The relationship between pressure and solute diffusion coefficient can be described as follows
DP — solute diffusion coefficient under pressure p;
R — gas constant;
T — temperature (k);
P — pressure (PA);
V0 — initial volume of liquid phase (M3).
According to the formula, the solute diffusion coefficient decreases exponentially with the increase of pressure. The experimental pressure value of pressure casting chromium white cast iron is in the order of MPa (MPA = 106pa). The ratio of diffusion coefficient after pressing to that before pressing is about 10-3, which indicates that the pressure strongly inhibits the diffusion of alloying elements.
The formation of eutectic structure is analyzed from the perspective of solute diffusion. As shown in the figure, Fe is enriched in carbide front liquid phase and Cr is enriched in austenite front liquid phase in chromium white cast iron. From the point of view of transverse diffusion, Cr is discharged from austenite and diffuses to the front of carbide during atmospheric pressure solidification. However, the solute diffusivity is very low under pressure, which makes the process difficult, and the enrichment of Cr in the front of austenite is intensified. The path of Fe and Cr is opposite, and the Fe atoms discharged from the carbide can not reach the austenite front smoothly under pressure. The enrichment of atoms leads to less Fe in the front of austenite and less Cr in the front of carbide.
In order to explain the formation of eutectic phase in chromium white cast iron by longitudinal diffusion, it can be concluded that the solute concentration at the front of solid-liquid interface is much thicker than that at normal pressure due to the decrease of solute diffusion coefficient due to the effect of pressure. The increase of liquidus temperature leads to the increase of the difference between liquidus temperature and actual temperature, that is, the increase of pressure widens the range of composition supercooling zone in the front of solid-liquid interface. Therefore, the increase of pressure can promote the growth of dendrite.