Superior stress relaxation resistance of 2.2 GPa martensitic steels and the effect of vanadium

For ultra-high-strength steels subjected to sustained loading, stress relaxation resistance is a core indicator determining their long-term service reliability. This work investigates the stress relaxation behaviors of two 2.2 GPa-grade martensitic steels (with and without V addition). Both steels were fabricated by flash heating and tempering treatments and exhibited high density dislocations in ultrafine-grained microstructure. Stress-relaxation testing results across various temperatures demonstrate superior relaxation resistance for these two steels. The thermal activation energy (ΔG 0 ) of dislocations at an extremely high stress of 1750 MPa of V-free and V-alloyed steel is 11.8 eV and 14.8 eV, respectively. The synergistic combination of high mechanical strength and stress relaxation resistance in the present martensitic steels is superior to that of other metallic materials reported in previous studies. Furthermore, the higher activation energy and relaxation resistance of V-alloyed steel is primarily associated with the increased density of dislocation obstacles from grain boundaries and nanoprecipitates.