The solidification structure of chromium alloys is closely related to the content of C and Cr in them. According to its different Cr content and application in different technical fields, it can be roughly divided into three categories
(1) Low chromium cast iron
Low chromium cast iron is defined as cast iron with Cr content of 2 ~ 5 wt.%, which is suitable for low temperature and low wear conditions. The production cost of low chromium cast iron is low because of its low alloy elements. The CR / C ratio of low chromium cast iron is low, so the carbide in the alloy is mainly m3c. Carbides in low chromium cast iron present continuous network structure, which separates matrix from matrix with good toughness. Low chromium cast iron has higher brittleness, lower toughness and worse wear resistance than other cast iron with high alloy content.
(2) Medium chromium cast iron
The cast iron with Cr content of 5 ~ 10 wt.% in alloy is called medium chromium cast iron. The most commonly used medium chromium cast iron with Cr content of 8 ~ 10 wt.% is mainly used for manufacturing wear medium of cement ball mill and coal mill in power plant. Compared with low chromium cast iron, with the increase of Cr content, the type of carbide in the alloy also changes. Besides m3c, M7C3 carbide is added in the alloy. Compared with m3c carbide, the hardness of M7C3 carbide is higher, so the hardness of medium chromium cast iron is higher than that of low chromium cast iron. Although the hardness increases significantly, the solidification structure of medium chromium cast iron is martensite + retained austenite, so the hardenability is poor. In order to improve its properties, high temperature air quenching and low temperature tempering are needed in heat treatment.
(3) High chromium cast iron
High chromium white cast iron is the most commonly used wear-resistant cast iron at present, which refers to the cast iron with Cr content between 12 ~ 28 wt.%. Under the harsh working conditions of wet grinding and high-speed slurry transportation, it has the performance of wear resistance and corrosion resistance, and is widely used in mining machinery, cement ball mill, active power generation coal mill and other high temperature and high wear conditions. High chromium cast iron is usually in the category of eutectic and hypereutectic solidification structure, and its C content is usually more than 2.7 wt.%. The excellent properties of high chromium cast iron are attributed to the combination of hard carbides (such as M7C3 and M23C6) and relatively ductile iron matrix (martensite and / or austenite). Such a matrix is conducive to the absorption of impact energy and improves the fracture toughness of the material, and the hard carbide plays an important role in bearing the wear stress. Although each phase in the solidification structure has excellent mechanical properties, the carbide / matrix interface has great influence on the overall properties of high chromium cast iron.
