Study On Casting Process And The Microstructure And Properties Of Large Ductile Iron Castings

Machine tool industry is the foundation of the equipment manufacturing industry,as the trend of componets is going to be high-performance, lightweight andsophisticated, higher precsion and performance are required for the new generation ofmachine tools. As we know, there are still gaps in chinese machine tools because thestability of precision and quality of demestic-made machine tools are worse than theimported ones, the application of demestic-made machine tools are limited in roughprocession and semifinishing. As one of the most important componets of machinetools, ram effects the precision and performance of the high-grade CNC machine tools.Therefore, independent design and manufacturing of high-quality ram is of greatapplication value for manufacturing high-grade CNC machine tools.The ram of model TK6920-20C020is researched in this paper, which isheavy-section ductile iron casting of material QT600-3. A T-type simulated block onbehalf of major thicknesses of the ram was made in order to study the relationsbetween microstructure property and wall thickness of ductile iron. The graphitemorphology, matrix structure and mechanical properties of simulated blocks ofdifferent wall thickness were analyzed. The results indicate that, the nodulizing gradeof graphite can be above grade2and the mechanical properties is able to get thestandard of QT600-3when the wall thickness of simulated block is less than80mm,however, the nodule diameter increases sharply and the grahpite morphology starts todeteriorate when the wall thickness of simulated block is more than80mm, as thestructure of matrix becomes mainly composed of ferriter, its mechanical propertiescan not meet the standard fo QT600-3. The test results are expected to provide theguidance on how to set chills in the actual production process of the ram.A three-dimensional solid model of the ram including its gating system wasestablished on the platform of UG，and then the casting process was simulated bymeans of software ProCAST in order to show the influence on the molten metal flowvelocity, solidification process and shrinkage defects by changing the composition ofmolten iron, pouring temperature and using external chills or not. The simulationresults show that, compared with the molten iron of composition3.0C2.1Si0.3Mn0.5Cu, the mold filling process of the molten iron of composition3.6C2.1Si0.3Mn0.5Cu is much smoother, moreover, it has less difference of solidification time and shrinkage defects. The quality of the ram can be improved byreducing pouring temperature, setting chills on the large setions and hot spots, whichcan shorten the solidification time and decrease the shrinkage defects. The sectionswhere the shrinkage defect may appear are according with the practical production.Further more, An optimized simulation scheme was presented which couldsuccessfully remove the shrinkage defects predicted in the past simulation schemes.On the basis of above experiments, the chemical composition, microstructure andmechanical properties of the pre-production were detected under recarburizingprocess by using cast iron or carburetant. Graphite form with different wall thicknessof casting was observed, and graphite core and graphite degeneration were analyzedby using SEM and EDX. The results show that the casting under carburtantrecarburization has better metallographic structure and mechanical properties. Underrecarburizing process by using cast iron, graphite form with different wall thickness isrelated to cooling rate, compared with the thin-wall sections where are not settedchills, the heavy-sections with chills have better graphite form and more graphitespheres, the graphite core is mainly composed of rare earth oxides which areintroduced by high melting-point Si particles. The graphite degeneration is caused byinclusions which may undermine the stability of the austenite shell. The inclusions arecomposed of Ce Sb Mg Cu，ect.