Effect of cooling rate on microstructure and mechanical properties of CB2 tempered martensitic steel

The cooling rate of casting has a significant effect on microstructure and mechanical properties of castings. The 9Cr-1.5Mo-1Co cast steel, referred to as CB2, is one of the most promising alloys for various cast components to be used under ultra-supercritical conditions. In this study, HRTEM, SEM, and XRD methods were used for qualitative and quantitative analyses of contents, phases, and sizes of the inclusions and precipitates, as well as microstructure observation of the tempered martensitic steel at different cooling rates. Traditional tensile tests were conducted to characterize the material mechanical properties. Results show that the size of the boron nitride and precipitate, the width of the martensite lath and the content of the S-ferrite are reduced as the cooling rate increases from 5-8 degrees C-min(-1) (CB2-S steel) to 50-60 degrees C-min(-1) (CB2-F steel). The precipitates are mainly composed of M23C6 and a small amount of M3C. The average diameters of the M23C6 precipitates in CB2-F and CB2-S are 202 nm and 209 nm, respectively. The inclusions are mainly composed of BN, Al2O3 and MnO2, and the inclusion ratios are 0.1969% for the CB2-F and 0.06556% for CB2-S steel. The average martensite lath widths of CB2-F and CB2-S steels are 289 nm and 301 nm, respectively. Furthermore, the M3C having a diameter of about 150 nm and a thickness of 20 nm is observed in the 8-ferrite of the tempered CB2-S steel. The presence of the 8-ferrite reduces the precipitation strengthening and dislocation density in CB2-S steel. In addition, the lower cooling rate stabilizes the 8-ferrite structure at room temperature.