Abstract: Effect of tempering time on microstructure and mechanical properties of A-100 ultra-high strength steel after quenching+cryotreatment was studied by means of mechanical properties test, microstructure observation and fracture failure analysis. The results show that, when the A-100 steel tempered at 482 ℃, the reversed austenite is precipitated at the grain boundary of lath martensite, and M₂C-type carbide is precipitated around martensite. With the increase of tempering time, the amount of reversed austenite increases and disperses more uniformly, and the grain size of the martensite increases. With the increase of tempering time from 5 h to 8 h, the proof strength, plastic extension decreases from 1890 MPa to 1728 MPa; the tensile strength decreases from 2045 MPa to 1988 MPa, decreasing by about 8.57% and 2.78% respectively. However, the fracture toughness increases from 98.75 MPa·m^1/2 to 114.3 MPa·m^1/2, increasing by about 15.74%. When increasing tempering time, the dimple area of impact fracture decreases, and the average dimple area accounts for roughly is 86.2% of the total area after tempering for 5 h, while decreasing by about 70.1% when tempering for 8 h.

Key words: A-100 steel, tempering time, fracture toughness, retained austenite