead times. Also, instead of a painted exterior, is it necessary for the unit to have a stainless steel exterior? While many companies offer both versions based on style and type of unit, the stainless versions often carry a premium, and taking a painted standard unit and converting to a stainless exterior will add significant increase in cost and delivery.

Standard products are available from most vendors of lab furnaces, while some manufacturers such as Thermcraft offer standard units as well as fully customized units that can be specified and engineered to exact customer requirements. Careful consideration will put you ahead of the curve when the time comes to begin the search for your new lab furnace.

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