Is there any way to reduce the risk of grain-boundary oxidation (GBO) during the carburizing procedure? There is now a method of atmosphere carburizing that will eliminate the risk of GBO other than low-pressure carburizing. This method of carburizing will take away the risk of intergranular oxidation and the progressive cracks.

The gas decomposition chemistry using propane will produce the following decomposition occurrence into methane and ethylene and further into methane, carbon and hydrogen (note the complete lack of oxygen).

 C
HCH+ CH₄→CH +2C +2H

Hence, intergranular oxidation and cracks are absent simply because there is no oxygen present or allowed to develop. Surface oxide or scale will be present only if it was not pre-machined away prior to the low-pressure carburizing procedure.Low-pressure carburizing will take away the risk of GBO and the progressive microcracks.

Salt-bath carburizing will also eliminate the risk of GBO and surface microcracks. But, once again, the carburized steel must also be clean of any prior surface oxide such as scale.

With atmosphere carburizing, one can reduce the GBO formation and the microcracks by paying close attention to the carburizing procedure.

  • Reduce the carburizing temperature (but then the process cycle times are extended).
  • Use a high atmospheric carbon potential. This will reduce the volume of carbon dioxide produced. Care must now be taken in controlling the atmosphere carbon potential in order to not over-carburize the newly formed case and create the potential for retained austenite and excessive surface carbide formations.
  • Lack of control on the formation of the carbon monoxide in atmosphere carburizing will allow the occurrence of GBO.