Microstructure And Mechanical Properties Of A Medium-carbon Bainitic Steel

Low-temperature bainitic steel is a high-performance structural steel whichexhibits an excellent combination of ultra-high strength, high ductility and goodtoughness. The key to the excellent mechanical properties of bainitic steel are thenanoscale bainitic-ferrite plates and carbon enriched austenite. The ultra-high strength isdue to the presence of a nanoscale bainitic microstructure and very high dislocationdensity, whereas their good ductility and toughness is attributed to the film-likeaustenite that exists between the bainitic-ferrite plates. In order to ensure thinbainitic-ferrite and film-like austenite, it is necessary to carry out the bainitictransformation at low temperatures. On the other hand, lowering the BSand MStemperatures requires having a high carbon content, at the expense of both toughnessand weldability.In this work, a steel with a0.30wt.%carbon content was designed based on phasetransformation theory. The toughness deterioration in high carbon nanostructuredbainite steels was effectively avoided. The phase transformation points of the steel weremeasured by a Gleeble3500thermal simulator. The microstructure, phase fraction wereanalyzed by OM (Optical microscopy), SEM (Scanning electron microscopy), TEM(Transmission electron microscopy) and XRD (X-ray diffraction). The mechanicalproperties and wear resistance were tested using micro-hardness, universal testingmachine, Charpy impact machine and abrasion tester. Through the studies, the effect ofa conventional bainitic transformation and a new multi-step low-temperature bainitictransformation on the microstructure and mechanical properties in a medium-carbonsteel were investigated. The main outcomes are as follows:The bainitic transformation was carried out in a temperature range of300-360°C.The microstructure was composed of bainitic ferrite, carbon enriched film-like austeniteand martensite/austenite blocks. The average size of untransformed austenite blocks andthe thickness of bainitic-ferrite decreased as the transformation temperature decreasedfrom360to300°C. The volume fraction of retained austenite reached20.0-27.7vol.%.The tensile strength of1324-1391MPa with an elongation of11.7-16.6%wereobtained. The room temperature Charpy-V impact energy reached34.4-39.0J.The results of the multi-step low-temperature bainitic transformation in the medium-carbon steel showed that, compared with the microstructure obtained byconventional bainitic transformation, the blocky microstructure was almost eliminateddue to the formation of higher volumes of nanoscale bainitic-ferrite plates and film-likeaustenite, which to a refinement in the average thickness of the bainite plates. Thesamples with refined microstructure showed superior mechanical properties. The tensilestrength increased from1380to1565MPa, elongation from11.7to12.6%and theroom temperature Charpy-V impact energy from39.0to45.0J.Results of bainitic steel and a quenching and partitioning steel in the abrasion testshowed that both steels have very high hardness (458-471HV1) and the abrasionweight loss decreased as the surface hardness increased. The Q-P treated steel has ahigher stability of retained austenite and thicker deformed layer that showed betterharden ability and a superior stirring wear resistance.