2013 Volume 99 Issue 11 Pages 642-650
In our previous paper, we reported that the surface carbon concentration (Cs) in carburized Nb-bearing steel decreases substantially with increasing the machining speed before carburizing. In the present study, a systematic study was made to clarify the effects of Cr,Nb addition and machining speed on the Cs. Cr oxide film was observed on the surface of specimens exhibited a large decrease in Cs. The coverage of specimen surface by Cr oxide film was increased with increasing Cr,Nb content and machining speed. Since Cs decreased with the increase in the coverage of Cr oxide film, it was considered that the decrease in Cs is due to Cr oxide film formed during carburizing. The carbon concentration profile was calculated assuming that carbon absorption was ceased after the complete coverage of specimen surface by Cr oxide film. It is confirmed that the present calculation fits well with the measured carbon concentration profile of low Cs. Moreover, it is shown that Cs becomes smaller as time period for Cr oxide film formation becomes shorter. The role of Nb is considered to be the retardation of recovery, recrystallization and grain growth through the pinning effect of Nb(CN) during carburizing. The high density of lattice defects introduced by machining is maintained by Nb(CN) and promotes faster Cr diffusion. In the present study, it has been clarified that in machined specimens of Nb-bearing steel, entire coverage of specimen surface by Cr oxide film during carburizing occurs in steels containing even a small amount of Cr like 1%.
