Effect of solidification pressure on eutectic structure of cast chromium white cast iron

The content and spacing of eutectic structure, the content and distribution of different phases in eutectic structure of two kinds of chromium white cast iron under Solidification pressure were studied.

(1) With the increase of solidification pressure, the eutectic structures of alloy 1 and alloy 2 are obviously refined. The eutectic structure of alloy 1 # changes from the larger lath structure to the finer one, showing short rod shape and equiaxed shape; In the solidification structure of alloy 2, massive and polygonal primary phases appear, and the eutectic structure changes from lath to curved panel, short rod and polygonal; No matter what kind of alloy, the distribution of phases in eutectic structure becomes more uniform.

(2) The eutectic spacing of solidification structure of Cr white cast iron decreases obviously with the increase of solidification pressure. When the solidification pressure is increased to 100 MPa, the eutectic spacing of the alloy decreases by about 30%, but when the solidification pressure is increased to 170 MPa, the eutectic spacing is almost the same as that of 100 MPa; When the solidification pressure is increased to 160 MPa, the eutectic spacing of alloy 2 decreases by about 50%, and the size distribution of eutectic phase is more uniform.

(3) The content of primary phase and eutectic structure in alloy 2 fluctuate with the increase of solidification pressure. With the increase of pressure, the primary carbides repeat from nucleation to growth to nucleation. With the increase of nucleation rate, the content of eutectic structure increases. With the increase of primary carbides size, the content of eutectic structure decreases.

(4) Based on the deduction of the quantitative relationship among the pressure P, △ T and RS in the process of eutectic solidification under pressure, it is concluded that with the increase of pressure, the overall undercooling △ t of eutectic solidification increases and the radius rs of eutectic dendrite tip decreases; The pressure increases the undercooling at the front of the unstable solid-liquid interface, leading to the breakthrough of plane growth, which makes the growth mode of austenite and carbide in eutectic structure change from parallel to alternate competitive growth.

(5) The effect of pressure on Solidification Microstructure of Cr white cast iron decreases with the increase of solidification rate. The effect of pressure on the morphology (eutectic spacing, eutectic volume and primary phase size) of the solidified structure is relatively small in the structure close to the die, i.e. with high solidification rate. In the solidification process of pressure casting Cr white cast iron, increasing the casting pressure can not change the heat field and heat flow in the solidification process. Therefore, increasing the preheating temperature of the die to reduce the cooling rate at the edge of the specimen is more helpful to obtain the ideal solidification structure of chromium white cast iron than only increasing the casting pressure.

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