Material selection for solidification structure test of chromium white cast iron by pressure casting technology

The raw materials used in the experiment are two kinds of alloy composition, including low chromium hypoeutectic white cast iron (alloy 1) and high chromium eutectic white cast iron (alloy 2). In order to reduce the difference of alloy composition caused by different heats, each kind of alloy is melted only in one heats, and two kinds of alloy are melted in two heats. Before each pouring, samples were taken in front of the furnace, and the alloy composition was detected by spectrum analyzer. According to the different pressure conditions, the alloy is poured three times at three pressure levels, and the alloy is poured five times at five pressure levels.

(1) Component selection

The composition selection of alloy 1 refers to the common low chromium white cast iron, and the high chromium white cast iron refers to the composition of KmTBCr26 in the national standard of wear resistant cast iron (GB / t8263).

(2) Organization selection

According to the calculation formula of carbon equivalent, the composition of the selected alloy is hypoeutectic alloy, which is far away from the eutectic point and belongs to hypoeutectic low chromium white cast iron; The composition of alloy 2 is close to eutectic point and belongs to near eutectic high chromium white cast iron.

The solidification structure of Cr white cast iron with different compositions can be hypoeutectic, eutectic or hypereutectic. The main reason for the selection of hypoeutectic alloy is that the amount of Cr in the alloy can be very low, which reduces the type of carbide while controlling the cost. When the CR / C ratio of the alloy is small, there is only m3c type carbide in the solidification structure, which is convenient to compare the phase transformation after pressure. At the same time, this type of carbide tends to grow in the form of straight rod, and its morphology is simple and easy to observe. In addition, the primary phase of hypoeutectic alloy is austenite, and carbide precipitates only in eutectic reaction. Using hypoeutectic alloy with low Cr / C ratio in the study, eutectic carbides with single type and shape can be obtained, which makes the study of solidification structure morphology and carbides more intuitive.

There is only eutectic structure in the solidification process of eutectic chromium white cast iron. Different from hypoeutectic and hypereutectic, there is no primary austenite and carbide in the solidification structure, which causes the segregation of solute composition and the solidification latent heat released by primary phase to affect the growth of eutectic structure. In addition, it can be seen from previous studies that the pressure may cause the eutectic point to shift. If the hypereutectic alloy is used, the carbide content may be too much due to the high content of C element, which is not conducive to the mechanical properties of the material. Therefore, selecting eutectic chromium white cast iron as the research object can cover the transformation process of eutectic and hypereutectic two different solidification structures, effectively reduce the workload and save the experimental cost.

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