The rate of diffusion of carbon into steel while in the austenite phase is concerned with carbon in solid solution in austenite. The diffusion rate of the carbon into the austenite phase is based on:
• Selected process temperature
• Selected process time at temperature
• Composition of the steel to be carburized
The induced carbon will diffuse into the steel surface where the surface generally maintains the highest carbon level, and all the carbon below the steel surface will be diluted because of the diffusion. This means that there will be a carbon gradient from the steel surface to the transition zone (that is the area below the effective case depth). This further means that the carbon gradient diffusion will be dependent also on the steel surface carbon potential.

This means that we now have an additional control aspect of carburizing. The required surface carbon potential to transform to martensite will be further dependent on steel selection and enrichment gas carbon potential.

Generally, one is looking for a surface carbon content between 0.80% up to an arbitrary figure of 1.30%. A further diagram is necessary to be consulted, and that is the limit of solubility of carbon in austenite in iron.

The graph shows the relationship of surface carbon potential in relation to a selected carburizing process temperature and in relation to control parameters such as dew point, EMF millivolt signal and CO in relation to CO2.

At a given temperature, the rate of carbon diffusion will increase as the temperature is increased with increasing carbon concentration. The core carbon potential of the steel being carburized is what will determine the core hardness (plus whatever alloying elements are present in the steel composition).

The calculation method of case-depth determination is based on the classical simplified formula of:
Total case depth = √ t x f

Where f is a temperature-driven factor and t is time in hours. The value of the factor f is as follows:

1600°F = 0.018”
1650°F = 0.021”
1700°F = 0.025”

Because carbon has mass, the weight of absorbed carbon can be calculated based on surface area and total depth of diffusion. This can then be a very useful control parameter for the carburizing procedure. Once the control parameter has been established (dew point, EMF, etc.) and if the observed value exceeds the line of saturation, then sooting is likely to occur within the furnace process chamber.

It is necessary then to monitor and control the enrichment gases such as the carrier gas and the enrichment gas itself. It is further necessary to control the resulting created gases such as CO, CO2, nitrogen, hydrogen and water vapor.

Once the process and control parameters are established, accurate control of the carburizing process can be achieved. Bear in mind that all of the above relates to atmosphere carburizing and not to low-pressure carburizing simply because one is working at partial pressures for carburizing. This will require a process-control system that is based upon surface area in relation to part geometry.