In Part 1, we began a discussion of furnace atmospheres that continues here.
1. Oxygen (O2)
Oxygen reacts with the iron in steel to form iron oxide (FexOy) and also reacts with carbon to lower the carbon content of steel. Oxidation of iron occurs in the presence of oxygen and increases in severity as the temperature is raised. In addition, oxygen will decarburize steel. If steel is to be kept bright during heat treatment and free of decarburization, oxygen (O2) should be avoided.
Oxidation in air involves reactions that are irreversible and, in general, not controllable due in part to the fact that the relative humidity changes constantly, affecting the oxygen potential. The result of thermal oxidation in air is almost always unsatisfactory with respect to thickness and composition uniformity.
(4) 2Fe + O2 → 2FeO
(5) 4Fe + 3O2 → 2Fe2O3
(6) 3Fe + 2O2 → Fe3O4
2. Water vapor (H2O)
Dew point is a temperature at which gas is saturated with water vapor at 100% relative humidity. In other words, dew point is a measure of the amount of moisture present in a gas (Table 1) and quantifies the concentration of water vapor in the atmosphere. In a furnace atmosphere, the water‐gas reaction (Equation 7) relates the concentration of H2, H2O, CO and CO2 in the atmosphere.
(7) CO + H2O = CO2 + H2 (water-gas reaction)
Dew-point analyzers look at the H2O/H2 ratio in the water-gas reaction. Infrared and oxygen-probe analyzers look at the CO/CO2 ratio in the water-gas reaction.
Water vapor (H2O) is a strongly decarburizing gas and, therefore, any constituent such as CO2 will have a tendency to form water vapor. Thus, CO2 must also be closely controlled. In addition, to prevent decarburization by water vapor, the carbon monoxide (CO) and hydrogen (H2) must be present in amounts to satisfy the equilibrium condition at each temperature.
Water vapor and carbon dioxide oxidize and decarburize steel. Hydrogen is formed when water vapor oxidizes iron. Therefore, to prevent oxidation and to keep iron bright, a definite excess of H2 over H2O vapor is required for each temperature. Water vapor present in a furnace atmosphere is oxidizing to iron and also combines with carbon in the steel.
The discussion continues in Part 3.