Effect of solidification parameters on Microstructure of cast chromium alloy

The effects of pressure on solidification structure, including nucleation rate, undercooling and diffusion coefficient of alloy elements, provide a good theoretical and practical basis for this study. Therefore, it is a new method to improve the solidification structure of chromium alloy by pressure casting.

The morphology and distribution of carbides are the decisive factors affecting the impact wear resistance of white cast iron, including primary and eutectic carbides. When the carbides are evenly distributed in the matrix, the properties of Cr based alloy are the best. However, the transformation of carbide morphology directly depends on the roughness or smoothness of solid-liquid interface, melting entropy, chemical composition and volume fraction of constituent phase.

A series of Hypereutectic coatings with different C content were prepared by Chia Ming Chang γ- The hypereutectic structure of Fe (Cr, Fe) 7C3 carbide. When the C content of the coating increases from 3.73 wt.% to 4.85 wt.%, the carbide content increases from 33.8% to 86.1%. The morphology of primary M7C3 carbide also changes from blade shape to rod shape, as shown in Figure 1. It is also found that the wear resistance of the alloy is related to the average free path of hardness (H) / primary M7C3 carbide( λ) Rod like M7C3 carbide has higher hardness and better wear resistance.

(a) SEM photograph of hypereutectic Fe-Cr-C cladding: (b) Microstructure of claddings with different carbon contents: 3.73 wt. %; (c) 4.85 wt. %

The crystal orientation and grain boundary misorientation of three kinds of Fe-Cr-C alloys with different carbon concentrations were analyzed by electron backscatter diffraction. When the C content is 2.3 wt.% ~ 5.9 wt.%, the morphology of primary phase changes from non facet dendrite to facet polygonal structure, as shown in Figure 2. Primary (Cr, Fe) 23c6 and (Cr, Fe) 7C3 carbides have strong texture, single crystal structure and small angle, forming polygonal growth mechanism. However, the primitives with relatively random orientations α The phase has polycrystalline structure, which is composed of large angle boundary caused by dendrite growth mechanism.

The eutectic carbides formed in as cast hypoeutectic fe-25cr-5mo-0.82c alloy were characterized by K wieczerzak. It was found that non-equilibrium solidification promoted the pre precipitation of carbides with high carbon content. The alloy was melted in high purity argon arc furnace and cast in water-cooled copper mold. The results show that the microstructure of the alloy is composed of ferrite (Fe Cr Mo solid solution) and complex eutectic carbides (containing Mo and Cr). Cold casting in copper mold makes the alloy in non-equilibrium solidification state and promotes the pre precipitation of carbides with high carbon content, such as M7C3 and M23C6, rather than M6C.

The interfacial form between liquid and solid: (a) Non-faceted growth; (b) Faceted growth

Luan y found that when the cooling rate reached 1.57 ℃ / S ~ 3.82 ℃ / s, spherical and rod-shaped MC nuclei were formed, which determined the final morphology of carbides. Because MC carbide core is rich in V and C, the liquid melt around the core is lack of V and C, which makes V and C diffuse to the vicinity of the core. During the growth process, MC carbides are formed at the positions rich in V and C, while austenite is formed at the regions deficient in V and C. MC nucleus and austenite nucleus grow together. In the process of binary eutectic reaction L → MC, chrysanthemum like carbides are formed by spherical MC nuclei, while dendritic carbides are formed by rod like MC nuclei, as shown in Fig. 3

Schematic illustration of the growth of chrysanthemum-like and branch-like carbide

To sum up, the commonly used methods of microstructure modification of chromium based alloys mainly focus on adding alloy elements, improving cooling rate, semi-solid + pressure casting, heat treatment and so on. The evolution of carbide morphology by these methods is inseparable from the analysis of carbide nucleation growth and solid-liquid interface morphology. It shows that the change of external process conditions is ultimately through the internal influence on the solidification process to change the final microstructure of the alloy, thus further affecting the mechanical properties of the alloy.