Furnace atmosphere burnout is a controversial subject among both captive and commercial heat treaters.

This is a discussion on furnace burnout and the reasons for the process of burnout, particularly in a carburizing furnace (such as an integral-quench furnace), an endothermic gas generator and low-pressure carburizing furnaces.

Why is furnace burnout needed?

The general carbon potential with a heat-treatment furnace is approximately 0.8-1.00% C. The endothermic gas generator is generally operating around 0.30% carbon potential (depending on the steel being carburized and thermally treated).

The insulation construction within the furnace comprises of the hot face (process chamber) of 4.5-inch-thick refractory brick (typically 2200°F brick). This is backed by a similar-sized refractory brick of a slightly lower grade than the hot-face brick. The complete refractory brick is further backed to the furnace casing with low-mass ceramic fiber, which is both fibrous and porous.

The objective of the refractory insulation is to keep the furnace atmosphere inside of the process chamber, maintain as much of the thermal energy in the process chamber and to keep the cold face (outside steel casing) to a maximum temperature of 150°F.

The refractory brick or hot-face low-mass ceramic fiber is porous. This means that there will be a temperature gradient from the hot-face chamber insulation to the cold-face furnace casing. In addition, there will be a diffusion of the process atmosphere gas from the process chamber into the refractory material.

As the carbon-rich gas diffuses into the refractory brick, the gas naturally cools. This means that the carburizing gas cools within the refractory, and carbon will begin to precipitate into the refractory.

This means that a solid mass of soot/carbon will begin to precipitate within the refractory. This will cause a change in the refractory brick insulation conditions. The carbon buildup within the refractory can also create the potential for electrical tracking of the heating-element electrical input and perhaps cause electrical shorting.

A cleaning operation is required in order to correct this problem. This is the “burnout,” and we will discuss it further next week.