Figure 1. Schematic representation of carbon transport in carburizing[1]

I have always been confused by the term “carbon flux” and how it relates to the carburizing process. Can you explain this in laymen’s terms and provide a short explanation of the various factors that influence carburizing?

An important characteristic of gas carburizing is that the carbon content of the case can be controlled by suitable adjustment of the gas composition. In simplest terms:
  • Carburizing Thermodynamics asks, “Will carburizing take place?”
  • Carburizing Kinetics defines “the rate at which carburization will happen” (and the mechanisms by which carburizing reactions will occur).
The carburization of steel is based on two fundamental concepts:
  • Carbon flux – the transfer of carbon to the steel surface from the source supplying the carbon
  • Diffusion – the movement of carbon into the steel itself (influenced by the properties of the steel)
Let’s consider the mechanism of carburization from the standpoint of these fundamental concepts. The controlling factors are:

1. Factors that control the flow of carbon into the steel:
a. Concentration limits
b. Carbon gradient
c. Case depth
d. Added carbon
e. Rates of carbon demand
f. Diffusion coefficients
g. The ferrite phase
h. The cementite phase

2. Factors that influence the transfer of carbon to the steel surface:
a. The carburizing reaction
b. Atmosphere conditions

In other words, the three stages of carbon transfer during the process are:
1. Carbon transport from the atmosphere through the boundary layer
2. Chemical reaction at the steel surface
3. Carbon diffusion into the bulk of the material

Total carbon transfer from the atmosphere to the steel is thus determined by the limiting process, which kinetically becomes the rate-controlling stage of carburizing. Figure 1 schematically shows the mechanisms of carbon transfer during carburizing and the primary control parameters: the mass transfer coefficient (β) defining carbon atoms’ flux from the atmosphere to the steel surface and the coefficient of carbon diffusion in steel (D) at austenitizing temperatures.

Finally, the primary function of a carburizing atmosphere is to provide an adequate supply of carbon to the steel surface. Reactions occurring at the steel surface are influenced by:

1. The carburizing agent (hydrocarbon) responsible for the actual transfer of carbon.
2. The extent to which various carbon compounds may supply carbon is related to the conditions that determine the equilibrium with the surface carbon of the steel. That is, the equilibrium composition of the reactants and products is a function of temperature and also the carbon concentration (gradient) at the surface. The equilibrium composition is, therefore, a condition involving transfer or flow of carbon.
3. The maximum availability of carbon in a given hydrocarbon (carbon compound) represented by the excess above the composition of the steel surface. The availability of a given agent decreases with increasing surface concentration and is lowest at the upper limit of carbon solubility (saturated austenite).