It can be
noted that the formation of carbides is more uniform in the fine grain samples compared with the coarse ones. The carburized layer
in the PC sample is obviously thicker. The grain microstructure condition,
treatment condition, carburized layer thickness and surface
hardness for the samples are summarized in Table 2. It shall be
noted that the PC sample has the thickest carburized layer and
highest surface hardness, with a value of 64 μm and 1512 HV respectively.
The introduction of force to the surface of the carburized
material is expected to deform the asperities plastically, which consequently
accelerates diffusion of carbon atoms into the material. This
eventually leads to a thicker carburized layer and higher surface hardness
for the material. Although the fine grain microstructure also contributes
to the increment in carburized layer and surface hardness
(comparing samples C and D), it is notable that the effect of superplasticity
is more pronounced (comparing sample D and E).
Fig. 4 shows high and low magnification of the carburized layer for
fine grain DSS. The cross-section microstructure consists of a mixed
phase structures, particularly near surface region. High carbon supersaturation
would be expected at the upper part of layer. At