Abstract: Effect of tempering temperature on microstructure and mechanical properties of A330M ultra-high strength steel in the range of 180-380 ℃ was studied by means of scanning electron microscope (SEM), transmission electron microscope (TEM) and mechanical properties test. The results show that the mechanical properties of the A330M steel are obviously affected by the tempering temperature, and the impact absorbed energy decreases with the increase of tempering temperature. When tempered between 180-380 ℃, the impact fracture morphology of of the tested steel is dimple, quasi-cleavage and intergranular fracture in turn with the increase of tempering temperature, and the ductile fracture changes to brittle fracture. After tempering at different temperatures, the microstructure is mainly composed of lath martensite and retained austenite, martensite lath precipitates a large amount of parallel aciculate ε-carbides, with the increase of tempering temperature, ε-carbide size increases, higher tempering temperature will lead to further precipitation of cementite, resulting in tempering brittleness, reducing the impact property of the tested steel. When tempered at 220 ℃, excellent strength and toughness matching can be obtained, basically eliminating residual stress, with good tempering stability, tensile strength up to 2207 MPa, impact absorbed energy up to 34 J.

Key words: tempering temperature, ultra-high strength steel, microstructure, ε-carbide, mechanical properties