Effect of cyclic quenching treatment on microstructural evolution and properties of ductile cast iron

Due to the coarse grain microstructure and low hardness of ductile cast iron (DCI), this material is unable to meet the performance requirements of bearing structural materials. To obtain ultra-fine-grained and ultra-hardened DCI, the influence of a cyclic quenching treatment on the evolution process and properties of DCI after three cycles of quenching at 860 °C for 30 min was investigated through morphology and grain-size characterization, X-ray diffraction analysis, and mechanical-property determination of DCI. It was found that the microstructure and performance depended strongly on the process of the cyclic quenching. With an increasing number of cycles, the grain size of austenite was significantly refined from 41.2 to 12.3 μm, and the length and width of lath/plate martensite, as well as the lath and twins, are decreased. In the meantime, the morphology of retained austenite changed from blocky to granular, together with the stability increased. The experimental sample obtained a performance with a hardness higher than 60 HRC after cyclic quenching treatment due to the refined grain size and increased dislocation density.