Abstract: Effects of cryogenic treatment, quenching and tempering, and quenching and tempering+cryogenic treatment on microstructure and mechanical properties of the X70 pipeline steel were studied by means of optical microscope, scanning electron microscope, microhardness tester, and universal electronic tensile test machine. The results show that the microstructure of the X70 pipeline steel without treatment is ferrite+pearlite. After cryogenic treatment only, the microstructure is finer and more uniform, the content of pearlite decreases, the microhardness and tensile strength of the X70 pipeline steel increase respectively by 2.73% and 9.58%, and the tensile fracture morphology is equiaxed dimples, showing a ductile fracture characteristic. After quenching and tempering, the microstructure of the X70 pipeline steel is ferrite+tempered sorbite+bainite, the hardness and tensile strength increase by 13.62% and 5.64%, respectively, compared to that of the untreated specimen, and the tensile fracture is distributed with dimples and a small amount of quasi cleavage morphologies, showing a decrease in toughness. After quenching and tempering+cryogenic treatment, the microstructure of the X70 pipeline steel is ferrite+tempered sorbite+bainite, the microstructure is finer and more uniform, the ferrite content increases, the hardness and tensile strength increase by 22.52% and 16.76%, respectively, compared to that of the untreated specimen, and the cryogenic treatment improves the plasticity of the quenched and tempered specimen, thus the tensile fracture exhibits ductile fracture characteristics.

Key words: X70 pipeline steel, quenching and tempering, cryogenic treatment, microstructure, mechanical properties