Additionally, another interesting phenomenon is that
the size of Cr3C2 skeleton and pores in the cross section image is smaller
than that in the surface image. The variance is from the environment
and time for the grain growth of Cr3C2 during the carburization. The
Cr2O3 on the surface will prefer to react with the carbonaceous gas. If
the carburization reaction for the Cr2O3 on the surface is initiated, it
will have enough time to grow to big size. However, for the Cr2O3 beyond
the surface, the liberated [C], which is from the decomposition of
CH4, has to transfer to the reaction interface through surface of the carburization
reaction. Thiswill decrease the carburization chance of Cr2O3
and reduce the growth time of Cr3C2 grain.