I am very interested because we are having similar problem in one of our plants. We have reduced the occurrence of this problem by improving our pre-washing system (degreaser) and instituting better control of our machining coolants (soluble oils and other oils). We are now looking into mechanical cleaning before heat treating (polishing, green shot peening, etc.) since we suspect that a mechanical barrier is forming during turning (we are using ceramic inserts), preventing carbon penetration.
AnswerSome cutting fluids (particularly those that are boron-based) are tenacious and, if allowed to dry on the surface of a part, pose a difficult cleaning problem. If not fully removed, they have the potential to retard carburization. These fluids often in combination with cleaning systems (aqueous or solvent) that aren't properly maintained result in surface residues, which produce areas that will not properly carburize. Degreasers are especially suspect if the fluid is not properly filtered or replaced when contaminated.
There have also been reported cases where certain stop-off paints that have not been allowed to properly dry, under certain conditions, "blistered" or "burst," spreading contamination to adjacent areas. I have heard of instances where mechanical barriers from ceramic inserts (such as you reported) or tumbling medium have also caused carburizing problems.