| With the rapid development of high-speed railway,wind power,petrochemical and other industries,nodular cast iron should not only have good mechanical properties,processing properties,shock absorption properties and low cost advantages,but also should have sufficient low temperature impact performance,so as to avoid the accident caused by low temperature brittle fracture,so as to adapt to more and more stringent service environmental conditions.QT400-18 L has been widely used because of its excellent low temperature impact performance.In this paper,ductile iron with ultra-high low temperature impact toughness was obtained by studying the effect of annealing treatment on the microstructure and properties of QT400-18 L,and its low temperature impact fracture behavior was discussed.In this thesis,the heat treatment process of QT400-18 L was optimized,and single stage and two stage graphitization annealing processes were used at high temperature(920℃),middle temperature(840℃)and low temperature(750℃).The results show that: After annealing at low temperature(750℃×3h+700℃×3h),the ferrite QT400-18L(-50℃)shows the outstanding mechanical properties,namely,the tensile strength,elongation and Brinell hardness are 402 MPa,20.3% and 133 HBW,respectively.The impact energy at the temperature of-50℃ reaches 13.5 J.Charpy and oscilloscopic impact tests were conducted on the ferrite QT400-18L(-50℃),The charpy impact test results indicate that its ductile-brittle transition temperature is Approximately-75℃.The results of oscilloscope impact tests show that the temperature has significant influence on the crack initiation and crack growth ability of ductile iron.Above the tough-brittle transition temperature,the crack initiation work of ductile iron changes little with the temperature;but,the crack propagation work decreases with the temperature.Below the ductile-brittle transition temperature,the crack initiation work and crack propagation work decrease sharply with the decrease of temperature.According to the impact fracture morphologies of the ferrite QT400-18L(-50℃)obtained by a series of low-temperature impact tests,the fracture behavior was invesigated at the Microscopic level.Based on the morphologies of cleavage planes,the fracture modes at different temperatures were analyzed.It is further verified that its ductile-brittle transition occurs at approximately-75℃,and the corresponding microscopic fracture morphology also changes from a dimple to a fluvial cleavage plane.By observing the graphites on the fracture surfaces of the V-notch samples,it is concluded that the main damage modes of the graphites are layered cracking,core cracking,graphite-matrix interface disintegration,irregular graphite and matrix cracking.Above the ductile-brittle transition temperature,the cleavage fracture originates from the graphites.The presence of graphites and inclusions leads to the cleavage fracture within the ductile-brittle transition temperature.Twins lead to the cleavage fracture below the ductile-brittle temperature. |
