Study On Microstructures And Mechanical Properties Of High-carbon Alloyed Steel By Low-temperature Isothermal Transformation

The aim of the present dissertation is to investigate the low-temperature isothermaltransformation kinetics, microstructure and mechanical properties of high-carbonSi/Si–Al-rich alloyed steels.Time-temperature-transformation (TTT) curves of high-carbon Si–Al-rich alloyedsteel was calculated using JMatPro and MUCG83softwares, respectively. Then differentisothermal transformation treatments of the commercial9SiCr and new designedhigh-carbon Si–Al-rich alloyed steels were carried out. The phase composition andmicrostructural morphology were studied by X-ray diffractometer, optical microscope andtransmission electron microscope. Mechanical properties of these two steels were alsomeasured. The fracture surface observations were operated by scanning electronmicroscopy and the fracture mechanism was also analyzed.Results show that an ultrafine carbide-free bainitic microstructure composed oflath-like bainitic ferrite and retained austenite can be obtained in commercial9SiCr steelby austempering at200°C for8h after austenization at870–950°C, and the thickness oflaths ranges from100to200nm. The maximum Charpy–U impact absorbed energy of29J is achieved in the sample austempered at200°C for8h after austenization at910°C,which is3.6times the value of the sample with low-temperature tempered at200°C (8J),while the hardness is slightly lower than that of the low-temperature tempered sample.Cracks are propagated by a combination of brittle quasi-cleavage fracture and ductiledimple fracture during impact fracture.A nanostructured bainite microstructure can be obtained in high-carbon Si–Al-richalloyed steel by austempering at220–260°C for4–24h after austenization at1000°C,which is composed of38–57nm-thickness laths of bainitic ferrite and retained austenite.The volume fraction of retained austenite and the hardness reach to12.5–18.0vol.%andHRC56.8–59.4, respectively. The hardnesses of samples austempered at220and240°Care slightly lower than that of the low-temperature tempered sample (HRC61.7). The highyield strength1534–1955MPa and high ultimate tensile strength2080–2375MPa areobtained and accompanied by the elongation6.7–7.8%and the Charpy–U impact energy 7.8–22.2J, which are more than the low-temperature tempered sample (ultimate tensilestrength1448MPa and Charpy–U impact energy2.1J). Fracture surface observations ofthe austempered samples indicate that both the impact and tension fracture are in a mixedmode of the brittle quasi-cleavage fracture and the ductile dimple fracture, while theductile dimple fracture is predominant in the tension crack propagation. But in thelow-temperature tempered sample, both the impact and tension fracture are the mode ofthe brittle quasi-cleavage fracture. The wear resistance of samples austempered at thetemperature ranged in220260°C are more than that of tempered one by5121%. Inaddition, we also examined the worn surface microstructure of the samples withlow-temperature tempered and austempered at240°C, indicating that the single-phase αnanograins were formed in the worn surface of both samples. The mean size of nanograinsin the worn surface of240°C-austempered sample (~15nm) is far less than that of thelow-temperature tempered one (~30nm). The S-N curves of high-cycle bending fatiguefor all the austempered samples have a plateau after107cycles. The fatigue limits (nofailure in107cycles) are determined to be1033–1156MPa. All the fatigue cracks initiateat tensile surfaces of bending fatigue samples. The fatigue fracture mode of the fatiguecrack propagation regions is transgranular failure, which have the secondary cracks, and ofthe final fracture regions is brittle quasi-cleavage fracture.