Low-Temperature Bainitic Microstructures And Mechanical Properties Of Si-Al-Bearing Low-Alloy High-Carbon Steel

The aim of the present dissertation is to investigate the low temperature bainitic transformation kinetics, microstructure characteristic and mechanical properties of high-carbon Si-Mn-Cr-W-Al steel by austempered at low temperature.Time-temperature-transformation (TTT) curves of the high-carbon low-alloy steel were calculated by using a software for phase diagram calculation and properties simulation of metal materials. The transformation points, Ac1, Accm and Ms, were determined using thermal dilatometry. The phase composition and microstructure feature of samples austempered with different processes were studied by X-ray diffractometry (XRD), optical microscopy (OM) and transmission electron microscopy (TEM). Tensile, notched impact and wear resistant properties were measured. In addition, the tensile and impact fracture mechanisms and sliding wear mechanism were briefly analyzed by scanning electron microscopy (SEM).Results show that the low-temperature bainitic microstructure composed of lath-like bainitic ferrite with thickness of 50?90 nm and film-like retained austenite can be formed in the steel austempered at 220, 240 and 260°C. The retained austenite fraction and the thickness of bainitic ferrite decrease with the isothermal temperature increasing.The mechanical properties of the austempered samples are much better than that of low-temperature tempered sample. The tensile strength, elongation, and room-temperature impact energy of U-notched specimen are ranged 2080–2375 MPa, 6.7%?7.8% and 7.8?22.2 J, respectively, for the austempered samples, and 1448 MPa, ~ 0% and 2.1 J for the low-temperature tempered sample. As isothermal transformation temperature decreasing, the tensile strength increases and the elongation depresses. The room-temperature impact energy of 240°C-austempered sample is slightly higher than that of 260°C-austempered sample, and much higher than that of 220°C-austempered sample. The dry sliding wear resistance of the austempered samples is much higher than that of the low-temperature tempered sample, and increases with isothermal transformation temperature decreasing. The relative wear resistance of 220°C-austempered sample comparing to low-temperature tempered sample reaches to 1.51.