Directional solidification technology for solidification structure of chromium alloys

Directional solidification (UDS) is a method to produce eutectic composites or in-situ composites by applying high directional temperature gradient to the specimens during alloy solidification. UDS has the flexibility to adjust the solidification rate (R), solid-liquid interface temperature gradient (GL) and alloy composition (C0). It is a unique technology to study the solid-liquid interface behavior during alloy solidification. With this technology, the growth, segregation and interface migration of materials can be better studied, in order to have a better understanding of the solidification process of materials.

In addition to the type of carbides, the orientation of carbides is also the main structural feature affecting the abrasive wear properties of white cast iron. Liming Lu prepared Fe-Cr-C eutectic alloy with industrial raw materials. A new type of fiber reinforced eutectic composite was prepared by UDS technology. The strength of the composite reached 2300 MPa, which was about 7 times of that of high chromium cast iron with the same composition. Under the conventional solidification conditions, the eutectic of high chromium cast iron grows uniformly in all directions, and finally the flower like structure is obtained (as shown in Fig. (a)). Under the action of UDS, the eutectic group grows preferentially in the direction opposite to the temperature gradient, forming a parallel fiber structure, as shown in figure (b). By comparing the relationship between fiber morphology and solidification rate (R), it is found that fiber diameter, spacing and regularity decrease with the increase of solidification rate, as shown in figure (c). With the increase of solidification rate, the diameter, spacing and regularity of the fibers decrease gradually, and the volume fraction of the fibers keeps about 32%. The results show that the tensile strength of Cr eutectic composite changes parabola with solidification rate, and the maximum strength is 2300 MPa. When the solidification rate is 58 mm / h, the strength level of unidirectional solidification composite is about 7 times that of ordinary high chromium cast iron with the same composition.

Because UDS preparation technology usually needs both heating and cooling functions to meet the required temperature gradient, the equipment is more complex, and the experimental parameters are not easy to control, so it has not become a common method to control the solidification structure in engineering. Although people try to understand the use of UDS technology to prepare chromium eutectic composites fundamentally, more knowledge is still needed to promote the practical industrial application. In addition, the master alloys used in the past UDS technology are all made of high purity materials and special smelting process. However, in the actual solidification process, impurities will accumulate on the S / L interface, which may have an important influence on the structure of the obtained fiber, which is not conducive to obtaining the ideal solidification structure.

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