When oxygen diffuses into the steel surface during the carburizing process, it is usually along the austenitic grain boundaries. The origin and source of the oxygen comes from the endothermic process gas plus enrichment gas plus natural dilution air. The process atmosphere is typically made up of hydrogen, carbon monoxide, methane, carbon dioxide and water vapor – from humidity in the air – plus the appropriate enrichment gas.
The type of oxide formation will depend on the alloy content of the steel. The grain-boundary oxides form oxides of chrome, molybdenum and vanadium, etc. The degree to which the oxides form will depend on carburizing process temperature and, of course, the time at temperature.
The grain-boundary oxides will usually be seen simply by pre-grind and polishing perpendicular to the cross section without etching. The IGO will show at approx 800-1,000x magnification as what appears to be cracks penetrating into the carburized-steel surface.
After the unetched examination, the etching of the cut-and-polished surface can take place. The simple choice of etchant is a 3-5% nital etch with a 5-7 second (maximum) soak in the etchant. Intergranular oxidation can be as much as 0.010 inch (0.25 mm) deep, but generally on a case depth of 0.040 inch, it could be expected to see around 0.0015-0.002 inches.
Another feature of grain-boundary oxidation is that there may well be areas of lower-than-expected case hardness through a case-hardness traverse. If the surface hardness is lower than expected (or what is considered to be normal), then the investigation should focus on the atmosphere carbon potential in relation to the steel surface-carbon potential.
Intergranular oxidation can also produce a reduction in the fatigue strength of the carburized steel as well as being a contributing factor to early and premature failure of the carburized surface.
The ability to minimize (not eliminate) the grain-boundary oxidation will be with very careful and accurate control of the process-gas chemistry and carbon potential or to substitute the process gases with a non-oxidizing process system using process gases that do not produce any residual oxygen in the process chamber. Another alternative that can be considered to reduce the risk of intergranular-oxide formation and cracking would be to use steels with an analysis such that the problem is less likely to occur.
The only way to completely eliminate any grain-boundary oxidation is to use the low-pressure carburizing process, but this will incur a cost investment, which means higher process costs. Low-pressure carburizing will generally eliminate the need for careful atmosphere control.
In order to reduce the risk of intergranular oxidation and potential for surface cracking, it is necessary to pre-clean the work surfaces prior to carburizing. Surface contaminants such as cutting fluids can contribute to intergranular oxidation and cracking. A simple weekly check should be performed on the pre-wash unit to ensure that the oil skimmer is functional (if one is fitted) as well as the quality of the pre-wash solution in terms of PH value.
It is essential to good-quality carburizing to ensure that the surface to be treated is completely free of any oxide contamination and other potential surface impurities.