Microstructure Evolution and Hardness of Hypereutectic High Chromium Cast Iron after Tempering

The effect of the tempering process on the microstructure evolution and hardness of 35 wt.%Cr-4 wt.%C Hypereutectic High Chromium Cast Iron (HHCCI) was studied by means of the scanning electron microscope and transmission electron microscope. The results show that with the increase in tempering temperature, the primary and eutectic carbides of HHCCI have no significant change, and the precipitation of secondary carbides in the matrix increases at first and then decreases. From the TEM analysis results, the secondary carbides after tempering are M₂₃C₆, which are rectangular, and with the extension of tempering time, a small part of eutectic carbides decompose and dissolve into the matrix, and the secondary carbides continue to precipitate and grow, and the length can reach several microns to more than 10 microns. With the increase in tempering temperature, the hardness of HHCCI increases, and the hardness tends to decrease when it exceeds 500 °C. After tempering for 6 h, the precipitation of secondary carbides in the matrix reaches the maximum, and the hardness of HHCCI reaches the maximum. After tempering holding time for more than 6 h, the hardness of HHCCI decreases continuously due to the aggregation and growth of secondary carbides.