Grain-boundary oxidation or intergranular oxidation (IGO) is caused as a direct result of oxygen diffusion into the surface of the steel. The source of this oxygen is the decomposition of the furnace atmosphere. The objective of the gas decomposition is to produce an approximate atmosphere composition from the endothermic gas generator of:

• 20.0% CO
• 1.0% CO2 (max)
• 2.0% Ch4
• 0.80% moisture (water vapor)
• 40.0% H2
• 36.20% nitrogen

At the process furnace, the hydrocarbon enrichment gas is added for the carburizing procedure, and the carbon potential of the atmosphere is controlled according to the steel being carburized and the final surface carbon requirement.

The gases that will cause the IGO are CO2 (carbon dioxide) and water vapor (H2O).

These gases will provide the oxygen source to diffuse into the steel surface. In addition to the oxygen diffusion, the oxygen will react with the alloying elements that are in solution to cause further oxides to develop.

The size of the oxygen atom in relation to the iron atom is 1/3 smaller, and it will therefore penetrate into the lattice structure very slowly. The depth of penetration will depend on:

• The selected process temperature
• The time at the selected process temperature
• The steel’s alloying elements
• The process gas composition

A typical depth based on a process temperature of 1700˚F (930˚C) and a cycle time of seven hours at temperature would be 85 microns.

IGO cannot be seen on the surface of the carburized component. It can only be seen by a destructive cut followed by polish and then observed microscopically at a magnification of 400-500X.

The single most effective method of stopping IGO is low-pressure carburizing.

The problem that IGO is likely to create if it is not addressed is that the surface is a potential crack propagation site. To remove the surface IGO, the depth will need to be determined by destructive examination of a component or an appropriate test coupon. Surface stock can then be removed by a grinding operation.