What does it mean when we hear the term “internal oxidation” when carburizing? The principle cause of internal oxidation is the diffusion of oxygen into the grain boundaries of the steel being carburized. This is also known as grain-boundary oxidation.

Internal oxidation and grain-boundary oxidation can generally only be seen by microscopy (say 500X or greater magnification). The phenomena cannot be avoided when gas or pack carburizing. It does happen, and it will happen.

The depth of the penetration of oxygen around the grain boundaries is dependent on:
  • Prior grain size
  • Carburizing process temperature
  • Time at carburizing temperature
  • Steel chemistry
  • Process gas analysis (carrier gas as well as enrichment gas chemistry)
If the process is gas carburizing, the control of carbon dioxide (CO2) is critical. The higher the CO2, the greater the risk of grain-boundary oxidation.

The grain-boundary oxide formation can contribute to the formation of micro cracks occurring just below the surface of the carburized case. However, the root cause of the microcrack formation is not necessarily as a result of the formation of grain-boundary oxidation.

Grain-boundary oxidation will occur just below the surface of a carburized martensitic case. The grain-boundary oxidation may lead to microcracks occurring at the surface due to the high surface carbon concentration (0.80% plus).

When using atmosphere carburizing techniques that employ an endothermic gas generator system, the reduction of grain-boundary oxidation can only be reduced and not eliminated. So what can be done about grain-boundary oxidation?

The problem can only be controlled and not eliminated, so one can only employ strict process-control methods for the carburizing procedure, including:
  • Control of incoming steel
  • Control of prior grain size
  • Careful control of the gas analysis dew point from the endothermic gas generator
  • Careful control of the gas analysis dew point from the enrichment gas analysis with low CO2 values
  • Process time
  • Process temperature
  • Part surface cleanliness
  • Mechanical removal (machining) of the steel surface after carburizing
The only way to eliminate internal grain boundary oxidation is to use low-pressure carburizing techniques or plasma carburizing techniques.

It is known that the industrial gas companies are working on the development of an oxygen-free process-gas delivery system that will require only the retrofitting of the gas flow control panel, meaning the atmosphere carburizing furnace will not be a redundant or obsolete piece of equipment.

It will, of course, depend on the metallurgical requirements of the component being carburized as to which method of carburizing is employed based upon whether we can live with the grain-boundary oxidation.