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

摘要

The cooling rate of casting has a significant effect on microstructure andmechanical 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 ultrasupercriticalconditions. In this study, HRTEM, SEM, and XRD methods were used forqualitative and quantitative analyses of contents, phases, and sizes of the inclusionsand precipitates, as well as microstructure observation of the tempered martensiticsteel at different cooling rates. Traditional tensile tests were conducted to characterizethe material mechanical properties. Results show that the size of the boron nitride andprecipitate, the width of the martensite lath and the content of the δ-ferrite are reducedas the cooling rate increases from 5-8 °C·min-1 (CB2-S steel) to 50-60 °C·min-1 (CB2-Fsteel). The precipitates are mainly composed of M23C6 and a small amount of M3C. Theaverage diameters of the M23C6 precipitates in CB2-F and CB2-S are 202 nm and 209nm, respectively. The inclusions are mainly composed of BN, Al2O3 and MnO2, and theinclusion ratios are 0.1969% for the CB2-F and 0.06556% for CB2-S steel. The averagemartensite 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 isobserved in the δ-ferrite of the tempered CB2-S steel. The presence of the δ-ferritereduces the precipitation strengthening and dislocation density in CB2-S steel. Inaddition, the lower cooling rate stabilizes the δ-ferrite structure at room temperature.