Effects Of Process Factors On Microstructures And Properties Of High Si-Mo Ductile Iron

High Si-Mo ductile iron has high heat resistance and good thermal fatigue resistance,which is widely used in manufacturing of high-performance automotive exhaust manifold and turbocharger shell.But the higher Si and Mo contents in ductile iron make it difficult to control the spheroidization rate,the size and distribution of spheroidal graphite and intergranular carbide,which decrease the high temperature mechanical properties of the alloy.Under producing conditions,taking effective measures to improve the spheroidization rate and content of ferrite as well as refine the nodular graphite and reduce the number of grain boundary carbides have a positive effect on promoting the level of automotive engine manufacturing industry in our country.This paper aims at optimizing the spheroidization and inoculation technology,and solid-state phase transformation process for high Si-Mo ductile iron by changing the technological factors such as nodularizer varirties,inoculant treatment and changing shake-out time of green sand mold.The effects of processing method on microstructure and properties of the ductile iron were researched by using the quantitative metallographic technique.Elements segregation behavior is analyzed by color etching technique and micro-zone energy spectrum analyses,further explore the correlation between the process parameters and the microstructure and properties of high Si-Mo ductile iron.The results shows that:Excessive amounts of RE elements in the nodularizer not only deteriorate the graphite morphology,decrease the spheroidization rate and increase the pearlite content,but also reduce the mechanical properties of the alloy.The spheroidizing effect of nodularizer MgRE7-2 is better than that of nodularizer MgRE8-4.With the increasing of inoculant mass fraction,the spheroidization rate and the content of ferrite increase,the contents of pearlite and cementite tend to reduce,the mechanial properties present a trend of increasing at first and then decreasing.The recommended inoculant mass fraction is 1.0%～1.2%.The iron liquid after the spheroidizing and inoculation treatment process is poured into the casting mold.In the solidification and solid-state phase transformation,delaying the shake-out time of green sand mold can increase diameter of graphite sphere,reduce the contents of pearlite and cementite in high Si-Mo ductile iron,but the spheroidal graphite sphericity has no obvious change.When 1.4%MgRE7-2 nodularizer is used and Si-Ba inoculant mass fraction is 1.0%,shaking out 30 min later after pouring,the content of ferrite is 75%,pearlite and cementite are7%and 0.2%respectively in the ductile iron,the tensile strength of the alloy reachs 639.2 MPa,and the elongation 8.1%.During solidification,affected by Mo element,the Si element mainly distributes in the solidification end zone,Mo and Mn elements have the positive segregation charecteristic resulting in the formation of intergranular molybdenum and silicon-riched carbide.