When (for example) we are carburizing at 1700°F with a carbon potential of 0.90%CP, the atmosphere gas will not only diffuse into the steel being treated but also into the refractory insulation system. This is simply because of the porosity of either refractory brick of low-mass ceramic fiber. The purpose of the refractory insulation is to contain the process temperature within the process chamber of the furnace in use.
The estimated cold face when one has the process chamber at 1700°F is 130-150°F. This means that with a 10-inch insulation thickness, the refractory material is protecting the outside furnace wall, the room temperature around the furnace in question and, of course, the heat-treatment associate.
As the temperature is diminishing across the 10-inch insulated furnace wall, the process gas temperature is also diminishing. As the gas is cooled by the refractory, the carbon present in the gas will begin to precipitate out and begin to form (if it can be expressed this way) a wall of precipitated carbon within the refractory insulation material. The refractory insulation is now being changed in its composition by the precipitation of carbon from the diffused atmosphere. This means that the insulation characteristics of the refractory material are changing.
It then becomes necessary to periodically reverse the condition and come back to a “clean” refractory insulation. This is accomplished by the procedure of “burnout.” If the burnout is not done at all, the refractory insulation material will not behave as it did when the furnace was first started up. The burnout procedure is quite simple to conduct.
The furnace should be free and empty and at a temperature of approximately 1550°F. It should also be clear of any work from the process chamber. Then an air blast of shop air is blown into the process chamber. This will ignite the diffused carbon in the refractory insulation.
The carbon that is now emerging from the insulation material does not burn vigorously but with a gentle glow on the insulation hot face. The time that it will take to complete the burnout period will be determined by the last burnout. Generally, it should not take much more than 3-4 hours.
What will be seen on the temperature-control instrument is that the process chamber will begin to raise its temperature. This is caused by the ignition of the diffused carbon into the insulation. When the burnout is nearing completion, it will be seen that the process temperature will have peaked out and is now cooling. When the furnace temperature reaches its original setpoint temperature of 1550°F, the burnout is complete.
Now we have to condition the furnace with the new process atmosphere. The refractory insulation will now begin to soak up the carbon (as before), which will diffuse into the insulation. It is recommended that you do not try to carburize until two austenitize and quench loads have been processed.
The furnace will now be in a condition to successfully carburize once again with a reasonably accurate level of control of the carbon potential.