Intergranular oxidation (IGO) is a phenomenon that occurs as a result of gas carburizing due to the process-gas decomposition. The oxygen atom is approximately 35% smaller than the iron atom. The solubility of oxygen in iron is relatively slow, which means a slow diffusion of oxygen into the steel surface during the carburizing procedure.

The rate of diffusion of oxygen into the steel surface will be very dependent on the oxygen potential of the furnace process atmosphere and the process temperature.

During the carburization process, the oxygen atoms that are released as a direct result of the process-gas decomposition begin to diffuse slowly into the steel surface and migrate to the grain boundaries of the steel. Once into the steel, the oxygen will then combine chemically with elements in the steel that have an affinity for oxygen.

Carbon and hydrogen will also diffuse into the steel surface more quickly (faster than the oxygen atoms) to begin to interact with the carbide-forming elements and begin to form the austenite, which will subsequently transform to martensite.

The IGO can be seen microscopically as what appears to be cracks at the immediate surface when the sample is just pre-ground and polished. The apparent cracks are up to 0.005-0.007 inches deep. However, this will depend on the time at carburizing temperature and, of course, the gas analysis.

Formation of the IGO cannot be stopped, simply because of the process gas chemistry. The only way to eliminate the IGO is by removing it at final grind. But then one needs to know what extent the depth of the IGO is.

IGO is a natural result of atmosphere carburizing. The only way that the IGO will not be formed will be with low-pressure carburizing (vacuum carburizing), providing that there are no oxygen-bearing materials being carried into the process chamber. With vacuum carburizing, there is simply no IGO because there is no oxygen in the process gas.