Simulation Study on Thermo-Mechanical Controlled Process of 800 MPa-Grade Steel for Hydropower Penstocks

The thermo-mechanical controlled process (TMCP) of 800-MPa-grade non-quenched tempered steel used for penstocks was simulated on a Gleeble-3500 thermo-mechanical simulator. The effect of the finish cooling temperature (FCT) which ranged from 350–550 ◦C on the microstructure and mechanical properties was studied. The microstructure of TMCP specimens is primarily composed of lath bainite (LB) and granular bainite (GB). The decreased FCT can induce the increase of LB and the decrease of GB in the volume fraction, and the decrease in the amount and the size of Martensite/Austenite (M/A) constituents with a more dispersive distribution. The LB has higher strength and hardness than GB, and the GB with fine and dispersive M/A constituents has excellent impact toughness. The minimum values of the yield strength, tensile strength and hardness, and the maximum value of the impact absorbed energy are obtained for the FCT of 450 ◦C. For the FCT over 450 ◦C, the yield strength, tensile strength and hardness are increased slightly, but the impact absorbed energy is rapidly decreased, which is mainly attributed to the formation of block M/A constituents. When the FCT is around 400 ◦C, the optimal combination of yield strength and impact toughness is obtained, which meets the technical requirements of 800-MPa-grade hydropower penstock steel.