The Influence Of Low Temperature Impact Toughness And The Fracture Behavior Of Ferritc Ductile Iron

Due to its excellent ductility and moderate strength, ferritic ductile iron has been widely used in producing some core components of wind power equipment such as the hub of a wind turbine. But long time exposure to cold air brings brittleness to the components. Therefore, the low-temperature impact toughness of ductile iron is a critical factor that limits ductile iron’s widespread application to wind energy in cold areas. Most of the research has focused on the exploration of mechanical properties at low temperature. But none of them gives the explanation on microcosmic mechanism of ductile iron during low temperatures. The role of graphite and grain boundary on low temperature impact toughness hasn’t analyzed. In this study, the influence of low temperature impact toughness and the fracture behavior of ferritc ductile iron are discussed.The cleavage fracture resistance of 400-18L ductile iron is reduced with the decrease of impact temperatures. Above ductile-brittle transition temperature (DBTT), most of the total fracture energy was expended during the crack propagation process. Below the DBTT, both crack initiation energy and crack propagation energy decrease obviously. Three-dimensional reconstruction of impact fracture morphology was accomplished by using confocal laser scanning microscope. The results of quantitative fractography indicate that cleavage fracture produces flatter fracture surfaces accompanying with less absorbed energy during the impact fracture process. It indicates that fracture roughness has a close relationship with crack propagation energy at low temperature. In addition, the value of Rc/RO linearly decreases with the decrease of temperature, while the logarithm of impact absorbed energy linearly decreases with the increase of fractal dimension.SEM and TEM were used to analyze the size of ferritic grains and the cleavage facts. Experimental results show that size of cleavage facts decreases with the ferritic grain grade. Cracks mainly form at grain boundaries under low temperatures due to dislocation pile-up. Inclusions on grain boundaries lead to the formation of microcavities on dimple fracture. OM and SEM were used to analyze the fracture mechanism of graphite nodules. Experimental results show that the energy absorbed by impact test decreases significantly with the decrease of graphite grade. Triaxial stress coefficient increases with the ratio of graphite size and graphite spacing. In addition, the impact fracture behavior of ductile iron has been investigated that cracks initiate from the interface between nodular graphite and the matrix.By using in-situ fracture metallographic observation method, crack initiation and propagation of QT400-18L ferritic ductile iron under different temperatures were analyzed. Above DBTT, graphite nodules play the role of crack blunting and reducing crack propagation rate; in DBTT range, the fracture morphology shows mixed fracture with cleavage and dimples, which are related to graphite nodules; below DBTT, deformation twins lead to the nucleation of microcrack and result in cleavage fracture, the deformation twinning could possibly play a significant role in the ductile to brittle transition of 400-18L ductile iron.The morphology and distribution of dislocation near impact fracture surfaces at different temperatures are observed by TEM, it shows that the inhomogeneity of the dislocation distribution is increased with the decrease of temperature. The dislocation density near impact fracture surfaces at different temperatures is calculated by XRD diffraction method. As the temperature decreases, both the density of dislocation and the number of mobile dislocation are decreased. Based on dislocation theory and fracture transformation mechanism of ferritic ductile iron, the theoretical formula of ductile to brittle transition temperature is deduced, which indicates that the ductile to brittle transition temperature is closely related to grain size, graphite size and graphite spacing. A constitute model of metal flow is established based on dislocation dynamics and is applied in simulating the plastic deformation of QT400-18L ductile iron. The flow stress is considerably influenced by athermal and thermal-activated factors. The thermally-activated stress exerts as the temperature is below a critical value, namely, ductile to brittle transition temperature (230K). The comparison shows that the simulation result is consistent with the experiment result.