Study on Microstructure Difference of Thickness Section of 1000 MPa Grade Ultra-thick Steel for Hydropower Engineering

The thickness cross-section effect of 1000 MPa grade ultra-thick hydropower steel causes uneven microstructure and mechanical properties, and then affects the service behavior and reduces the product service life. Taking the samples from the 120 mm thick ingot at different thickness positions, the microstructure, hardness test and static CCT curve test of the steel for hydropower engineering were carried out, and the evolution of microstructure and properties of the steel at different cooling rates were investigated. The results show that the cooling rate of 4 ℃/s is the critical cooling rate to obtain full martensite. A large number of carbide precipitated particles are distributed in the matrix when the cooling rate is slow, and the volume fraction of precipitated particles gradually decreases with the increase of the cooling rate. The microstructure of 1000 MPa grade 120 mm thick hydropower steel shows a transition trend from surface of martensite to core of granular bainite, and the hardness gradually decreases from the surface to the core. The cooling rate from the surface to the core of the full-thickness sample is roughly inferred to be 0.1-2 ℃/s, and the surface cooling rate is close to 2 ℃/s, while the core cooling rate is close to 0.1 ℃/s.