A Research On Microstructure, Properties And Solidification Characteristic Of Ductile Iron By Investment Casting

Investment casting has the characteristics of high dimensional accuracy, good surface finish and being able to produces complex shape and ultra-thin section castings, investment casting has already become an important preparation means of ductile iron parts. Since 1960s, the investment casting technology of ductile iron has rapidly developed abroad, and it also has developed rapidly in China in the recent years. The production and complexity of ductile iron castings produced by investment casting are all significantly increased.However, the composition, pouring conditions, nodulizing treatment and inoculation treatment of investment casting are different from sand casting. First of all, in the investment casting, the carbon equivalent is lower. Secondly, because of the hot shell in the investment casting, the cooling rate decreases, the tendency of shrinkage enlarges. So it will be of great theoretical and practical significance for establishing process and the control of the performance of ductile iron to research on microstructure, properties and solidification characteristic of ductile iron by investment casting.By taking engine castings of fast train as the research object, through computer simulation and actual cooling curve of ductile iron and the testing of organization and performance, this paper researched the microstructure, properties and solidification characteristic of ductile iron by investment casting. The results showed that the shrinkage tendency of the ductile iron declined when the pouring temperature increased from 1320℃to 1420℃, and the shrinkage tendency increased when the temperature of shell dropped. The shrinkage tendency was minimal when the pouring temperature was 1420℃and the temperature of shell was 700℃. It is seen from the cooling curve that compared with the casting made from cold shell, the casting made by the hot shell has higher eutectic temperature, longer eutectic reaction time and the eutectic platform is more stable.It was found that when the content of V, Ti elements is high, the spheroidization was poor. There were a large amount of distorted graphite nodules in the microstructure. The graphite form was grade 3 to 4 in Chinese graphite nodule standard; also there were a lot of carbides in the metal matrix. Therefore, in the investment casting, pig iron of non-v, Ti and low sulphur and phosphorous should be chosen. In the production of ductile iron in investment casting, using low-RE Mg-nodulizer and secondary inoculation, prepared ductile iron with whole round graphite of uniform size. Increasing the intensity of mold and the baking temperature could reduce casting rejects and improve the quality of ductile iron castings. Cold shell castings had a large number of nodular graphite with small size, the size degree of the nodular graphite of cold shell castings was grade 7 while the size of hot shell was grade 5. In the matrix of cold shell castings, the content of cementite was more than 2%, meanwhile, the content of cementite of hot shell castings was less than 1%. The tensile strength and elongation were more than 600MPa and 3%, respectively. It can meet the requirements of microstructure and properties for engine castings.