As far back as 1943, F.E. Harris developed a formula for the calculation of time in relation to temperature for the depth of carbon penetration into steel during the carburizing process. He presented his findings, which were published in Metal Progress August 1943. The formula was somewhat complex and it was simplified to the following:

Total case depth = √t x f

where t = time and f is a temperature driven factor

Remember that the formula is for total case depth and not effective case depth and also that the formula is based on plain-carbon carburizing steels and not alloy steels. However, it can serve as a guide to obtaining the required case depth.

Remember to also consider the time that the steel being carburized is at the selected carburizing temperature. Extended period of time at elevated carburizing temperatures will promote grain growth.

The formula developed by Harris has led to many different methods of carburizing, each one being particular to both the steel composition and the part geometry:
  • Carburize with a constant process-gas flow with enrichment gas for the carburizing
  • Boost and diffuse – This process involves initially boosting the atmosphere carbon potential up to say 1.30% and turning off the enrichment gas once that atmosphere carbon potential has been achieved. Then the surface carbon is allowed to diffuse down to the required case depth, bearing in mind the required surface carbon content necessary for the component to transform the surface into martensite.
  • Pulsed boost and diffuse – This means allowing the atmosphere carbon potential to boost (say to 1.10% CP) and then shut off the enrichment gas for a predetermined length of time. This will assist in carburizing into blind holes. As the gas pressure in the process chamber increases with flow, it will flush out stagnated gas (such as can collect in a blind hole) and replace it with fresh enrichment gas. This procedure will carry on for the length of the process cycle at the carburizing temperature.
There are “many variations on a theme” (so to speak) for the atmosphere carburizing procedure.

How does one check the process-gas carbon potential within the process chamber of atmosphere carburizing systems? There are a number of simple methods of establishing the carbon potential (CP):
  • Dew point is one simple method of establishing the CP simply by sampling the process gas from the process chamber. The process gas will contain moisture (water gas equation), and it is then possible to evaluate the process carbon potential.
  • Shim stock analysis – This method relies on the gain in weight of a shim of a known carbon potential, if the furnace is in a carburizing condition. It is simply a shim (known carbon and known weight) that is inserted into the process chamber through a gate valve located on the side of the carburizing furnace. The shim is left in the atmosphere to carburize for, say, 30 minutes. It is then removed, washed and reweighed. If the furnace is carburizing, there will be a gain in weight. Conversely, if the furnace is decarburizing, there will be a loss of weight (due to the shim decarburization)
  • The analysis system that will give a result in real time (the two methods above, although accurate, do not give a real-time result) is the oxygen probe. This is a very accurate and real-time control parameter with the carbon potential (or millivolt) that is displayed on the control instrument.
The above methods of control do work, and each in its own right is an effective control method for process-gas carbon-potential evaluation.