Study on strengthening and toughening mechanisms of 1000 MPa grade hydropower steel with low-temperature high toughness

This study investigates the high-toughness performance of 1000MPa grade hydropower steel under ultra-low temperature conditions, along with the microstructural evolution and strengthening mechanisms during quenching and tempering processes. The results indicate that the hot-rolled steel exhibits an average lath martensite size of merely 0.22 μm, while approximately 4% untransformed ferrite and about 2.49% retained austenite were preserved after quenching. At a tempering temperature of 610 °C, the steel demonstrates pronounced secondary hardening behavior, primarily attributed to the precipitation of M 2 C and M 2 (C,N) carbonitrides within this temperature range. Even at -150 °C, the steel maintains excellent low-temperature impact toughness, achieving impact energies of 67 J, 134 J, and 142 J, owning to the ultrafine-grained structure(1.27 μm ) tailored through composition and process optimization. When tempered near the Ac 1 temperature, the dislocation density decreases from 5.33×10 8 mm -2 to 3.21×10 8 mm -2 , while recovery and recrystallization of ferrite increases the fraction of high-angle grain boundaries (HAGBs) from 49.3% to 56.3%. With increasing tempering temperature, crack initiation energy rises from 46 J to 69 J. The synergy of various microstructural mechanisms leads to superior macroscopic toughness in the steel. The strengthening mechanisms in this experimental steel result from a combination of multiple hardening effects, with grain refinement being the dominant contributor, accounting for more than 50% of the total strengthening. Tempering temperature significantly influences the strengthening mechanisms, particularly under high-temperature tempering, where the contribution from precipitation strengthening increases markedly, from 5.3% to 18.6%.