In Part 3, we continue our discussion of furnace atmospheres.
1. Carbon Dioxide (Chemical Symbol: CO2)
Carbon dioxide (CO2) is one of the reaction products when a fuel is burned in air. CO2 oxides iron at elevated temperatures. To prevent oxidation, it is necessary to have an excess of carbon monoxide (CO). Therefore, CO is a desirable constituent to prevent oxidation. CO2 is not only oxidizing to steel, but it is extremely decarburizing. To prevent decarburization, CO2 must be controlled very closely. The actual amount depends upon the CO content, temperature and the carbon content of the steel. Carbon dioxide is a mildly oxidizing gas and forms oxides with iron at elevated temperatures. At temperatures greater than 540°C (1000°F), the following reaction may occur:
(8) Fe + CO2 = FeO + CO
At temperatures lower than 540°C (1000°F), the following reaction may occur:
(9) 3FeO + CO2 = Fe3O4 + 3CO
Decarburization may also result by the reaction like:
(10) Fe3C+CO2 = 3 Fe + 2CO
(11) C + CO2 = 2CO
At temperatures above 760°C (1400°F), CO2 can react with carbon to produce CO.
2. Carbon Monoxide(Chemical Symbol: CO)
Carbon monoxide is a highly reducing gas. The reversible reaction of CO to form carbon and CO2 is of particular interest in a furnace atmosphere. CO has a high carbon potential and becomes increasingly more stable at elevated temperatures. It is only at lower temperatures in the range of 480-730°C (900-1350°F) that CO will create carbon in the so-called “carbon reversal” reaction (Eq. 12). Carbon in the form of soot can have a negative influence on the heat-treatment process.
(12) 2CO = C + CO2
Methane (CH4), propane (C3H8), butane (C4H10) and propylene (C3H6) are common hydrocarbons.
Hydrocarbons are used both as fuel gas and introduced directly into the atmosphere as a source of supply of carbon in the furnace. Their chemical activity depends on their thermal decomposition and tendency to form nascent carbon at the surface of the steel. Reactions involving these gases are also dependent on the temperature of the furnace and of the workload. Under certain circumstances when hydrocarbons dissociate, free carbon in the form of soot can be created, which acts both as a barrier to slow or prevent carburization (i.e. carbon pickup by the steel) and as an insulator retarding the cooling rate during quenching.
The discussion concludes in Part 4.
1. Herring, D.H., Heat Treating and Atmosphere Generation, Chartered Institute Gas Consultancy Program, Institute of Gas Technology, 1979.
2. Korla. S.C., Atmosphere in Furnaces, Lecture 35, NPTEL (National Programme on Technology Enhanced Learning) , IIT Kanpur, India.