(1) Solidification structure of specimens at different positions under atmospheric pressure
The solidification structure of alloy 2 under normal pressure is shown in Fig. 1. The samples at different positions have similar composition, which is composed of primary carbide and eutectic structure. The number of primary carbides in the upper and bottom samples is more than that in the middle samples. The tips of primary carbides in the upper and middle samples are sharp, while the bottom samples are round and blunt. In addition, the sizes of primary carbides are relatively large, and there is no obvious difference except for a few smaller ones in the bottom sample.
The difference of eutectic structure in the sample is obvious. The proportion of eutectic in the middle part of the sample is larger than that in other parts of the sample, so is the eutectic spacing. The white eutectic carbides in the eutectic structure are mainly equiaxed, short rod and curved plate strip in the top sample, while in other samples, they are mostly long laths or interconnected into a network.
(2) Effect of solidification pressure on solidification structure of middle sample
Fig. 2 shows the solidification structure of the specimen in the middle of the test block under different pressures. Generally speaking, the solidification structure of the middle sample is composed of primary carbide and eutectic structure. The size and quantity of primary carbide are relatively large when the pressure is less than 100MPa. With the increase of pressure above 100 MPa, the primary carbides change from long strip to polygon, but the amount does not increase significantly. Compared with the samples at other positions, the size of primary carbide does not decrease significantly under pressure, and there is a significant size difference compared with eutectic carbide. The results show that the eutectic structure in the middle part of the sample is refined obviously under the action of pressure, and the eutectic spacing and the ratio of grain length to grain diameter are reduced obviously. The morphology and distribution of eutectic carbides changed from strip shape to fine equiaxed shape, and the distribution tended to be uniform. In conclusion, the effect of pressure on the solidification structure of the middle sample is very obvious. When the solidification rate is a certain value and small, the pressure can optimize the solidification structure of chromium white cast iron. But for the primary carbide, even increasing the pressure is not very effective for its size refinement under the condition of small cooling rate.
(3) Effect of solidification pressure on solidification structure of bottom sample
Fig. 3 shows the solidification structure of alloy 2 at the bottom of the test block under different pressures. The samples of Cr white cast iron at the bottom have finer solidification structure than those at other positions, which is also composed of primary carbide and eutectic structure. Among them, primary carbides appear in a large number in the solidification structure without pressure, and show smaller polygon. With the increase of pressure, the size of primary carbide further decreases, but it does not continue to decrease with the increase of pressure. When the pressure is 160 MPa, there is no significant difference even with the non pressure. The number of primary carbides under pressure is completely different from that at other locations. The increase of pressure does not make the number of carbides increase significantly. The eutectic structure of the bottom sample does not change significantly in the state of no pressure and pressure, and the eutectic spacing does not decrease with the pressure, and the eutectic distribution is relatively uniform.
