As was discussed last month, the heart of any vacuum furnace is its hot zone. If properly designed, constructed and maintained it will help ensure that your furnace performs in an optimal manner. One of the most important aspects of the hot zone is its insulation and the choice of materials used in its construction. The focus here will be on graphite-lined hot zones.
A popular alternative to all metal lined hot zones utilizes graphite-based insulation. A typical design for a maximum operating temperature of 1315ºC (2400ºF) consists of a 50 mm (2 in.) thick graphite material, typically in the form of (felt) blanket or board (Fig. 1). It is not uncommon to have two or more layers of material. For those familiar with high-temperature atmosphere furnaces, this insulation thickness may seem surprising considering the process temperatures inside the furnace. However, the thermal characteristics of graphite materials make it possible. In addition, the outer shell of the furnace is typically water cooled and the fact that most processes run either under vacuum or in a partial pressure atmosphere further reduce heat transfer. If it were not for the cooling jacket, the exterior temperature could be in the range of 150 – 260°C (300 – 500° F).
The graphite blanket or board is usually installed in layers, and during installation, the layers are staggered to minimize the likelihood of continuous gaps from inside to outside and subsequent heat loss. A graphite felt lined hot zone may consist of individual layers either 6 mm ( ¼”) or 12 mm (½”) thick. Graphite board is typically in 25 mm (1”) layers. Blanket products can subsequently be covered by a graphite paint and board product can be supplied with a hot face covering of graphite foil or foil-bonded carbon composite. These improve reflectivity (i.e., reduced emissivity) of the heat from the graphite heaters back toward the load as well as protect the surface from damage by process or quench gases. There can also be intermediate layers of foil sandwiched between the layers of graphite (Fig. 2). These additional foil layers provide increased insulating efficiency. One test of four foil layers in between graphite boards showed an improvement of 10% to 35% as successive layers are added.7
In a graphite insulation system, the graphite blanket/board has a low thermal conductivity (Fig. 3), which resists heat transfer from the hot side to the cool side. With a thermal conductivity three times better (lower) than metal heat shielding, these systems provide superior insulating characteristics in comparison to all metal hot zones and are normally less expensive and take less time to install. In comparing graphite blanket to graphite board, the blanket option is generally less expensive, as durable, and arguably easier to maintain.
Graphite insulation comes in a variety of styles and configurations. The typical materials of construction include: graphite felt, rigid graphite board (coated, uncoated, and foil-bonded), graphite foil, carbon fiber reinforced carbon (CFC or C/C), and non-fibrous board (foil board). All of these products work by blocking the radiant heat emitted from the interior of the hot zone, which minimizes the heat transfer to the cage supporting the insulation and ultimately to the cold wall. Because of graphite’s low conductivity, these hot zones are typically more energy efficient than all-metal designs.
One of the disadvantages of graphite materials is that they have a high specific heat (cp), a property that defines how much thermal energy a material can hold. Compared to molybdenum, a common metallic heat shielding, graphite’s specific heat is more than double (Table 1). Therefore, a given weight of graphite will retain twice the heat energy as molybdenum. This results in a slightly reduced quenching capability because some of the cooling capacity of the quenching gas is needed to bring down the insulation temperature (Fig. 4). In addition to the thermal mass of the insulation containing excess heat, graphite is also (by design) a poor conductor, so its heat is trapped within the graphite fibers, delaying the release of its heat energy during quenching. The multiple separated layers of heat shielding used in the all-metal insulation design, on the other hand, allow the quench gas to freely flow between them, allowing for more rapid quenching.
Graphite blanket (felt) is a rayon-based insulation supplied in rolls (Fig 5) and is convenient to work with as it is easily cut with scissors or a box cutter. It can be formed and sewn into various shapes using carbon cord as a thread. Common sizes are 1.067 – 1.200 mm (42” and 47”) wide, with thicknesses from 3 – 13 mm (0.12” – 0.50”), and densities of 0.0005 – 0.001 g/cc. Graphite felt is ideal for many vacuum furnace applications as it exhibits minimal shrinkage at temperature, has low thermal conductivity and a high sublimation temperature (i.e., the temperature at which it will evaporate into a gas), namely 3600°C (6512°F). For applications where chemical purity is not critical, carbon felt is used as a less expensive alternative to graphite felt. It has a maximum temperature rating, however of only 1000°C (1832°F).
Graphite board (Fig. 6) is similar to graphite felt, except it is infused with carbon binders under compression and heated to form a rigid structure, in a process referred to as carbonizing and graphitizing. Graphite board is used at temperatures up to 2200°C (3992°F) and vacuum levels to 10-5 mbar (7.5 x 10-6 Torr). The board can be coated with graphite foil (aka foil-bonding) on one or both sides, to allow it to better reflect the radiant heat from the heating elements back into the hot zone, which improves its insulating effectiveness. The graphite foil also provides a barrier against gas penetration. Graphite boards can also be coated with a carbon composite fabric to provide extra mechanical stability and improved resistance to gas erosion, as experienced during high-pressure gas quenching for example. Sacrificial layers of carbon composite material or graphite-foil products are also used to cover the bottom third of a hot zone when performing brazing operations. Graphite boards can be molded into special shapes (Fig. 7) or machined for specific applications.
PAN vs Rayon Graphite Felt
Graphite felt is made from either of two source materials, polyacrylonitrile (PAN) or rayon, which are known as precursors. The first step in the process, carbonization, refers to the conversion of an organic substance into carbon material by heating it in the absence of air. Fossil fuels, for example, result from the carbonization of vegetation. After the precursors are carbonized, the resulting carbon is then heated to approximately 3000°C (5432°F) in the absence of oxygen. This causes the carbon crystallites to grow and rearrange in an orderly layered hexagonal structure of parallel planes (Fig. 8). This conversion is called graphitization, which causes a change in the physical properties of the material, giving it the desired strength and other properties.
The resulting graphite has a huge molecular structure with numerous, strong covalent bonds that require a lot of energy to separate the carbon atoms, giving graphite its temperature resistance properties. Graphitization also removes impurities from the material, and additional steps can produce graphite well over 99.9% pure. The high carbon content fibers resulting from graphitization are then used to manufacture flexible felt insulation, which can then be rigidized into boards.
Although graphitization is used for making both PAN and rayon based graphite, graphite that uses rayon as the precursor is a higher performing material than PAN, and has better insulating characteristics (Fig 9), particularly at higher temperatures. A 20% improvement in insulating efficiency has been documented with rayon in comparison to PAN7. However, rayon-based graphite is more expensive. One source advertised prices (in February 2018) of $30 per pound for PAN, and $44 per pound for rayon-based graphite.5
Gas Erosion and De-Binding
When considering graphite insulation one must be aware of the potential for erosion of the material due to abrasion from the quenching gas. This problem is most often observed in the pressure range of 6 – 20 bar along the exposed joints and penetrations through the boards when rapid gas quenching. For this reason, many of these areas are capped or covered by another material such as molybdenum or carbon composite. At a pressure of 20 bar, a quenching gas such as nitrogen has a weight of 51 kg/m3 (1.4 lb/ft3), so it’s easy to understand how high-velocity gas quenching can so easily erode the graphite board. In addition to the reduction in insulating efficiency due to loss of insulation thickness, the other problem resulting from gas erosion is the fine graphite powder that results. This powder will be carried by the quenching gas through the heat exchanger and blower of the gas quench system. It then collects inside the heat exchanger, decreasing its efficiency. In addition, the graphite powder can contaminate the work and interfere with the process being performed in the furnace.
Non-Fibrous Foam Board
In addition to erosion from the quenching gas, another source of fugitive graphite powder is vaporization of the binder from the board (aka de-binding). Without the binder, the graphite is liberated as a fine powder that can contaminate the process and clog the quench gas cooling system, in the same way, the abraded powder can.
An alternative to graphite felt is non-fibrous carbon foam board. This has received mixed reviews in the industry. Similar to graphite board, carbon foam offers high-temperature resistance up to roughly 2800°C (5072°F) and good resistance to thermal shock. Since it is not manufactured by adding binders to felt, there are no binders to evaporate out of the insulation, and de-binding is not a concern. Foam boards offer low thermal conductivity, high mechanical stability and a mostly closed cell design that impedes gas penetration. One drawback of this material in comparison to conventional carbon board is its lack of mechanical rigidity. It is fragile and breaks easily. It also has a higher thermal conductivity than rigidized graphite board, and only offers 25% of the thermal insulating capability. Therefore, it must be four times thicker to provide the same insulating quality.
Graphite Decomposition When Exposed to Oxygen
A major concern with the use of graphite insulation is the potential for it to decompose when exposed to air. If oxygen is present at temperatures above approximately 300°C (572ºF), the carbon that makes up the graphite will combine with oxygen to form CO and CO2 (Equation 1).
(1) C + ½O2 = CO and C + O2 = CO2
The result is that the graphite will slowly turn into CO and CO2 gas, which will be removed from the furnace by the pumping system. At first, pitting occurs (Fig. 10), often observed as a “sugar cube” surface appearance10 and as further decomposition ensues, the graphite continues to lose mass (Fig. 11) and simply disappears.
A majority of today’s vacuum heat treating furnaces are supplied with graphite hot zones given the tremendous versatility of this material choice. Graphite insulation has improved significantly over the past several decades with respect to thermal stability and product integrity over a wide temperature and application range. Just as with all metal-lined vacuum furnaces, a leak-tight vacuum furnace system ensures that air (oxygen) and water vapor are kept out of the hot zone. Clean, dry work entering the furnace extends the overall life of all hot zone materials.
1. VAC AERO International (https://vacaero.com)
2. Industrial Heating (http://www.industrialheating.com)
3. Graphite Store (http://www.graphitestore.com)
4. SGL Group (http://www.sglgroup.com/cms/international/home/index.html?__locale=en)
5. CeraMaterials (http://www.ceramaterials.com)
6. Graphite Concept Products (http://www.graphiteconcept.com/component/option,com_frontpage/Itemid,1/)
7. Herring, Daniel H., Vacuum Heat Treatment, BNP Media, 2012.
8. Fradette, Real, “Methods of Improving Vacuum Furnace Insulation Efficiencies”, Industrial Heating, September 2013.
9. Sinotek Materials (http://www.cn-materials.com)
10. Herring, Daniel H., Vacuum Heat Treatment, Volume II, BNP Media, 2016.
11. “Strength Degradation of Glass and Carbon Fibres at High Temperature”, S. Feih, E. Boiocchi, E. Kandare, Z. Mathys, A.G. Gibson and A.P. Mouritz