Effect Of Nano-SiO₂ On Microstructure And Mechanical Properties Of As-Cast Ductile Iron

As-cast ductile iron has good mechanical properties and low production cost.It is widely used in mechanical manufacturing.With the development of economy and technology,people demand higher performance.However,the traditional method to obtain high strength or elongation by adjusting the ratio of pearlite to ferrite has some disadvantages.Therefore,it is urgent to find a new way to improve the performance of ductile iron through continuous research.The two-dimensional lattice mismatch is 2.90%between SiO2 and ?-Fe,and the characteristic parameter of empirical electron theory is 1.759 according to the theoretical calculation,which are the basic conditions of heterogeneous nucleation of primary y-Fe.In order to increase the density of SiO2 particles and improve their wettability and compatibility with ductile iron matrix,SiO2 particles were modified by high energy ball milling.The modified nano-SiO2 particles were added into ductile iron melt by an additional method,and the amount of nano-SiO2 added was 0.25wt.%,0.50wt.%,and 0.75wt.%,respectively.The results showed that the microstructure of as-cast ductile iron at room temperature was obviously improved by adding different content of nano-SiO2,the spheroidization rate increased,the graphite content in unit area increased,and the average size of graphite nodule decreased.The effect of adding 0.50wt.%nano-SiO2 was the most obvious.The ferrite structure in the ductile iron matrix was obviously refined.With the increase of nano-SiO2 content,the ferrite grain size level increased first and then decreased,and the ferrite grain size grade of 0.50wt.%nano-SiO2 ductile iron was the largest.That is,the number of grains per square millimeter is the largest and the average grain size is the smallest.The interlamellar spacing of pearlite decreased obviously,but the content of pearlite decreased slightly.The tensile properties of ductile iron at room temperature-were improved by adding different content of nano-SiO2.With the increase of nano-SiO2 content,the tensile strength and elongation of ductile iron increased first and then decreased.The tensile strength of 0.50wt.%nano-SiO2 ductile iron is the highest,which can reach 471 MPa,which is 34 MPa higher than that of the sample without adding SiO2.The elongation was also the highest(26.42%),which was 8.29%higher than that of the samples without adding SiO2.The tensile strength and elongation are improved at the same time.When 0.50wt.%nano-SiO2 was added,the impact toughness of ductile iron at room temperature was increased by 7.67%.With the decrease of temperature,the impact properties of the unadded and added 0.50 wt.%nano-SiO2 ductile iron showed a downward trend.However,when 0.50wt.%nano-SiO2 was added,the ductile-brittle transition temperature of ductile iron changed from-50 to-60,so that the material could be used in lower temperature environment.