Last week in part 1, we discussed what happens when refractory is contaminated by the carbon-rich furnace atmosphere. To correct this problem, a cleaning operation of the refractory brick will be necessary to remove the carbon from the refractory and restore the insulation characteristics back to the original condition. This is done by simply igniting and burning the carbon from the furnace insulation material.

The procedure is known as a “burnout.” It is a simple procedure that requires a temperature reduction on, for example, an endothermic gas generator down to 950°C (1742°F). For an integral-quench furnace, the burnout temperature would be anything above 800°C (1472°F). This is followed by the removal of the process gas, which, in turn, is followed by the introduction of large volumes of air into the process chamber. The purpose of the air is to ignite the carbon in the surface of the hot-face insulation of the process chamber. This will cause the reversal of the carbon within the refractory to migrate to the hot-face surface and burn. Hence the term burnout.

This can be accomplished by simply introducing high volumes of air into the process furnace. (Some of the furnace equipment today is sold with an integrated air blower to assist in the burnout procedure.) What will be noticed on the furnace temperature controller is that there will be an increase in temperature. This shows that the burnout is proceeding. When the burnout is completed, the furnace temperature will begin to drop back to the original process temperature.

Once the burnout is complete, the process furnace chamber needs to be “conditioned.” This means that the carbon that has been burned out of the refractory is now (to some extent) put back. If one tries to carburize on a newly burned-out furnace, the refractory brick will soak up the carbon from the carburizing gas, and it will be difficult to control the carbon potential of the process gas.

It is common practice to do straight-hardening loads prior to returning to the carburizing conditions, and this is what is meant by the term conditioning of the furnace prior to returning to carburizing. A low-pressure carburize is burned out at a temperature below that of the LPC furnace’s maximum operating temperature. The purpose is exactly the same: to simply burn out the precipitated carbon/soot form the LPC furnace interior.

The question is often asked, how frequently should the procedure be conducted? The answer given by the equipment manufacturers is generally be once per week.