As shown in the figure, the hardness of the treated high chromium cast iron samples is significantly higher than that of the as cast ones. The hardness of the modified high chromium cast iron samples is slightly higher than that before modification. After heat treatment, the matrix structure changes from austenite to tempered martensite. The composite modification can greatly refine the primary phase and eutectic structure grain of high chromium cast iron, more uniform distribution and strengthening brought by fine grain, which can improve the hardness of high chromium cast iron.
There is no obvious increase in hardness before and after modification, which can be explained as the following two points:
- chromium is an alloy element that can strengthen the structure. The addition of a large amount of chromium has significantly improved the structure and distribution morphology of cast iron. On this basis, the function space of improving the structure of cast iron through composite modification to improve the hardness of cast iron is very limited. Therefore, the higher the chromium content, the greater the proportion of alloy strengthening, and the closer the hardness of cast iron before and after modification.
- although microstructure refinement brings about the effect of microstructure strengthening and improves the hardness of the material, the results of quantitative analysis of carbides show that the mass fraction of carbides before and after modification is 34.6% and 30.8% respectively for high chromium cast iron samples with 3.0% carbon content. The reduction of mass fraction of carbides in the microstructure will lead to the decrease of hardness of high chromium cast iron. The combined effect of increasing and decreasing on the one hand determines the final hardness of modified high complex iron. Therefore, the reason why the hardness of high chromium cast iron increased slightly after modification is that the hardness of high chromium cast iron increased slightly by fine grain strengthening than by reducing the mass fraction of carbides.