When high chromium cast iron is as cast, eutectic carbide (M7C3) with broken network distribution exists in its microstructure. The carbide of this shape will split the austenitic matrix and adversely affect its properties. Therefore, as cast high chromium cast iron has low hardness and poor wear resistance, which can not meet the requirements of practical industrial application. Therefore, it is necessary to change the matrix structure and the morphology, size and distribution of carbides through appropriate heat treatment process to obtain high hardness cast iron and make it have good comprehensive properties. The common heat treatment process of high chromium cast iron includes subcritical treatment (holding at 300 ℃ – 700 ℃ for 2-6h), that is, it is lower than the critical point of A1; Or destabilization treatment (holding at 800 ℃ – 1100 ℃ for 1 – 6h), or subcritical treatment + destabilization treatment [14-29]. Among them, destabilization treatment can significantly improve the hardness of cast iron and the mechanical properties of cast iron by obtaining martensitic matrix structure and dispersed and evenly distributed secondary carbides.
The effect of destabilization temperature parameters on the microstructure and hardness of high chromium cast iron is studied. By exploring the appropriate heating temperature, the quantity and morphology of secondary carbides precipitated in austenite dendrite of high chromium cast iron are optimized. At the same time, the MS point of high chromium cast iron is increased to obtain martensitic structure in the process of high temperature destabilization treatment, so as to improve the overall hardness of high chromium cast iron. At present, many researchers have done a lot of research work on the wear resistance of high chromium cast iron by destabilization treatment, and achieved very good results. However, there are few articles on its impact properties. Therefore, the effect of destabilization treatment on impact toughness will be further studied.
The effects of different destabilizing treatment temperatures on the microstructure, matrix type, secondary carbide type, size, shape and content of high chromium cast iron were studied. At the same time, the effects of different heat treatment temperatures on the mechanical properties such as hardness and impact properties of high chromium cast iron were also analyzed. The results show that:
(1) The heating temperature is 900 ℃ – 1000 ℃, and the matrix structure of high chromium cast iron is martensite. When the destabilization temperature is 1050 ℃, the matrix structure is martensite + retained austenite. Therefore, after destabilization treatment, the microstructure of high chromium cast iron is martensite + eutectic carbide M7C3 + secondary carbide + retained austenite.
(2) With the increase of heating temperature, the macro hardness of high chromium cast iron first increases and then decreases. The hardness reaches the maximum at 950 ℃. The secondary carbide in the structure is the mixture of M23C6 and m3c, and its content first increases and then decreases. At 1050 ℃, the content of secondary carbide is the least. With the increase of heating temperature, the secondary carbide changes from fine granular to spherical and rod-shaped.
(3) High chromium cast iron is a brittle material, the fracture mode is brittle fracture, the impact toughness is small, and the destabilization treatment temperature has little effect on its impact properties. At the impact fracture, there are microcracks in martensite matrix and eutectic carbide M7C3, which reduce the toughness and increase the brittleness of high chromium cast iron.