Effect Of Bi On The Graphite Morphology And Low Temperature Impact Properties Of Heavy Section Ductile Cast Iron

Ferritic ductile iron has the advantages of good toughness and not easy to produce brittle fracture,which can meet the requirements of large wind turbine wheel hub,gearbox,base and other parts at room and lower temperature in the working environment.The heavy section ferritic ductile iron castings(greater than 100 mm in thickness),due to the cooling rate,and the solidification graphite distortion may easily occur at the center of the heavy section casings.The reduction of graphite spheroids,the large diameter of graphite and the floating of graphite lead to the decrease of the mechanical properties,especially the decrease of plasticity.Therefore,the effect of chunky graphite on the properties of heavy section ductile cast iron is studied.It is possible to improve the mechanical properties of the heavy section ductile iron by improving graphite morphology.The method of melt heat preservation was used to observe the graphite precipitation process of heavy section ferritic ductile iron.The results show that when the holding time is less than 4 hours,graphite is spherical.When the holding time reaches to 4 hours,graphite is chunky.With holding time increasing,the vermicular and flake graphite appear.The distribution of trace elements around graphite spheroids in as-cast organization was analyzed by means of EDS.The results show that the elements such as Mg,La,Bi are not only formed with S element as graphite nuclei,but also adsorb on the surface of the nodular graphite.According to the Wulf principle,the trace element Bi adsorbs on each crystal plane,the growth rate of crystal faces tends to be consistent.In this'way,the shape of graphite will be more rounded.Through further analysis of the mechanism of action of Bi by electron theory,it is concluded that S(the structure formation factor)value increases after adding Bi,which means that the ability of spontaneous nucleation becomes stronger.According to the EET,the FC'D value increases,the resistance to C atom diffusion increases in the liquid iron,the graphite ball number increases.It is found that the graphite nodule number increases and the graphite spheroidal size reduces after adding Bi.Due to the presence of chunky graphite in the center of the heavy section castings,the tensile strength,impact toughness and elongation are 24.1%and 76.9%lower than those on the edge of castings.When the Bi content is not more than 0.011 wt.%,the morphology of graphite is improved and the mechanical properties are improved.Observed near the center of impact fracture found on the surface of the toughening,there are uneven size and distribution of dimples on the fracture with chunky graphite,and the graphite eutectic masses fall off.The results of a series of oscillographic impact tests for heavy section ferritic ductile iron castings show that temperature has a significant effect on the crack formation and crack propagation ability during the impact process,and when the ductile and brittle transition temperature is higher than the ductile-brittle transition temperature,The crack shape required for impact fracture of ductile iron does not change much,and the crack propagation work decreases slightly,while the crack shape success and crack propagation work decrease sharply below the ductile transition temperature.The impact fracture of heavy section ductile iron castings was quantitatively studied by 3D laser confocal microanalysis system.With the decrease of temperature,the fracture roughness index decreases and the change of fracture roughness index with temperature is consistent with the change of impact absorption energy.With the increase of the ratio of chunky graphite,the impact energy decreases gradually,and the fracture type of specimen is from ductile fracture through mixed fracture to brittle fracture.In situ metallographic observation of the fracture was carried out to study the initiation and propagation path of cracks and the evolution of the microstructure at different temperatures.The fracture morphology of impact specimen changes smooth with the decrease of temperature,and the crack is easy to propagate along the chunky graphite with the decrease of temperature.The stress between graphite was studied and compared with in-situ observation diagram.The results show that irregular graphite is beneficial to the formation and propagation of microcracks and may even be the source of cracks.The distribution of slip bands near the fracture surface at different temperatures is analyzed by SEM,and the distribution of slip bands decreases with the decrease of temperature.The dislocation slip band near the impact fracture gradually becomes shallow,and the slip resistance of the dislocation increases.The dislocation configuration near the impact fracture at different temperatures is observed by TEM.The dislocation configuration near the impact fracture at different temperatures from free state to cellular structure to dislocation entanglement is studied.