Strengthening, Toughening And Low-cycle Fatigue Behavior Of Lowcarbon Carbide-free Bainitic Steel

In this paper, three low-carbon carbide-free steels with different aluminum content(0Al, 0.6Al and 1.2Al, wt. %) were designed and manufactured. The effect of heat treatment process parameters, aluminum content and loading rate on the microstructure, microstructural evolution and mechanical properties of low-carbon carbide-free bainitic steel was investigated by optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, digital image correlation method and mechanical property tests, et al., which revealed the mechanism of strengthening and toughening of this steel; in addition, low-cycle fatigue behavior of low-carbon carbide-free bainitic steel was also studied by low-cycle fatigue test.The effect of holding time and austempering temperatures on microstructure and mechanical properties of the 1.2Al steel was investigated. The results show that, at austempering temperature of 320 oC, with increasing in the holding time, the amount of carbide-free bainitic ferrite and retained austenite increases at first and then keeps unchanged, the impact toughness and total elongation also increase at first and then keep unchanged; in addition, with increasing in the holding time, although the yield strength almost does not change, the tensile strength tends to increase slightly and then keeps unchanged. With decreasing the austempering temperature from 350 oC to 300 oC, the amount and size of retained austenite, and the thicknesses of bainitic ferrite laths all exhibit decreasing tendency and accordingly, yield and tensile strength increase slightly, the total elongation decrease significantly, while the impact toughness increases at first and then decreases.The effect of aluminum content on the the microstructure and mechanical properties of low-carbon carbid-free bainitic steel was investigated. The results show that, with increasing the aluminum content, the thicknesses of bainitic ferrite laths decrease obviously, the impact toughness increases significantly; in addition, with increasing the aluminum content, the amount and size of retained austenite exhibit a decreasing tendency, the tensile strength and total elongation decrease, but the impact toughness increases. This indicates inconsistent effect of aluminum content on tensile strength, tensile ductility and impact toughness of the steel investigated.The effect of loading rate on fracture absorption energies of the two types of samples of the 1.2Al steel was investigated. The results show that the crack initiation, propagation and total absorption energies of U-notched samples increase significantly with increasing the loading rate; in comparison, for pre-cracked samples, although the crack initiation energy almost does not change with loading rate, the crack propagation and total absorption energies remain to increase with raising the loading rate. The result is mainly due to that the amount of austenite-to-martensite transformation decreases with increasing the loading rate.The low-cycle fatigue behaviors of the 1.2Al steel austempered at 300 oC and 350 oC were studied. The results show that both samples austempered show similar cyclic stress response behavior, i.e., initial cyclic hardening followed by cyclic softening or by cyclic saturation and softening till failure, depending on the given total strain amplitude applied. The analysis suggests that the major cause for the initial cyclic hardening is neither strain-induced martensitic transformation nor the the increase in dislocation density. The interactions of the extremely high density of dislocations initially existent in the starting microstructure and the decrease in the density of mobile dislocations is the main reason for the initial cyclic hardening; however, the cyclic softening after the initial cyclic hardening is attributed to the annihilation and the rearrangement of the large amounts of dislocations. In comparison with the sample austempered at 350 oC, the sample austempered at 300 oC exhibits a longer fatigue lifetime at lower strain amplitudes; however, at higher strain amplitudes, the sample austempered at 300 oC shows a shorter fatigue lifetime. This main reason is that the sample austempered at 300 oC has a higher yield strength than the sample austempered at 350 oC, while the latter shows a higher tensile ductility than the former.