Composition design, Microstructure and Properties of Ultra-High-Strength Steel using for energy storage flywheels

According to the properties request of energy storage flywheels running in high speed, chemical composition of a new ultra-high-strength steel has been designed. The designed steel specimen was prepared using intermediate frequency induction furnace and its transformation point, which had been simulated and calculated through J-Mat software in advance together with cooling curves, was investigated using by thermal dilatometer. Then the microstructure and mechanical properties of the designed steel have been evaluated by means of OM, SEM, durometer and universal material tensile tester. Simulation results showed that the pearlitic transformation of designed steel occurred at 687-453 degrees C and bainite transformation at 453-340 degrees C. Martensitic transformation started at 340 degrees C and terminated at 220 degrees C. The experimental results indicated that the casting microstructure of the designed steel was a duplex structure consisting of martensite and acicular bainite with a small amount of retained austenite. The austenization temperature ranged from 698 degrees C to 790 degrees C. The superior comprehensive mechanical properties of tensile strength of 1900MPa and elongation of 6.65% as well as the microstructure of tempered martensite was obtained after heat treatment.