Effect of solidification pressure on growth mode of primary carbide in chromium white cast iron

In alloy 2, the nucleation and growth of primary carbides are in the high temperature liquid phase, and there is no other relative interference, so in a low stress field, there are fewer defects in the primary carbide crystal. On the contrary, eutectic carbides in alloy 1 and alloy 2 grow in high stress field, so there are many crystal defects. The growth mode of primary carbide is obviously different from that of eutectic carbide, and the same is true under pressure. Generally, the morphology of solidified M7C3 carbide is hollow hexagonal structure. With the increase of pressure, the hollow hexagonal structure of M7C3 gradually changes into symmetrical solid hexagonal structure, and there are some stepped grooves on the surface of the head of carbide growth zone.

When the pressure increases, the stress change on both sides of the interface caused by interface bending can be expressed as:

Where:

σ—— Specific interface energy;

R — curvature radius of interface;

V — molecular volume.

The pressure makes the solute distribute unevenly on both sides of the curved surface, resulting in the chemical potential difference, which accelerates the spiral growth of carbide. The exchange of impurity atoms on the surface of the crystal nucleus accelerates the growth of spiral dislocation rotation steps, releases latent heat, and reduces the melting point of the liquid, forming a low melting point liquid phase wrapped in the carbide center, which reduces the low melting point liquid phase region.

Schematic diagram of primary carbide growth

In the process of internal cladding, the degree and size of forming a complete hexagon are also related to the pressure. When the pressure increases, the long-range diffusion of C atom becomes difficult. At this time, the nucleation rate has a greater contribution to the growth of carbide. The critical radius of carbide nucleus is small, the nucleation rate is high, and the coverage area of primary carbide is small and the coverage speed is fast. Therefore, when the pressure is high, the primary carbide of hypereutectic high chromium cast iron will present a regular hexagon with small size as shown in the figure. M7C3 carbide has higher microhardness and obvious anisotropy in transverse direction rather than longitudinal direction. The refined M7C3 increases the probability of the transverse surface on the wear surface, which also contributes to the improvement of the hardness of the alloy.

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