The Effect of Thermal Processing and Chemical Composition on Secondary Carbide Precipitation and Hardness in High-Chromium Cast Irons

The excellent abrasion resistance of high-chromium cast irons (HCCIs) is given by an optimal combination of hard eutectic and secondary carbides (SC) and a supporting matrix. The tailoring of the microstructure is performed by heat treatments (HTs), with the aim to adjust the final properties (such as hardness and abrasion resistance). In this work, the influence of chemical composition on the microstructure and hardness of HCCI_26%Cr is evaluated. An increase in the matrix hardness was detected after HTs resulting from combining precipitation of M23C6 SC during destabilization, and austenite/martensite transformation during quenching. Kinetic calculations of the destabilization process showed that M7C3 secondary carbides are the first to precipitate during heating, reaching a maximum at 850 degrees C. During subsequent heating up to 980 degrees C and holding at this temperature, they transformed completely to M23C6. According to the MatCalc simulations, further precipitation of M23C6 occurred during cooling, in the temperature range 980-750 degrees C. Both phenomena were related to experimental observations in samples quenched after 0-, 30-, 60- and 90-min destabilization, where M23C6 SC were detected together with very fine SC precipitated in areas close to eutectic carbides.