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
carburizing stage, the austenitic grains (γ) are transformed to carbon
expanded austenite (γ′) while ferrite grains (α) are first transformed
to austenite and then changed to carbon expanded austenite due to
carbon diffusion. Carbide precipitates appear in the layer when the
solubility of austenite is above the boundary solubility in austenite.
As can be seen in the figure, the carburized layers produced below
the critical temperature are free from precipitation and contain supersaturated
carbon. On the other hand, the layers produced above
the critical temperature revealed a mixed phase structures, particularly
in the upper part of the layer. The cooperation with the existence
of coagulated and uniformly distributed carbides gives a high hardness
on the surface.