Gas Fired Vacuum Furnace
Heat Treating in the 21st Century.
The Role of Gas Fired Vacuum Furnaces History,
Comparison w/ Electric and Future Outlook.
by Klaus H. Hemsath, PhD and Mark K. Hemsath, MBA
ASM Heat Treating Society Conf. Proceedings, Sept., 1997
In 1989, the authors, through their association with Indugas, Inc., and with the support of Columbia Gas, GRI (Gas Research Institute), and Southern California Gas, installed the first gas fired vacuum furnace in a heat treat facility in Cleveland, Ohio. The furnace was designed for low temperature duty with a maximum allowable furnace temperature of 1250°F. This new furnace was not only the first commercial gas fired vacuum furnace to be installed in the world, this furnace was also designed to carry out vacuum plasma nitriding, or ion nitriding. The gas fired ion nitrider was followed by another furnace development intended to operate at a temperature of 1750°F. This second furnace was successfully tested in the laboratory, also through the support of GRI, Southern California Gas and Consolidated Natural Gas. The authors turned over the marketing of the new design to an unaffiliated furnace manufacturer for market penetration efforts.
The many years of design and development allowed the authors to learn from the early challenges associated with the design and to design an even better performing, more reliable, more dependable, and lower cost furnace. A design objective here was to decrease equipment cost, yet increase performance. The result has culminated in a whole generation of new, gas fired vacuum furnaces.
Versions of the new vacuum furnace have included designs for a new type of tempering furnace, a new annealing furnace, and a new furnace for carburizing and through-hardening.
The Temper and Annealing units have been successfully built, tested and installed, with the first unit approaching two years of service. The carburizing furnace design has a unique integral quench, as well. The quench is designed to exhibit excellent agitation and extremely short transfer times. Transfer times of less than 10 seconds are a design feature. The gas fired vacuum furnaces are offered in either a horizontal or a vertical configuration.
The authors also have designed the furnaces for high volume shops, so, in addition to the usual conventional material handling configurations an entirely new integrated, high volume heat treat facility configuration has been designed that offers unmatched production capacity for the investment. The design of the integrated facility incorporates a high volume material handling system, as well.
The initial gas fired furnaces designed and built by the authors over the last ten years have performed at or above expectations under industrial operating conditions. The performance has been excellent. The first “prototype” has been operating since 1989 for seven days per week with virtually no downtime. The first unit of the second generation has been operating for almost two years with no service calls. The designs have proven to be sound furnaces with very high “up time” and low operating costs.
The History of Gas-Fired Vacuum Furnace Design Successes
The Gas-Fired Ion Nitriding Furnace
The gas fired design improved conventional ion nitriding furnaces significantly. First, the new furnace has insulated furnace walls instead of a cold water-cooled shell, saving energy and improving heating performance. Next, the gas-fired heating system can bring a load to temperature without using the plasma discharge for heating purposes, or having to use additional electric resistance elements for heating.
In FIGURE 1: GAS FIRED ION NITRIDER a picture of the first gas fired ion nitrider is shown.
In FIGURE 2: GAS FIRED 1750°F FURNACE a picture of the high temperature first-generation gas fired furnace is shown. The furnace design offered two entirely new process features; very uniform part temperatures and fast gas firing.
The gas-fired furnace features combine to offer substantial quality and cost improvements. Temperature gradients within work loads are greatly lowered due to uniform convective heating, resulting in uniform layer thickness on all parts. Operating cost are decreased by using relatively inexpensive natural gas rather than electricity for heating. Although energy costs are not the main feature, energy costs for heating can typically be reduced by more than 50%.
More importantly, this new design has improved performance features to offer. By using natural gas and convection for heating the load can be brought to temperature much faster than in a conventional, electrically heated ion nitrider. A good size ion nitrider typically comes equipped with a 150 to 200 kW power supply. When using plasma heating the power cannot be maintained during 100% of the time due to the tendency of the plasma to degenerate into concentrated arcs in the presence of minor amounts of contamination on the surface of parts. It is not unusual that less than 100 kW are used on a time averaged basis during the initial stages of the heating cycle. Heating rates in the conventional electric design further are reduced by water cooling of the vacuum shell and by the desire to minimize the effect of excess radiation on the parts during heat up. Radiation losses from the load to the water cooled furnace walls additionally reduce the heating rate. Effective heat input into the load is, therefore, comparatively small and heating rates are modest.
A typical 2000 pound load needs approximately 600,000 Btu to raise its temperature and the temperature of the furnace to 1250°F. The gas fired furnace can deliver the heat in less than one hour. With a 200 kW power supply the electric furnace will need more than two hours to supply the energy. Temperature uniformity for the gas fired furnace will be superior despite the shortened heating time.
The extensive field testing of this new furnace design has demonstrated that gas fired vacuum processing is superior to electric operation. Product quality is improved, operating costs are decreased, productivity is increased, initial equipment costs are very competitive, and cooling water consumption is substantially reduced. As a side benefit, installation costs are also less with gas-fired units. In all, the gas-fired Ion Nitrider is the ideal furnace design for plasma case hardening.
Gas-Fired Vacuum Tempering Furnace
In FIGURE 3: VACUUM TEMPERING FURNACE an early version of the gas fired vacuum tempering furnaces is shown. This furnace comes equipped with four gas fired radiant tubes with a combined heat input of 1.25 million Btu/hr. This furnace has an improved feature over the design discussed in the previous section. The furnace employs patented rapid air cooling tubes. These tubes assist in the cooling of load and furnace after heating cycle completion. This feature improves furnace productivity dramatically.
The gas-fired vacuum furnaces operate differently than electric versions. Vacuum is used to evacuate the furnace of impurities, but then an inert (or reactive) gas is admitted into the furnace to allow for convective heating. A high volume fan then helps to recirculate the hot gases through the load.
In a typical gas-fired vacuum furnace heat treat cycle the furnace is vacuum purged with a high volume vacuum pump. The furnace is then refilled with clean nitrogen and the heating cycle begins. The recirculation fan assures high rates of heat removal from the heating tubes and helps transfer the heat quickly and uniformly to the load. A small amount of hydrogen can be used to keep the parts extremely clean.
Heating takes place by convective heat transfer, which is further assisted by the muffle which separates the heating tubes from the load, assuring proper recirculation and radiation “hot spot” minimization. This mode of heat transfer, heating by convection, is by far the most effective at low process temperatures (below 1100°F). Conventional, electrically heated vacuum tempering furnaces heat with radiation only. The radiation mode of heating is very slow at low temperatures and will create very non-uniform temperatures with all but the most unusual load shapes (gas turbine housings and similar geometries are well suited for vacuum heating). Tests showed that temperature uniformity requirements of AMS (Aerospace Material Specification) 2750 could be met easily. The maximum allowable temperature differential for heat treating of steels of +/- 25°F is readily met. The furnace operates easily within the tighter +/- 10°F specification.
The cooling tubes accelerate the otherwise slow cooling process and assure that low break out temperatures for high quality surface specifications can be approached in times that are short when compared with cooling times achievable in conventional vacuum tempering furnaces. Cooling is again assisted by the recirculation fan. Due to the cooling tubes, high pressure cooling is not required. Consumption of cooling water is very small and can be reduced to almost negligible levels with a specially designed water cooling system that is offered as an option. Production rates with heavy, dense loads of the gas-fired unit can not be approached by electric furnaces.
Gas-Fired Vacuum Annealing Furnace
In FIGURE 4: GAS FIRED VACUUM ANNEALING FURNACE a horizontal configuration of the high temperature gas fired vacuum furnace is shown. In this picture the unusual door handling mechanism is shown. This mechanism makes handling of the door very simple and requires a minimum of effort for moving. Heat input and heat removal capabilities were increased further to maintain high heating and cooling rates when heating to temperatures of up to 1750°F. When fully equipped this design operates with the low cooling water consumption of two (2) GPM. This furnace type is ideally suited for general solution heat treating, annealing, normalizing, spheroidizing, and stress relieving operations. The configuration of this furnace is well suited for heavier loads such as foil annealing, wire annealing, and stress relieving of weldments. For critical annealing operations the cooling rate can be programmed and controlled cooling rates can be automatically implemented with this furnace.
The pioneering effort, which started in 1985 by Indugas, Inc. and the authors, has led to a line of furnaces that are rugged, offer superior quality features, are comparatively inexpensive to operate and are becoming better understood and accepted in the marketplace.
At process temperatures below 1750°F the gas-fired design performs better than any of the conventional vacuum furnaces. It heats faster, achieves better temperature uniformity, is more flexible in carrying out cycles, costs less to operate and is less expensive to buy.
The market for this furnace line is not the electric vacuum furnace market. The market is high quality, low cost, productive heat treating. Electric vacuum furnaces are ideal for temperatures above 2100°F. Batch heat treaters can now obtain the cost per pound of basic atmosphere furnaces with the quality of the vacuum furnace, and more.
The furnace design promises to be the ideal furnace for implementing more demanding future heat treating specifications. The furnace is ideally suited for modern manufacturing operations where it can be easily integrated into just-in-time manufacturing lines. Varied manufacturing lines making a wide variety of parts with varying cross sections and weights can be accommodated easily. The restrictions inherent in continuous furnace lines are eliminated. The very high production capacities of the furnaces can cover load ranges of 2,000 to over 10,000 pounds per charge. Multiple furnace installations can be designed that can produce as much as 50,000 tons per year. Load dimensions are 36″, 48″ and 60″ square by 56″ to over 100″ long.
Comparison with Electric
The electric vacuum furnace has a long history. In the early days of the electric vacuum furnace, the vacuum was seen as a way to produce clean parts in the absence of oxygen or water vapor. Atmosphere furnaces were either direct fired, which meant that the atmosphere had products of combustion (water vapor, trace to high oxygen and other elements), or atmosphere furnaces used an atmosphere such as nitrogen. In the early days process atmospheres were largely generated, resulting in high levels of impurities, making atmosphere heat treating less desirable.
The electric vacuum furnace has a number of features that make it a less-than-ideal furnace. In spite of these shortcomings, vacuum furnaces have proliferated because no competition existed. Driven largely by funding through Defense and aerospace spending, electric vacuum furnaces were perfected over the years. Today, the electric vacuum furnaces have many plusses, but some distinct flaws. Some of the positive features are:
- Good cleanliness through tight furnaces with powerful vacuum pumps.
- High processing temperature ranges with good equipment life.
- Excellent cooling when high pressure quenching with nitrogen and massive heat exchangers is used.
The negatives should be reviewed, as well:
- Cold wall design makes uniformity and heat up rates difficult, especially at lower process temperatures.
- Heating by line of sight radiation only causes large temperature gradients during heat up.
- Heating to low temperatures is difficult by radiation only.
- Installation is expensive due to the large required electric power supplies.
- Water usage is very high.
- The hot zones need to be maintained and are expensive.
- The initial acquisition costs are very high.
By comparison, a gas-fired unit offers the following benefits:
- Operating costs are much lower.
- Heating rates are much faster.
- Temperature uniformity in most loads (especially dense loads) is much better.
- Overall productivity is higher due to convection heating which allows for densely packed loads. High heat input, and higher loading capacities increase production even more.
- Capital costs are lower.
- Installation costs are lower.
Since few gas-fired units have been placed, there must be valid reasons for this. After all, the first unit was put into production in 1989!
Gas fired vacuum furnaces have one major drawback; their maximum process temperature has been limited to 1750°F. These furnaces are, therefore, not suited for austenitizing of many tool steels.
However, improper positioning of the product, limited marketing efforts and lack of educated sales personnel have been the largest reasons these units have not caught on better.
Comparison of Operating Costs
Variable operating costs for gas fired vacuum furnaces are lower because the costs for energy and water are lower. As a rule of thumb; a gas fired vacuum furnace will consume twice as much raw energy (due to heated exhaust gases). However, the cost for gas energy is roughly $4.50 per million Btu while the cost for electricity is on an average $20.50 per million Btu (at $.07 per KWh). Overall heating costs with gas are, therefore, about half of that with electric ($9.00/$20.50 = 0.44).
Heating Rate Comparisons
The gas fired vacuum furnace uses convection rather than radiation as the heating mode. The heating rate, especially at lower (less than 1250°F) and intermediate temperatures (less than 1750°F) is much higher than with radiation resulting in faster heating rates. With compact, heavy loads, this difference becomes even more pronounced.
The net heat input (after allowing for exhaust gas heat losses) of gas fired furnaces is usually two to three times as high as that of electric furnaces. In combination, the extra heat input and the convection which allows the heat to be transferred to the load, results in much higher productivity potential.
Temperature Uniformity Comparisons
Convection heating creates very uniform temperature profiles even in comparatively dense loads.
Whether the uniformity tests are performed to AMS 2750 or on a densely packed load, the gas-fired, convection units have constantly shown excellent uniformity. Two gas fired furnaces are especially advantageous when fixtured loads with many individual pieces are processed. Radiation heating of parts on the inside of a load will be affected by the shadowing effect of parts on the outer periphery of the load leading to very large temporary temperature differentials.
The AMS uniformity test does not recognize this peculiarity. Therefore, many critical parts that are processed under vacuum may not have the anticipated uniformity and consistency of metallurgical properties. Gas fired vacuum furnaces on the other hand are ideally suited for large, well fixtured loads.
Cooling Rate Comparisons
Gas fired vacuum furnaces are equipped with convection fans and cooling devices. Therefore, the cooling rate is excellent. Electric vacuum furnaces, when outfitted with optional high pressure gas quenches, can cool very quickly, as they utilize convective cooling with an external or internal heat exchanger.
In the gas-fired units cooling rates can be controlled, allowing for excellent, controlled microstructure formation on materials (such as spheroidization on steels) where slow cooling rates are critical. Quenching is a separate issue to be discussed later.
Capital Cost Comparison
Even with the many advantages of the gas fired units, their acquisition costs can be much less. A gas fired unit with the same load size, i.e. 36″ wide x 36″ high x 56″, is usually less expensive to buy. In addition, the low cooling water usage (meaning a water cooling tower system is not needed, or an existing system can be used since there is minimal added load) adds up to less capital dollars needed. As well, the relatively easy installation of a gas unit compared to the large electrical contractors and service required, means a less costly installation. The combined effects of lower capital costs and higher annual production will result in significantly lower fixed costs for the gas fired furnace.
Fast heating, fast cooling, and heavier loads add up to more pounds processed per year. With the gas fired vacuum furnaces heat treat cycles can be shortened compared to electric furnaces resulting in an overall substantial increase in annual production for gas fired vacuum furnaces.
Higher heat inputs, high convection and uniform heating means that the load capacity can be increased as well, without a major acquired cost penalty. When combined with matching material handling equipment, furnaces can be built that can process very heavy loads (7,500 to 10,000 pounds)
A major operating cost advantage for the gas fired vacuum furnace emerges when one combines savings in fixed costs (which include charges for capital costs) and variable costs. Major cost savings and superior product quality make the gas fired vacuum furnace the choice, not an optional consideration, for intermediate and low temperature, high quality processing.
Future Outlook and Future Furnace Developments
The quest for better performing and lower cost equipment never ends. Product quality needs to be improved. Processing costs must be lowered. And, possibilities for operator error must be eliminated.
Future furnace creations will have some or all of these features:
- More automation of material handling.
- Very sophisticated and integrated control and data logging computer systems.
- Automatic programming and mathematical modeling of heat treat cycles.
- Central quench facilities for use by all furnaces.
- High pressure gas quenching with hydrogen.
- Extended use of overhead cranes for high volume shops.
- Heavier furnace loadings with larger furnaces.
As a result of these improvements the typical heat treat shop will begin to look more and more like a modern machining cell. High automation, large volumes and minimal labor will lead to lower costs and high repeatability. Heat treat shops will be much cleaner because electric and gas fired vacuum furnaces do not leak gases into the shop environment.The parts that arrive for heat treatment will be fixtured carefully in a staging area (through higher convection furnaces, fixturing will be less demanding and less labor intensive). The fixtured load will be clearly identified and all pertinent cycle data will be down loaded from a central computer server. A process control computer will execute the heat treat cycle. The computer will analyze the specified cycle data and select the right furnace, quench, washer, and temper furnace. Scheduling of equipment will be performed by computer. Human interference will be reduced to a minimum. Most of the human contribution will relate to supervisory control of facility operations, to load fixturing, and to material handling monitoring.
Quench facilities will be shared by a large number of furnaces. Four types of quenches will be used; water quench, polymer quench, oil quench, and/or high pressure gas quench. With these various quenches a virtually complete control of quench and cooling rates can be achieved. High pressure hydrogen will become very popular as a way to fill the gap between oil and pressurized argon.
Gas quenching deserves a further discussion here, due to advances that will happen in technology.
High alloy steels need to be heated to high temperatures to achieve full conversion to austenite. For many applications it is desirable to cool the alloy comparatively fast to obtain the most desirable properties.
In most applications the cooling with liquid media is too fast, the cooling with gaseous media is too slow. High pressure gas cooling attempts to bridge the gap between liquid and gas cooling rates. High gas pressure, high gas velocities, and high thermal conductivities of gases increase gas cooling rates. High pressure argon (10 bar or 150 psi) has been used to achieve high gas cooling rates in the past.
Argon is not necessarily the most suited gas nor is 10 bar a very high pressure. It is reasonable to expect that especially hydrogen is going to be used more frequently and that quench pressures will continue to climb.
Gas cooling rates or gas quenching rates will become faster and may eventually approach the cooling rates of polymers and oils.
Heat transfer analysis shows that the cooling rate of an argon atmosphere can be accelerated when the atmosphere is replaced with hydrogen. Hydrogen at 5 bar has about four times the heat transfer rate of Argon at 5 bar. Hydrogen and mixtures of hydrogen and nitrogen at high pressures will achieve very high cooling rates. The cooling rates are higher and the costs for the atmosphere supply are lower. Argon and Nitrogen have very similar heat transfer properties.
Hydrogen has not been used very much due to a number of reasons. Safe handling of hydrogen requires special procedures. Equipment features must be adjusted to the particular properties of hydrogen and its mixtures. However, the heat transfer advantages of hydrogen are so pronounced that its utilization will increase.
Equipment design must be adjusted to accommodate the singular and special properties of hydrogen.
The appearance of the future heat treat shop will be very clean. There will be predominantly batch furnaces with no open flames. All effluents will be collected and treated prior to discharge, and proper equipment design and controls will eliminate soot formation.
A complete record of the actual heat treat cycle will go with each completed load, with the possibility to send to the customer. Quality control records are accurate, detailed, and complete.
Heat treating in the future will be carried out in large, well equipped facilities. This future type of heat treat facility will perfectly interface with the modern approach to just-in-time machining. The heat treat cycle becomes part of the overall manufacturing process and constitutes an operations step with a comparatively long duration.
Advantages of Overhead Crane Operations in the Future
Overhead material handling has a number of benefits. Due to slow advancements in crane automation technology, this area has been ignored. In the future shop, load sizes and furnace configurations will lend themselves ideally to overhead material handling.
Every floor location can be accessed at any time from overhead. Load capacity of overhead cranes is virtually unlimited for heat treat type operations. Overhead cranes can be easily programmed to access any floor position. Crane equipment is now becoming available to allow for complete automation.
Portable furnaces moved by overhead cranes have been used for decades in steel mills and specialized heat treat facilities. Technology for moving and connecting furnaces is well established.
Separating the quench from the furnace is not yet common but does not present unusual difficulties. Vertical transfers of loads from furnace to quench are much more desirable from an equipment standpoint. The use of an inert purge medium will avoid oil fires and will keep quench vapors from escaping into the shop environment.
The quench process is very short when compared to the heating or heat treat cycle. A single quench can, therefore, serve several furnaces. Therefore, all that is required is that the cooling capacity for the quench fluid is sized properly.
The use of one quench for several furnaces has major advantages. It reduces costs, it provides flexibility, and it reduces required floor space. Design of a high capacity single purpose quench removes several design constraints that must be accommodated in integral quench designs. The overall goal is to reduce acquisition cost, reduce floor space and to reduce operating costs.
Heat treating is entering a new technology phase. Computer control, gas fired vacuum processing, advanced quenching, accelerated cooling, high speed material handling, and large capacity equipment combine to improve heat treating in many major ways:
- product quality is improved
- scrap rates are reduced
- material properties are more uniform
- operating costs are lowered
- environmental conditions are enhanced
- safety is augmented
- working conditions are more comfortable
With all these advancements available, heat treating is well positioned to accept the challenges of the future.
The conclusion is that for processing temperatures below 1850°F, electric vacuum furnaces should become extinct.
Summary and Conclusions
Gas fired vacuum furnaces have added new capabilities to the heat processing of metals. Improved product quality, reduced operating costs, higher productivity, and less need for carefully fixtured loads have expanded opportunities for batch vacuum processing to become the method of choice for all heat treat shops, large or small.
Today, a well proven line of equipment exists. It is now the equipment manufacturers’ duty to educate the furnace buyers and users of the successes and history.
Most processing in electric vacuum furnaces should be replaced by the lower cost gas units. Much processing being done in less expensive atmosphere furnaces can be done in gas fired vacuum units without a cost penalty, but with quality improvement. The new gas fired equipment can be installed in new facilities and in existing ones. Alongside atmosphere units or along with electric vacuum furnaces, in either case, economic benefits can be realized.
The gas fired designs should find rapid acceptance and will allow the trend toward integrated, automated material handling to become a full reality. Combined with better quality, these batch facilities should replace many versions of continuous processing lines, as well, and reduce costs. In the future heat treating shop, all control and material handling operations in a large heat treat facility can be executed by a few advanced supervisory and process control computers, with minimal labor inputs. After fixturing, a load will be loaded into the furnace, heated, soaked, and quenched without human interface.
Gas fired vacuum units allow all the future needs to be obtained today: product quality is improved; costs are reduced; and productivity is increased.
Source: Technical Papers Heat Treating in the 21st Century, The Role of Gas Fired Vacuum Furnaces History, Comparison w/ Electric and Future Outlook. by Klaus H. Hemsath, PhD and Mark K. Hemsath, MBA ASM Heat Treating Society Conf. Proceedings, Sept., 1997.
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