Advantages of using carbon/carbon furnace tools to replace heavy metal furnace tools in brazing furnace

Today's brazing furnace usually needs to use many heavy stoves in the brazing room. These stoves include all heavy metal structures in the high temperature zone of the brazing furnace (such as furnace grate, charging tray, charging rack, charging basket, etc.).
All parts and stoves placed in the high temperature zone shall be heated to the brazing temperature, then safely cooled to room temperature, and deformation shall be avoided as far as possible.
If the furnace in the furnace chamber is heavy, it will not only prolong the brazing process time, but also occupy a large part of the production capacity.
Everything comes at a price. Brazing enterprises have to pay a considerable part of the cost to heat these heavy stoves to the brazing temperature, often repeated many times every day, day after day, year after year. These costs have obviously swallowed up the profits of brazing enterprises.
In fact, I have seen some brazing enterprises: the furnace tools account for more than 90% of the total weight of the materials they put into the furnace, and less than 10% are the parts to be brazed and provided to customers.
Many such brazing enterprises are not so much brazing as heat treatment of stoves.
I said to some factory managers, "Let me see if I am right: although you are not aware of it, you are actually carrying out heat treatment on the furnace. Of course, you did put some parts that need brazing in the furnace, but the benefits thus obtained were offset by heat treatment. Isn't that right?"
This can often cause them to think, and let them understand why their so-called "brazing furnace" is so inefficient.
To help change this situation, carbon fiber reinforced carbon (carbon/carbon) is increasingly used to make brazing stoves. The carbon/carbon stove not only greatly reduces the weight of the furnace, but also has the additional advantage of long-term thermal stability.

Basic introduction
The internal structure of the carbon/carbon stove includes the carbon fiber as shown in Figure 1. They are added to special carbon matrix materials to form a composite structure with high strength.

Figure 1. Typical carbon fibers used in the manufacture of carbon/carbon stoves
The strength of carbon fiber itself is very high, while the matrix carbon material has special heat resistance, chemical resistance, low thermal expansion, high thermal conductivity and high electrical conductivity.
The length of the carbon fibers used (usually cut very short) and their arrangement direction in the carbon matrix can be changed as required to obtain the desired properties.
Compared with other furnace materials such as pressed graphite, ceramics or metals, carbon/carbon materials are characterized by high strength, light weight and good flexibility. It is suitable for use in a wide temperature range, not only the performance is not affected, but also the shape is stable (no deformation).
Carbon/carbon materials can safely withstand temperatures in excess of 3500 ° F (2000 ° C) in any non oxidizing environment (vacuum, argon, nitrogen, etc.). They should not be used in hydrogen or any atmosphere with high oxygen content at high temperature.
Handling of stoves
Production personnel need to correctly handle carbon/carbon stoves, which really requires mastering relevant precautions and receiving appropriate training.
Dropping carbon/carbon stoves randomly or onto the ground may damage carbon/carbon materials and shorten the service life of stoves.
It should be remembered that the carbon/carbon material will not have the same depression or deformation as the metal stove, but although it is not easy to break due to its good flexibility, the production personnel should still learn how to store, handle and use the carbon/carbon stove.
Like all materials, carbon/carbon stoves will expand and contract during heating and cooling, but it should be able to maintain shape stability better than metal stoves (because its deformation resistance is better than metal).
It can be seen from Figure 2 and Figure 3 that the metal grate has been used for several months and has produced obvious deformation due to repeated access to the furnace, while the carbon/carbon grate for the same purpose has remained flat without any deformation after being used for the same long time.

Figure 2. This 66 lb (30 kg) grate was significantly deformed after 9 months of repeated heating/cooling (no correction was made during use).

Figure 3. This 5lb (2.5kg) carbon/carbon grate is used in the same way as the metal grate in Figure 2 without any deformation.
Since many of the metals you braze may react with the carbon/carbon stove that carries them, you need to put a certain separation layer between the metal parts and the carbon/carbon stove, such as placing ceramic material sheets or ceramic fiber cloth under the brazed metal parts.
Figure 4 shows an example of placing ceramic parts on grate bars.

Figure 4. Ceramic parts sleeved on the grate
It can be seen that the ceramic pieces sheathed on the grate can reliably prevent any metal placed on them from reacting with the stove under them.
Because carbon/carbon materials are carbon with purity greater than 99%, and pure carbon is easy to react with any metal containing iron (such as stainless steel), any reaction between carbon and steel may generate low melting point eutectic at a temperature lower than 2100 ° F (1150 ° C).
If the metal carried by carbon/carbon furnace is partially melted, it will cause direct contact. You need to pay attention to this, and take precautions to ensure that the metal does not directly contact the carbon/carbon furnace during the heating process in the furnace.
Other metals that react easily with carbon are titanium and chromium.

Design of carbon/carbon furnace
Figure 5 is an example of carbon/carbon brazing furnace design.

Figure 5. The furnace should be carefully designed to ensure uniform pressure on the heat exchanger. As the temperature rises, the metal heat exchanger expands and the carbon/carbon spring is compressed.
The furnace is used to fix the parts to be brazed between the top plate and the bottom plate made of carbon/carbon materials.
The coil spring is also made of carbon/carbon material, which is used to apply uniform pressure during brazing. The carbon/carbon spring is made of unidirectional carbon fiber composite material. Even if it is used hundreds of times at a brazing temperature up to about 2350 ° F (1300 ° C), the correct pressure can be applied to the surface of the component.
Figures 6 and 7 show two examples of carbon/carbon springs (coil spring and plate spring).

Figure 6. Carbon/carbon coil spring

Figure 7. Carbon/Carbon Plate Spring
Comparison of carbon/carbon spring and counterweight pressing
Brazing enterprises generally place some metal blocks (counterweights) on the parts to compress the metal parts below, so that they can keep close contact throughout the brazing process. This often adds hundreds of pounds to the amount of material being heated.
By using light carbon/carbon springs instead of counterweight compression parts, a large part of the furnace weight can be eliminated.
In addition, the heating rate of the carbon/carbon furnace itself is about 3 times faster than that of the metal (because the thermal conductivity of the carbon/carbon furnace is very good), and the heating efficiency of the furnace charge is greatly improved, which is very beneficial to increase the output.

conclusion
For the same purpose, carbon/carbon stoves have many advantages over metal stoves, including:
• The total weight of furnace tools (grate, basket, tray, rack, etc.) is significantly reduced
• The output of brazing furnace is greatly increased (cycle time is shortened and operation times are increased)
• Significant reduction in energy costs for brazed parts
• Reduction in furnace repair/replacement due to fatigue or deformation
With the continuous development of carbon/carbon technology, it is expected that more and more users will choose to use carbon/carbon materials as a simple method to improve the brazing output and benefit, and the higher initial cost of carbon/carbon stoves compared with typical metal stoves will be recovered soon.
With the purchase and use of carbon/carbon stoves, the number of customers benefiting from this "new" technology will gradually increase, and their costs will steadily decline.