Abnormal Low Surface Carbon Concentration in Gas Carburized Nb-Bearing Case Hardening Steel after Severe Machining

The surface carbon concentration (Cs) of carburized Nb-bearing steel (SCM420Nb) was found to decrease substantially when carburized after severe machining. It was observed that the effect of machining condition is small in normal case hardening steel (SCM420), in which the Cs was in the range of 0.9~1.1%. However, the Cs in carburized SCM420Nb was reduced to 0.61% by mild machining, and 0.38% by severe machining. Since the Cs of severely machined SCM420Nb recovered to > 0.8% after the removal of surface layer by polishing, it was concluded that the abnormal decrease in Cs of SCM420Nb is mainly due to severely deformed surface layer produced by machining. Discussion was made on the abnormally low Cs observed in severely machined SCM420Nb steel with regard to the effect of solute Nb on the carbon activity and the effect of surface roughness. It was found that these effects are small compared with the observed reduction in Cs. Comparing the experimental and calculated carbon concentration profiles, Cs was expected to be reduced immediately after the start of carburizing. Finally, it was proposed that the oxide layer created on the surface during carburizing might be the cause of the abnormally low Cs.