Microstructures And Mechanical Properties Of Low-Temperature Bainite In Low-Alloyed Hi-Carbon Si-Containing Steels

The aim of the present dissertation is to investigate the formation, microstructures, mechanical properties, and the wear resistance of low temperature bainit in low-alloyed hi-carbon Si-containing steels 60Si2CrVA and 9SiCr. Transformation points, Ac1, Ac3, and Ms, of the steels were measured by thermal dilatometery. Different isothermal transformation treatments of the steels were carried out. The microstructural morphology and the phase composition were studied by optical microscope, transmission electron microscope, and X-ray diffractometry. In addition, mechanical properties and the wear-resistance under the condition of dry-slide friction and wear were also measured.The results show that the microstructures of the steels after isothermal transformation treatments are composed of two phases, carbide-free bainitic ferrite and retained austenite, with lath- and film-like morphologies, showing the features of low-temperature bainite, instead of the conventional lower bainite.The low-temperature bainitic microstructure can be obtained in 60Si2CrVA steel by isothermal transformation at 235℃for 8 h and 270℃for 3 h after austenizing at 1000℃for 20 min. The volume fractions of retained austenite are determined to be about 18.6 and 8.8% for 235℃- and 270℃-transformed samples. The corresponding mechanical properties are measured to be 1980 MPa of tensile strength, 9.5% of elongation, and 54 HRC of hardness for 235℃- transformed sample, and 1880 MPa, 10.5%, and 50 HRC for 270℃-transformed sample.9SiCr steel was isothermal transformed at 200℃for 8 h after austenizing at 870℃,910℃, and 950℃, respectively. The volume fraction of retained austenite is determined to be the maximum in the sample isothermal transformed at 200℃after austenizing at 910℃. The results of mechanical properties showthat the hardness of 9SiCr steel isothermal transformed at 200℃for different time after austenizing at 870℃and 950℃decrease firstly, and increase at certain time, and decrease again with the isothermal transformed time. The hardness of the sample by isothermal transformation at 200℃for 3 h after austenizing at 950℃is obtained to be the maximum of 64.8 HRC, which is reached the level of quenching. The impact energy of the steel with low-temperature bainitic transformation treatments is much more than that of quenching+tempering sample. The sample isothermal transformed at 220℃for 8 h after austenizing at 910℃is determined to be the maximum 1.51 of relative wear-resistance under the condition of dry-slide friction and wear at the load of 200 N and the rotation rate of 200 r/min.