Many years ago, F.E. Harris of MIT was tasked with improving productivity during the early part of World War II. His particular task was to investigate the process of gas carburizing. His work was published in the August 1943 issue of Metal Progress. Harris investigated the aspects of: time, temperature, steel chemistry and control of the furnace carburizing atmosphere.

He very quickly came up with a baseline material that was a plain low-carbon steel. His work took cognizance of the depth measurement, and he used the premise of total case depth rather than effective case depth. The original formula was quite complex. Once it was realized that one was only able to consider carburizing plain-carbon steels, however, the formula was simplified as follows:

Case Depth = Constant √t

“t” is the time at the carburizing temperature, given in hours

The constant value is temperature driven – the higher the process temperature, the greater the value of the constant. In other words, the higher the carburizing temperature, the faster the diffusion of carbon into the steel surface. The formula does not take into consideration alloying elements such as chromium, nickel, molybdenum, vanadium and others.

The constant values are as follows:

• Carburizing at 1550°F = 0.015
• Carburizing at 1600°F = 0.018
• Carburizing at 1650°F = 0.021
• Carburizing at 1700°F = 0.025
• Carburizing at 1750°F = 0.030
• Carburizing at 1800°F = 0.036
• Carburizing at 1850°F = 0.042
• Carburizing at 1900°F = 0.050

The formula does not discriminate according to process – the lower the process temperature, the slower the rate of diffusion. The formula can therefore be applied to lower-temperature diffusion procedures such as nitriding, ferritic nitrocarburizing, austenitic nitrocarburizing and carbonitriding.

Please bear in mind once again that the formula refers only to plain-carbon steels and does not consider alloy steels. Where the formula does not work is if the surface carbon content is less than the saturation of carbon in austenite. The result will be slightly shallower than if at the limit of solubility of carbon in austenite. This means that the atmosphere needs to be accurately analyzed and controlled to produce the appropriate formed case in the steel surface.

The various methods of control of the carbon potential of the process atmosphere are generally used as follows:

• Dew point – The final result using DP usually will take a maximum time of about 10 minutes.
• Shim stock analysis – Slowest method usually will take maximum of 50 minutes to accomplish the final carbon potential of the furnace atmosphere, but it is very accurate.
• Oxygen probe – Very accurate, and the results are given in real time.

It is very necessary that the atmosphere be monitored during the process to establish the carbon potential and so that the temperature can be accurately controlled.