Effect Of Several Technologies On Graphite Morphology Of Heavy Section Ductile Iron

Due to the slow cooling rate, there always appears deteriorated graphite such aschunky graphite in the center of the heavy section ductile iron casting, which could worsenthe mechanical properties. In the past40years, lots of researchers at home and abroad makemuch research on this topic and make much progress. But, it’s still a tough problem to figureout why the graphite gets deteriorated.In this thesis, a heavy section ductile iron casting with size of Φ590×800mm wasprepared, and the cooling curves at different locations in the casting were collected. Themethod of thermal analysis was used to analyze the characteristics of solidification. Then theimpact of cooling rate and Sb on the graphite morphology and properties was investigated. Anew technology was figured out and put into test in industry. In addition, the1500℃knockdown multifunctional resistance furnace was used to simulate the solidification processof heavy section ductile iron casting, and the effect of rare-earth elements, float siliconinoculation and pretreatment on the microstructure and properties was also investigated.(1) The cooling curves at the center of the casting, the location85mm distant from thecenter, the location170mm distant from the center and the location255mm distant from thecenter were collected and analyzed. The results show that it takes the longest53minutes forthe recalescence in the location170mm distant from the center with temperature rise of7℃,which means the eutectic reaction at this location is more notable than the other threelocations. This also confirms that most chunky graphite appears at this location.(2) As for the four castings without Sb, the casting under the lowest cooling rate ownsthe lowest nodularity index of30.5%, and the casting under the highest cooling rate owns thehighest of85.2%. The lower the cooling rate in the temperature range from1150℃to1160℃,the poorer the graphite morphology and mechanical properties would be. Thus it couldimprove the graphite morphology to minimize the holding time around1150℃.(3) As for the other three blocks with addition of0.03%Sb, no matter what the coolingrate is, the nodularity index values are all about90%. Even if the eutectic plateau length of thetwo blocks under relatively high cooling rate are different, the two blocks own nearlyidentical nodularity index and globular graphite average diameter value, because these two blocks possess the tolerated REE/Sb ratio for spheroidal graphite formation.(4) The addition of0.03wt.%Sb and the placement of densener can improve the graphitemorphology and properties of the heavy section ductile iron casting Φ590×800mm. Themechanical properties are all improved to a certain degree at most locations in the casting. Forexample, the tensile strength, hardness and elongation at the location170mm distant from thecenter is improved by12.5%、23.1%and46%respectively.(5) Several small-size castings ofΦ110×120were prepared and kept in the1500℃knockdown multifunctional resistance furnace to simulate the solidification process of heavysection ductile iron. The results show that the graphite morphology in the casting withresidual rare-earth elements of.014wt.%is the best among the three castings, which meansthe residual rare-earth elements of014wt.%is reasonable.(6) Several simulating castings were also prepared, float silicon inoculation with0.12wt.%long-time inoculants was used when the temperature of the melt dropped to1380℃.The results show that the deteriorated graphite was reduced, the elongation was improvedfrom4.95%to5.1%, and the tensile strength was improved from510MPa to538MPa.(7) When the simulating casting was pretreated with0.5wt.%micronsized siliconcarbide and0.33wt.%pure iron powders at the time2minutes before pouring, the nodulargraphite in the casting was increased, the elongation was improved from5.1%to5.95%, andthe tensile strength reaches510MPa.