Effect of Prior Heat Treatment and Machining Condition on the Surface Carbon Concentration in Gas Carburized Nb-Bearing Case Hardening Steel

In our previous paper, we reported that the surface carbon concentration (Cs) in carburized Nb-bearing steel (SCM420Nb) decreases substantially with increasing machining speed before carburizing. In the present study, a systematic study was made to clarify the effect of the hardness of starting material and the effect of machining condition (cutting depth, cutting speed, feed per revolution) on the Cs. It was found that the Cs in eleceropolished Nb-bearing steel was reduced from 1.0% to 0.7% with the change with microstructure from Ferrite + Pearlite to Martensite (in other words, with the increase in hardness). It was also found that the Cs in Ferrite + Pearlite Nb-bearing steel decreases almost linearly with the increase in logP (P:cutting power). To obtain normal Cs in high speed machined Nb-bearing steel, two methods were shown to be effective,① removal of severely deformed surface layer by polishing and ② oxidation treatment at about 600ºC in air prior to gas-carburizing. The formation of Cr oxide layer was observed only on the surface of specimens exhibited abnormally low Cs. Thus, it was concluded that abnormally low Cs observed in Nb-bearing steel was due to the formation of Cr oxide layer during gas-carburizing which prevent the carbon atoms enter into steels.