Microstructure, Mechanical Properties And Fatigue Crack Growth Behavior Of Carbide-free Bainitic Steel

Two kinds of low carbon carbide-free bainitic steels, one alloyed with aluminum andthe other without aluminum addition, were designed and produced. The effects ofisothermal temperature and holding time on microstructure and mechanical properties ofthe two steels were investigated by means of microstructural observations by opticalmicroscopy, scanning electron microscopy, transmission electron microscopy and X-raydiffraction, and mechanical property tests such as impact, tension and microhardnesstests. In addition, fatigue crack growth behavior of the Al-containing steel was alsostudied.The results show that carbide-free bainitic microstructures, composed of bainiticferrite and retained austenite, were obtained in both steels after austempering. Withincreases in austempering temperature, the amount of retained austenite increases, whileits carbon content decreases; concurrently, retained austenite tends to take small or largeblocks and the volume fraction of blocky retained austenite increases as well.Austempering in the temperature range from280to350℃, impact toughness of theAl-free steel increases with the decreasing austempering temperature, with the impacttoughness austempered at280℃the highest; however, the highest toughness of theAl-containing steel was obtained when austempered at320℃. Decreasing austemperingtemperature enhances yield strength and hardness of both steels. However, there is noobvious difference of tensile strength and elongation in the Al-free steel after various heattreatments. The Al-containing steel austempered at350℃shows the lowest tensilestrength but the highest elongation, as compared to austempered at other temperatures.The Al-containing steel demonstrates significantly higher toughness and slightly lowertensile strength and hardness than the Al-free steel. The differences of mechanicalproperties the two steels result from their dissimilar microstructures, especially the contentand morphology of retained austenite. More film-like retained austenite in a large amountin the Al-containing steel is helpful to improve toughness, but suppressing the effects oftransformation induced plasticity (TRIP). In comparison, more blocky retained austenitein the Al-free steel easily transform to martensite, which induces TRIP and retards the onset of tensile necking, ultimately leading to a higher uniform elongation. However, largeblocks of retained austenite in the Al-free steel play a damaging role by accelerating crackinitiation and propagation.The FCG behavior of the steel alloyed Al, austempered at320and350℃, wereinvestigated at two stress ratios (R), and it is demonstrated that the threshold stressintensity factor range (ΔKth) decreases with increasing R. At low R, the ΔKthof the steelaustempered at350℃is higher than that austempered at320℃; the reverse is true athigh R. Nevertheless, stress ratio and isothermal temperature play a minor role in the FCGbehavior in the Paris regime. It is considered that fatigue crack propagation in thenear-threshold regima is sensitive to microstructure, and thus crack closure andmicrostructural features cause the variation of ΔKth. On the contrary, fatigue crack growthbehavioe in the Paris regime is insensitive to microstructure and therefore, almost similarfatigue crack growth rates were obtained, regardless of the R value and heat treatment.