Effect Of Zinc On Microstructure And Properties Of Low-Temperature High-Ductility Ductile Iron

With excellent mechanical properties, the low-temperature high-ductility iron has been widely used in industry. The key parts of wind power equipment like hub and bearing pedestal, are made of this material. The future development trend of wind power puts higher requirements on the low-temperature toughness and corrosion-resistance.The element zinc was added based on the composition of QT400-18L, which is the low-temperature high-ductility iron. The main chemical compositions of the material are (wt%):3.7～3.9C,0～1.56Zn,2.3～2.5Si, Mn<0.3, S<0.025, P<0.07,0.02～0.04REr,0.03～0.06Mgr. The research purpose is study the effect of zinc on the microstructure, mechanical properties at normal temperature, low-temperature impact toughness and corrosion resistance of low-temperature high-ductility iron.The results show that, within the addition range (0～1.56wt%), the added zinc dissolved mainly in the ferrite.The microstructure of as cast material is consisted of globular graphite, ferrite and a small amount of pearlite, with no free cementite and phosphorus eutectic. All samples that added zinc were obtained with smaller graphite size within7～8grade and with higher spheroidization within1-2grade, as well as less pearlite (only2%～4%), than the sample without zinc. All the samples were endured740℃×4h graphitizing annealing process and completely ferrite matrix and graphite with higher spherical degree were obtained after graphitizing annealing.The hardness of the as cast samples decreases with the addition of zinc, and the more zinc added, the more reduce is gained. With same composition, the brinell hardness descends after graphitizing annealing. Whilst zinc unconspicuously improves the elongation, the addition of zinc slightly reduces the tensile strength, and all the samples alloyed with zinc still gain tensile strength higher than423MPa after heat treatment. With the425MPa tensile strength and24.0%elongation, the iron containing0.33%zinc exhibits the excellent combination properties.The impact energy is reduced with the reduction of the experimental temperature. The impact toughness of the low-temperature high-ductility iron that alloyed with zinc is higher than material without zinc. The iron containing0.33%zinc shows the best impact toughness, and the impact toughness values Akv of the samples reached16.5J at-20℃,14.5J at-40℃respectively.All graphite on the surface of the samples was not corroded in the3.5%NaCl solution. The corrosion course began with the corrosion in the grain boundary and the interface between matrix and graphite. Along with the increase of the corrosion of time, the internal of the matrix grain was also corroded. With corrosion rate lower than0.1g·m-2·h1, all samples are listed in the entire stability rank, and the corresponding corrosion level is one degree, which represents excellent corrosion resistance. The corrosion resistance slightly grows with the increasing of zinc content. When the content of zinc is1.56%, the highest corrosion potential Ecorr (-0.6808V) and lowest corrosion rate (0.052g-m-2-h-’), which suggests the best corrosion resistant among all samples.In conclusion, the addition of zinc improves performances of low-temperature high-ductility iron both on the microstructure and properties. Completely ferrite matrix and excellent combination properties were obtained, and both the corrosion resistance properties and the low temperature impact performances satisfy the applying conditions of marine and extremely cold wind plants. Take both the mechanical and corrosion resistance properties into consideration, the material with0.33%zinc content can be used to produce parts of wind generator in cold wind plants, and the ductile iron with1.56%zinc can be applied in off shore wind power equipment.