Study of Riser Design for High Strength and Heavy Section Ductile Iron

摘要

Ductile cast irons have been widely applied in the industry due to their high strength and ductility. In view of the stringent quality requirements for the castings, not only the mechanical properties must meet the specification, but also the shrinkage defects should be prevented. In this study a high strength grade (FCD-800-2) ductile cast iron was chosen as the target alloy, and a heavy section ductile iron casting (120mm cube) was selected for the study of the optimal riser design. Regarding the riser design, top risers were employed in this study. The experimental results indicate that under the condition of 1400℃ pouring temperature, the optimal riser design obtained is: D_r=70mm (φ), H_r=70mm, D_n=40mm (φ), H_n=20mm. On the other hand, as the insulated riser sleeve or exothermic riser sleeve was used, the optimal riser designs for both types are: D_r=54mm (φ), H_r=54mm, D_n=40mm (φ), H_n=20mm. Accordingly, a reduction of riser size (volume) as high as 55% can be obtained when insulated or exothermic riser sleeves were employed. Regarding the alloy design and heat treatment that are required to meet the FCD-800-2 specification: 480MPa in yield strength, 800MPa in tensile strength and 2% in elongation, the C and Si contents are set at 3.6% and 2.4%, respectively. Moreover, in order to attain a merely fully pearlitic matrix, Fe-Mn alloy and copper (Cu) were added. 25mm-Y blocks were cast for tensile specimens. The experimental results show that the optimal alloy design is 0.2%Mn, 1.0%Cu, with the as-cast mechanical properties of 500MPa, 797MPa and 6.3% for yield strength, tensile strength and elongation, respectively. Furthermore, as the optimal heat treatment condition of 890℃-1h/oil quench/600℃-1h/furnace cooling to RT was conducted on the specimens, the following mechanical properties: 720MPa in yield strength, 864MPa in tensile strength and 4.5% in elongation can be obtained, all meet the FCD-800-2 specification.