Abstract: Effect of tempering time on the microstructure and mechanical properties of steel used for deep-well tubing was investigated by using optical microscope, scanning electron microscope, transmission electron microscope, tensile testing at room temperature, and Charpy impact testing at 0 ℃. The results show that when the tested steel is tempered at 550 ℃ with a tempering duration of 35-75 min, a tempered martensite microstructure is obtained. With the increase of tempering time, the microstructure gradually recovers, the lath morphology gradually disappears. Within this tempering time, the mechanical properties of the tested steel remain stable, with tensile strength ranging from 955 to 958 MPa, yield strength from 903 to 908 MPa, product of strength and elongation of 18.7 to 19.1 GPa·%, and impact absorbed energy at 0 ℃ of 128.7 J to 133.3 J. Optimal comprehensive mechanical properties are achieved when the tempering time is 75 min, with tensile strength of 957 MPa, yield strength of 908 MPa, product of strength and elongation of 19.1 GPa·%, and impact absorbed energy at 0 ℃ of 133.3 J. Both the tensile fracture morphology and impact fracture morphology of the tested steel exhibit ductile fracture characterized by dimples.

Key words: steel for deep well tubing, tempering time, microstructure, mechanical properties