Microstructure And Mechanical Properties Of Large Diameter Resin Sand Centrifugal Casting Ductile Iron Pipe And Its Optimizing Process

The centrifugal casting pipe of ductile iron has many advantages,such as high strength,good plasticity and longevity. It is widely used as pipes of water supply and gas supply for cities. Because the nodularization degradation of ductile cast pipe occurs when using hot mold centrifugal casting process, the quality of pipes with a great diameter is not stable.This article analyzed the factors of nodularization degradation of ductile cast pipe, based on the samples got from mass productin lines. We did experiments in the factory to optimize the component of spheroidizer, inoculants and some other technical parameters. We found proper component of spheroidizer and inoculant for centrifugal casting pipe of ductile iron and produced DN1800mm ductile pipe, whose properties fully met the standard of ISO2531-98(E).When the diameter increases from DN800mm to DN1200mm, the quality of spheroidization decreases. The number of graphite spherulites decreases and the diameters of graphite spherulites enlarge. The proportion of ferrite increases while the roportion of pearlite decreases.To the same diameter pipe, the quality of spheroidization decreases from the ektexine to the inner wall. The number of graphite spherulites decreases and the diameters of graphite spherulites enlarge and the shape of the graphite spherulite becomes badly gadually.At the inner wall, the graphite spherulites become large and many chunk and vermicular-like graphite particles and cementite phase appared.We chosed Mg-Si-Fe-RE alloy as spheroidizer and optimized the component 6f Ba,Ca in the spheroidizer. We found that two kinds of spheroidezers which include Ca3.6Ba2.5 and Ca2Ba3.5 have a big number and small diameter of graphite spherulites. There is almost no degraded graphite. And the samples have good mechanical properties.We chose 75Si-Fe alloy as inoculant and added zirconium into it. We studied the relationship between the content of zirconium and the quality of spheroidizatin . We found that when the content of zirconium is 0.1-0.3%, the quality of spheroidization is good. It can increase the number of graphite spherulites and the proportion of ferrite. As the increase of the content of Zr, the shape of graphite become bad gradually and the diameter of graphite nodules become larger gradually and the proportion of ferrite enlarges. When the content of Zr reaches 0.8%,the basic structure is almost pearlite.Based on the experiments .we did, we used the speroidizer and inoculant in the mass product ion line and we found proper component of spheroidizer and inoculant for centrifugal casting pope of ductile iron. The component of spheroidizer is:Mg7-10%, RE2-4%, Ca 2.0-3.5%,Ba 2.5-4.0%. The percentage of spheroidizer added into the liquid iron is 0.8-1.5%(wt).We use 75Si-Fe alloy as inoculant and add zirconium(Zr 0.05-0.3%) into it. The percentage of inoculant added into the liquid iron is 0.8-1.0%. We use late inoculation method to prevent inoculation degradation.Based on the micro-structure of ductile pipe, we chose different heat treatment method to treat different content of cementite. Because of the enhanced quality of spheroidization, the time needed in heat treatment deduced. Such improvement resulted the decrease of the product cost.