The sample size of high chromium cast iron is 10 × ten × 5 mm3 cube. The martensitic structure of high chromium cast iron was obtained by holding the sample at 900 ℃ – 1000 ℃ for 1H and quenching it to room temperature. Because aging mainly occurs in the newly formed untempered martensite, the quenched sample is stored at room temperature for several days. In order to prevent sample oxidation, the quenched sample is wrapped with dust-proof cloth and placed in the sample box, in which desiccant is placed. In order to observe the microstructure changes of high chromium cast iron before and after aging, the quenched sample was corroded with 4 vol.% nitric acid alcohol and marked with microhardness tester. The changes of matrix structure before and after the experiment were observed by scanning electron microscope.
The figure shows the chemical composition distribution of C and Cr atoms along the tissue after natural aging. At the boundary a between the matrix and eutectic carbide, the carbon content of the matrix (1 wt.%) is slightly higher than that far away from the matrix (0.7 wt.%). In the experimental results obtained after direct quenching at 950 ℃ in Chapter 3, the carbon content of the matrix at the boundary is approximately equal to that far away from the matrix. That is, after natural aging treatment, the carbon content at the interface between eutectic carbide and matrix of high chromium cast iron increases slightly.