Single chamber vacuum furnaces can be divided in three different basic models. Horizontal vacuum furnaces are loaded from front and meet most applications. The space-saving design has low demands on the location for installation.
Vacuum bottom loader furnaces (Figure 1), which are loaded from underneath, are often used for the heat treatment of larger, rotationally symmetrical components. These kinds of systems are mainly used by manufacturers of engine sections for the aerospace industry or large pelleting dies. Beside the higher investment costs, these systems also require hall height and cranes for loading.
Vacuum shaft furnace systems, which are loaded from above, are in more rare cases.
Among other applications, they are used in the heat treatment of components where the ratio of diameter to lengths is very large, as for example in broaches and extruder screws. The installation of this kind of system is often carried out in pits.
Depending on the basic model single chamber vacuum furnaces can be designed for maximum batch weights from 50 kg up to 15.000 kg.
Figure 1: Vertical Bottom loader vacuum furnace, Example (Copyright: IVASchmetz GmbH)
Figure 1: Vertical Bottom loader vacuum furnace, Example (Copyright: IVASchmetz GmbH)
Principle design of vacuum chamber furnaces
Vacuum furnaces are electrically heated and designed with double-walled, water-cooled furnace shell. This reduces the thermal load on the metal furnace housing, since thermal losses are conducted to the cooling water in the vessel.
Inside the furnace there is the hot zone chamber with insulated lining and heating elements as well as the rapid cooling system with heat exchanger. The high-performance radial fan and electric motor are also installed inside the furnace.
Vacuum pump systems
Vacuum furnaces typically generate a vacuum between 10-1 and 10-6 mbar. For hardening of steel to achieve a metallically bright surface, a vacuum in the 10-2 mbar range is usually sufficient. A mechanical vacuum pump system consisting of a rotary vane pump and a roots pump is used here. To achieve higher vacuums, multi-stage mechanical pump systems or high-vacuum pumps such as oil diffusion pumps, turbomolecular pumps, or cryogenic pumps are installed. Such equipment permits to evacuate the furnace up to 10-6 mbar range.
Hot zones
The hot zones of most vacuum furnaces for heat treatment are lined with graphite insulation. The heating chambers can be rectangular or round.
The process temperatures for heat treatment of metal workpieces are usually up to 1250°C. Vacuum furnace systems are generally designed for a maximum working temperature of 1350°C. However, there are also brazing and sintering processes that require temperatures up to approximately 1600°C.
The working temperature also determines the thickness of the heating chamber insulation. For example, 40 mm insulation is standard in graphite heating chambers of vacuum furnaces. In the temperature range up to 1600 °C, insulation thicknesses of 60 mm are used for example.
Reinforced insulation in modified heating chambers is also increasingly being used for greater energy efficiency. This contributes to a reduction in energy consumption of approximately 10 % or more.
For special applications and where a hydrocarbon-free atmosphere is required, all-metal-insulated heating chambers (Figure 2) are used. These heating inserts utilize several consecutive radiation shields made of molybdenum and heat-resistant stainless steel for insulation. The application temperature determines the number of radiation shields and their material. These heating chambers can also be rectangular or round.
Figure 2: Molybdenum insulated hot zone of horizontal vacuum furnace, example picture
(Copyright: IVASchmetz GmbH)
Figure 2: Molybdenum insulated hot zone of horizontal vacuum furnace, example picture (Copyright: IVASchmetz GmbH)
Heating systems and versions
It is important to ensure uniform heating of the load to avoid distortion even in the first phase of the heat treatment process. The heating capacity of a vacuum fur-nace system ranges from approximately 50 to over 1.000 kW, depending on the size of the work chamber.
In practice, a single zone heating system has become established for most processes and system designs. For components or batches with a highly asymmetrical structure, heating systems can also be implemented in several separate zones (e. g., bottom, sides, top). In addition, also door and rear wall heating, center heating or combinations of separate heating zones can be configured.
Heating in the high temperature range occurs solely through radiation, in vacuum atmosphere. In the lower range, the advantage of hot gas convection can be beneficial.
Batch cooling in single chamber vacuum furnace
Depending on the requirements of the heat treatment batch and the materials, different cooling mechanisms can be used.
With controlled heat-down, the batch temperature is slowly reduced according to a gradient.
With vacuum cooling, the batch is cooled via the empty losses to the water-cooled furnace vessel. The heat energy is transferred very slowly. Heating chamber dampers can be opened to accelerate the process if necessary. Gas cooling with stagnant protective gas forces the heat transfer from the batch to the water-cooled furnace vessel. Even here heating chamber dampers can be opened to accelerate the process.
The high pressure gas quenching generates fast batch cooling speed. More subsidiary for some annealing or brazing processes, it is of high importance and necessity for the hardening of steel components.
High pressure gas quenching
In high-pressure gas quenching, the furnace system is flooded with protective gas (usually nitrogen for hardening steels, frequently argon for titanium alloys) to a preselectable pressure. Furnace systems are designed for maximum quenching pressures in the range of 1,5 up to 15 bar (absolute). The cooling fan unit, consisting of a threephase motor and fan wheel, generates a cooling gas flow which streams through opened heating chamber flaps or nozzles over the batch. The heat energy of the batch is captured and dissipated via the protective gas to the gas/water heat exchanger.
Hardening of middle- and high-alloyed steel components has various requirements to the gas quenching system.
Steel quality and its alloy composition, as well as component thickness and batch size, determine the required minimum cooling rate, which should not be higher than metallurgically required. The rate of batch quenching is determined by the furnace’s technical parameters like cooling gas pressure, gas flow rate (motor power and fan impeller design), gas/water heat exchanger (design and performance), and also the peripheral cooling water supply (volume and temperature, etc.). In general, a uniform and low-distortion heat treatment is desired. Component geometry and batch structure impose corresponding requirements here. The homogeneity of batch cooling is influenced by the furnace’s technical parameters, including the distribution of the cooling gas flow. This is determined by the arrangement and design of the inlet and outlet openings, such as gas distribution plates or nozzles.
Single chamber vacuum furnace with rectangular hot zone and straight cooling gas stream system 2R or system 2x2R
For the majority of typical heat treatment processes in single-chamber vacuum furnaces, the flow-through principle in a rectangular heating chamber has become established. For high cooling homogeneity, large gas inlet and outlet openings, sized appropriately for the charge, are required. These gas inlet and outlet openings are equipped with special gas distribution plates.
The homogeneity of the gas quenching process can be increased with the 2 R system (Figure 3) or 2x2R system, which feature programmable vertical and/or horizontal reversal of the cooling gas flow. Multiple changes in the cooling direction from horizontal to vertical flow are just as possible, as is even the implementation of rotating cooling.
Figure 3: Principle vertical cooling gas stream in rectangular hot zone with system 2R
(Copyright: IVASchmetz GmbH)
Figure 3: Principle vertical cooling gas stream in rectangular hot zone with system 2R (Copyright: IVASchmetz GmbH)
The gas direction change can be timeor temperature-controlled, depending on the requirements of the batch being heat treated.
For example, temperature-controlled gas flow direction change is regulated via programmable temperature difference within the batch, measured by load thermocouples.
Quenching is more uniform, hardness variation within the batch and distortion of individual components are significantly reduced. Furthermore, faster and therefore more economical batch cooling is achieved.
Vacuum furnace with separate quenching area system 2 PLUS
A way to increase the quenching rate in a vacuum chamber furnace is to use the system with separate quenching area without vacuum- and pressure-tight gate in the common furnace vessel. This furnace type with System 2 PLUS (Figure 4), incorporates the essential features of a single-chamber design with rectangular hot zone. The batch is heated to temperature in the heating chamber (either by convection or radiation) and, once the hardening temperature is reached, is automatically transported to the quenching area to be cooled there. A slide valve opens the thermal barrier between the hot and cold zones. The batch is moved into the cooling zone within a few seconds. The slide valve immediately closes the heating chamber again.
Figure 4: Principle vacuum furnace with separate quenching area system 2 PLUS (Copyright: IVASchmetz GmbH)
Figure 4: Principle vacuum furnace with separate quenching area system 2 PLUS (Copyright: IVASchmetz GmbH)
The quenching area is designed for optimal cooling gas flow, e. g., the cooling chamber is adapted to the usable batch volume. The cooling gas flow needs to absorb and dissipate only the energy of the batch and the transport unit. Unlike in a single-chamber furnace, the insulation, heating rods, and gas distribution devices of the hot zone are not actively cooled.
In the vacuum chamber furnace with the separate quenching area, cooling times can be doubled compared to a single-chamber furnace at similar quenching pressures. Operating costs and runtimes are reduced because the heating chamber does not need to be actively cooled during operation. This is particularly beneficial for heat treatment processes that require several tempering’s. The heating chamber only needs to be cooled down at the end of the treatment, when the batch is removed from the furnace.
The vacuum furnaces with a separate quenching zone also offer the option of running load thermocouples for measuring and documenting component temperatures throughout the entire cycle – heating, transport, and cooling. Precise temperature control ensures maximum reproducibility. This system type is used where components and batches to be hardened require a linear cooling gas flow with a generally higher quenching rate.
Single chamber vacuum furnace with round hot zone and optimized nozzle cooling system RD PLUS
For some vacuum processes, such as the hardening of very large and massive components like die-casting molds, and the resulting necessary high quenching speeds, a round hot zone with allround radial nozzle cooling can be advantageous. The cooling gas flow impinges on the surface of the batch at 360 ° and is discharged to the rear of the hot zone. The optimized RD PLUS nozzle cooling system (Figure 5) compensates thermal resistance of the load support structure, base grids, etc. with intensified quenching from the bottom side. Appropriately dimensioned cooling gas outlet openings are in the rear area of the hot zone. The gas flow is extracted through these outlet openings to the integrated gas/water heat exchanger (Figure 6).
Figure 5: Round hot zone with optimized nozzle cooling system RD PLUS, example picture (Copyright: IVASchmetz GmbH)
Figure 5: Round hot zone with optimized nozzle cooling system RD PLUS, example picture (Copyright: IVASchmetz GmbH)
Figure 6: Principle round hot zone with optimized nozzle cooling system RD PLUS
(Copyright: IVASchmetz GmbH)
Figure 6: Principle round hot zone with optimized nozzle cooling system RD PLUS (Copyright: IVASchmetz GmbH)
In addition, the system can also generate an additional cooling gas flow through the heating chamber from front to back side. Optionally openable cooling gas inlet nozzles are installed in the hot zone door area for this purpose.
Vacuum furnaces with the RD PLUS nozzle cooling system and its further developments are in international operation with usable space dimensions from 900 x 1200 x 900 mm (W x L x H) and now up to almost 2000 x 3000 x 1500 mm (W x L x H) and batch weights of up to 15.000 kg.
Low pressure carburizing in single chamber vacuum furnace with system CARB PLUS
Low pressure carburizing (LPC) in single chamber vacuum furnace with system CARB PLUS (Figure 7) is carried out using acetylene. This case hardening procedure is operated under low pressures and temperatures until 1070 °C.
Figure 7: Principle vacuum furnace with low pressure carburizing system CARB PLUS
(Copyright: IVASchmetz GmbH)
Figure 7: Principle vacuum furnace with low pressure carburizing system CARB PLUS (Copyright: IVASchmetz GmbH)
Most uniform carburizing and case hardening depth are realized even in densely packed batches. These results are achieved also in the narrowest bores and complex geometries. Even here the controllable overpressure gas quenching reduces component distortion. The fully automatic process with carburizing, hardening and tempering creates metallic bright surface result without oxidation.
Cryogenic treatment in vacuum furnace with system COOL PLUS
Subzero treatment is an established additional process step for the hardening of tools. This low-temperature treatment significantly improves tool properties through targeted optimization of metallurgical structures, such as the reduction of residual austenite. The integration of the subzero system into the vacuum chamber furnace enables an unmanned, fully automatic process with hardening, subzero cooling and tempering. The workpieces do not contact the ambient atmosphere between cryogenic treatment and the subsequent tempering (Figure 8). No surface corrosion can occur due to the process. This ensures the bright metallic surface quality which is typical for vacuum heat treatment. The subzero gradient, which can also be controlled and selected using load thermocouples, enables gentle, controlled treatment. Consequently, the risk of stress cracking caused by abrupt subzero cooling, as with other cryogenic technologies, is significantly reduced.
Figure 8: Principle heat treatment process with integrated cryogenic system COOL PLUS
(Copyright: IVASchmetz GmbH)
Figure 8: Principle heat treatment process with integrated cryogenic system COOL PLUS (Copyright: IVASchmetz GmbH)
The operating principle is the injection and gasification of liquid nitrogen into the graphite-insulated heating chamber of the vacuum furnace. During gasification, the nitrogen volume increases approximately 700 times. In the hot zone, the still cold, gaseous nitrogen is evenly distributed via the convection system. The heat energy is extracted from the batch and exhausted.
Conclusion
Modern vacuum chamber furnaces, with their wide range of possible characteristics and features, can be designed for a wide variety of heat treatment processes and batches.
Smaller systems with usable space dimensions of, for example, 300 x 300 x 300 mm (W x L x H), are used flexibly for R&D purposes, as well as for smaller batches or slight components such as the hardening of surgical instruments.
In large single-chamber vacuum furnaces, for example, 4.000 kg plate heat exchanger batches are brazed in large series. In systems with usable batch weights of up to 15.000 kg, corresponding large components are hardened using high-pressure gas quenching.
Author
Björn Eric Zieger
IVA Schmetz GmbH
+49 (0) 2373 686-0
info@ivaschmetz.com
