High strength carbides in cast chromium alloys

When the volume fraction of carbide with high hardness in the solidification structure of Cr based alloy is 20 ~ 35% and evenly distributed in the matrix, the wear resistance of the alloy is the best. These carbides can exist as eutectic carbides or primary carbides. The morphology of eutectic carbide in hypoeutectic white cast iron is usually coarse and continuous. The material has the weakness of low toughness and brittle fracture due to its splitting effect on the matrix. In the further development and application, how to obtain fine primary carbides in the alloy solidification structure, and how to improve the nucleation of eutectic carbides or hinder the growth of eutectic carbides are of great significance to improve the wear resistance of chromium alloys.

The quantity, nature, size and distribution of carbides in chromium alloys are closely related to the type and composition of carbides. The type of carbide is not only related to the ratio of chromium and carbon content in the alloy, but also closely related to the solidification process of chromium based alloy.

Cr is the most added element in chromium alloys because it has a strong tendency to form hard carbides. In addition, the alloy element Cr is relatively easy to obtain and the cost is relatively low, and the ideal alloy structure and mechanical, physical and chemical properties can be obtained after adding CR. By comparing the morphology and distribution of carbides in the solidification structure of Cr Based Alloys with different CR contents, it is found that the type of carbide changes from m3c to M7C3 with the increase of Cr wt.%. The morphology of carbides changes from continuously distributed flake and network to intermittently distributed chrysanthemum, curved surface strip and hexagonal rod. It can be said that Cr element is the key factor affecting the type and morphology of carbide in CR based alloy.

In addition to the effect of Cr, the content of C also plays a decisive role in the solidification structure of Cr Alloys. Previous studies have shown that the position of Cr atoms in carbides is easily replaced by Fe atoms in chromium alloys, which is the main factor affecting the number of carbides, but basically does not affect the type of carbides. The type of carbide is related to the ratio of Cr and C in the alloy. When the content of Cr is more than 11 wt% and Cr / C is more than 3.5, M7C3 carbide will appear in the solidification structure of the alloy. With the increase of Cr / C, m3c and M7C3 carbides will be replaced by M23C6 carbides. Different types of carbides have different mechanical properties. The microhardness of M7C3 is 1300 ~ 1800 HV, and that of m3c is 840 ~ 1100 HV. The hardness of the latter is much smaller than that of M7C3. In the sliding wear process, the eutectic M7C3 carbide can successfully prevent the plastic deformation of the matrix when interacting with the worn surface. Therefore, how to obtain more M7C3 carbides is the key to improve the mechanical properties of chromium based alloys, and it is also the current research hotspot.