Precipitation Behavior Of Graphite In Heavy Section Ductile Iron And Physical Simulation Of Its Casting

Heavy section ductile iron casting used widely in nuclear and wind power industry as a result of its excellent properties and low cost. Chunky graphite in heavy section ductile iron casting is still unsolved till now. In the present study, precipitation behavior of graphite and formation mechanism of chunky graphite in heavy section ductile iron are investigated based on experiments in laboratory and foundry. Meanwhile, effects of trace elements on graphite morphology are analyzed. Based on experimental results, solidification of a 120t spent fuel cask are studied by physical simulation method and forced cooling system is designed to reduce its solidification time. Finally, microstructure and mechanical properties of simulative casting are assessed comprehensively.A simulative apparatus is designed to study precipitation behavior of graphite in heavy section ductile iron and its influcing factors are discussed. Results show that graphite is spheroidal as the melt holding time is less than 240min. Subsequently chunky graphite precipitates directly after holding for 240min. It should be noted that vermicular graphite forms around eutectic chunky graphite cells after holding for 360min. When holding time reaches 420min, graphite morphology is flake-like together with chunky graphite. Chunky graphite precipitates from melt directly based on experimental results. HRTEM shows the microstructure of chunky graphite is the same with that of spheroidal graphite and its growth direction is along [0001]. As a result of concentration fluctuation and temperature fluctuation, togerther with effective nucleus, undercooling is small when graphite nucleates at the beginning of solidification. Graphite precipitates as spheroidal because of spheroidal elements is enough. Although concentration of spheroidal elements decrease with prolonged solidification time, growth pattern of graphite remains shperoidal under the condition of the residual concentration of spheroidal elements. However, concentration fluctuation and temperature fluctuation decrease gradually, thus undercooling needed becomes larger when graphite nucleates, which leads graphite to precipate as chunky. Besides, deleterious element, such as S in the melt promotes graphite to branch frequenlty, which also leads graphite to precipate as chunky.Effects of RE and Sb on graphite morphology and nodule count in ductile iron are investigated. Results indicate that the graphite nodule count reaches the maximum and mean diameter reaches the minimum with 0.014%Ce residual existing in ductile iron with Ce concentration from 0.005% to 0.020%. Spheroidal graphite is refined with Sb addition when cooling condition is the same and thus nodule count is increased. Chunky graphite emerges in the microstructure with Sb free with 90mm thickness specimen while fully spheroidal graphite is obtained with Sb addition. Nodule count of graphite is increased further with Sb and RE addition. Inreasing nodule count is effective to avoid chunky formation in ductile iron. The value of FC′D is 2.11874 and 5.22889, respectively, without and with Sb addition based on empirical electron theory of solids and molecules(EET), which indicates that diffusion resistance of C is enhanced because of Sb addition. The interface of graphite and melt is adsorbed with Sb, which also increases diffusion resistance of C. Meanwhile, nucleation rate is increased as a result of Sb addition. Thus, nodule count of graphite is increased, which eleminates formation of chunky graphite. Besides, the value of S' is increased, which promotes pearlite formation together with increased DFC′based on EET. Accordingly, proper concentration of Sb is needed to avoid chunky graphite and pearlite formation.Physical simulation method is adopted to study solidification of a spent fuel cask with its weight of 120t. A simulative ductile iron casting with 500mm thickness is produced according to its real solidification conditions in spent fuel cask. Results show that temperature distribution and solidification time is almost identical between simulative casting and spent fuel cask based on numerical simulation and experimental results. Microstructures change little at its isothermal position. Forced cooling system with chills and water is designed and its technologies is optimized by numerical simulation to ensure solidification time of silulative casting is less than 240min. Graphite is spheroidal and chunky graphtie is eliminated in the microstructure of simulative casting. Besides, matrix microstructure is almost ferrite. Tensile strength, yield strength, elongation, and impact toughness at -40℃in the last solidified region is 418MPa, 293MPa, 9% and 9.5J/cm2, respectively. Forced cooling, Melt treatment and casting technology for the simulative casting is applicable to a spent fuel cask with its weight of 120t.