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 process gas, which is endo plus enrichment gas plus natural dilution air. The process atmosphere is typically comprised of hydrogen, carbon monoxide, methane, carbon dioxide plus water vapor (which comes 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 boundaries 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 can usually be seen simply by pre-grind and polishing followed by etching. The etchant is a 3-5% nital etch with a five-second soak in the etchant maximum. The intergranular oxidation (IO) can be as much as 0.010 inch (0.25mm) deep, but generally on a typical case depth of 0.040 inch, IO could be expected to be around 0.006-0.008 inch (0.15-0.20mm).
A resultant crack may propagate deeper without it being made visual with the etchant. Another feature of the grain-boundary oxidation is that there may well be areas of lower-than-expected case hardness through a case-hardness traverse. The intergranular oxidation can also produce a reduction in the fatigue strength of the carburized steel.
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. Alternatively, process gases could be substituted with a non-oxidizing process system using process gases that do not produce any residual oxygen in the chamber. Another alternative that can be considered to reduce the risk of IO formation and cracking would be to use steels with an analysis such that the problem is less likely to occur.
The only way, of course, to completely eliminate any grain-boundary oxidation is to use the low-pressure carburize process, but this will incur a cost investment, which means higher process costs.
In order to reduce the risk of IO 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 the IO and cracking. A simple weekly check would be on the pre-wash unit to ensure that the oil skimmer is functional (if one is fitted) as well as checking the pH value of the pre-wash solution. It is essential to good-quality carburizing to ensure that the surface to be treated is completely free of any potential oxide contamination and other surface impurities.
What Promotes Internal Oxidation While Gas Carburizing?
By David Pye
David Pye is the owner and operator of Pye Metallurgical International Consulting, Saint Anne's on Sea, Lancashire, U.K. He has 25 years of practical experience in captive and commercial heat treatment, metallurgical laboratory operation and industrial furnace sales. He also has teaching experience on a very wide range of heat-treatment and metallurgical subjects.
He can be reached at firstname.lastname@example.org.
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