Fig. is a picture of the effect of modification on the microstructure of as cast high chromium cast iron: Fig. a and Fig. b are the as cast microstructure of high chromium cast iron without modification agent respectively; Figure c and figure d respectively show the as cast structure of the modified high chromium cast iron.
It can be seen from Fig. 1a and Fig. 1b that the typical as cast microstructure phases are austenite and eutectic carbide. A large amount of chromium element causes the morphology of network carbide in the original cast iron to change, and its eutectic presents a cluster Daisy shape. Carbide fibers grow along the center to the boundary and gradually become coarse. Especially on the boundary of the crystal zone, it is observed that the transverse section of the carbide grows into a thick hexagonal block, while its longitudinal section grows into a continuous rod. A small number of austenite or austenite transformation are surrounded by carbides with a center similar to the hexagonal block, so there are many holes in the center.
It can be seen from Fig. 1c and Fig. 1D that the modifier has a very obvious refining effect on the structure of high chromium cast iron. The chrysanthemum like carbides in the eutectic group almost disappear, but small and isolated carbides are uniformly distributed, but the hexagonal massive carbides still exist. Because high chromium cast iron with near eutectic composition is used in this test, and the composition is uneven during solidification, some areas show the solidification characteristics of hypoeutectic high chromium cast iron, while some areas show the solidification characteristics of hypereutectic high chromium cast iron. Therefore, the grain refinement mechanism can be analyzed from the following three aspects:
- refining effect on primary austenite: under certain thermodynamic conditions, rare earth surface active elements will react with s, O and other elements to form a phase with high melting point and stable chemistry. As a heterogeneous nodule, this micro phase increases the solidification nucleation rate, and the primary austenite nucleation increases, so the primary austenite structure can be refined, The refinement makes the tendency that the residual molten iron is separated in the subsequent eutectic reaction process further strengthened, and the refinement of eutectic structure is also realized;
- refinement of primary carbides: the non-ferrous metal Ti in the modifier reacts with C to form a compound with a coherent relationship with (Cr, Fe) 7C3, which can be used as the crystallization core of (Cr, Fe) 7C3. During the solidification and cooling of the solution, a large number of (Cr, FE) 7C3 small particle crystals are formed.
- refining effect on eutectic carbides: carbides (Cr, Fe) 7C3 will grow preferentially along [0001]h, and grow in fibrous form during eutectic solidification, and finally show Daisy cluster eutectic clusters. However, after the composite modifier is added, the highly active rare earth elements spontaneously segregate in the liquid phase in front of the growth trend of eutectic carbides during eutectic reaction, and the liquid complex, iron, carbon and other atoms are blocked from entering the eutectic of carbides smoothly. The growth rate in the preferred direction is greatly reduced, the rare earth elements are successfully inhibited, and the type eutectic carbides tend to grow, In addition, it can also promote the growth of eutectic austenite dendrites around the carbides, and limit the space for the growth of carbide fibers. Therefore, the chrysanthemum shaped eutectic clusters disappear and are replaced by dispersed, isolated and small carbides.