The Effect Of Silicon,Nickel Content And Cooling Rate On Microstructure And Properties Of Low Temperature Ductile Iron

As a kind of alloy material which can show good impact performance at low temperature,low-temperature ferritic ductile iron has been widely used in wind power components and high-speed railway steering axle boxes and other harsh working environments.The toughness of metal materials tends to decrease with the decrease of temperature.Although the as-cast QT400-18 L material used in engineering can achieve impact energy of not less than 12 J at-40℃,with the further decrease of temperature,The impact toughness of ductile iron is drastically reduced and cannot meet the requirements for use under special conditions.Therefore,exploring the basic theory and preparation technology of ultra-low temperature as-cast high-toughness ductile iron has important theoretical significance and engineering application value.Based on the QT400-18 L ferritic ductile iron metallurgy theory,the basic chemical composition of ferrite ductile iron and the low alloying treatment of molten iron were used to obtain ultra-low temperature and high toughness ductile iron material,and the chemical composition was explored.Correlation with the impact energy of low-temperature alloys was studied.The influence of cooling rate on the microstructure and properties of the alloy was studied.The physical mechanism of the impact energy of low-temperature ductile iron was revealed.The results show that:On the basis of the as-cast-40 ℃ ferrite ductile iron chemical composition,by appropriately reducing the silicon content,while using the nickel element to low-alloy the molten iron,and with the appropriate charge composition and smelting process,can be cast State-60 ℃ high toughness ductile iron material.Under non-equilibrium solidification conditions,the dendrites of austenite in ductile iron grow in a halo-dendritic manner.The solidification structure of the alloy consists of eutectic clusters,austenite dendrites,intergranular austenite and a small intergranular inclusion..The as-cast normal temperature structure consists of ferrite,pearlite and graphite sphere.It is worth mentioning that,unlike the all-ferrite low-temperature ductile iron pursued by the industry,a small amount of pearlite is allowed in the matrix of the test.The mechanical properties require that the tensile strength and elongation of the alloy at room temperature are 402.0MPa～405.4MPa and24.2%-24.8%,respectively,and the impact energy at-60°C is above 12 J.In the low-temperature and high-toughness ductile iron,when the Ni content is zero,as the Si content increases,the number of graphite balls increases,the distribution tends to be uniform,the ferrite content increases and the pearlite content decreases,and the normal tensile strength and elongation of the alloy.The rate decreases first,then increases first and then increases and then decreases.The low temperature impact energy tends to decrease.When the Si content is constant,the graphite spheroidization rate and spherical diameter increase gradually with the increase of Ni content.The tensile strength is increased,and the elongation is lowered,and the low-temperature impact work is lowered.As the cooling rate decreases,the number of dendrites decreases,the graphite sphere diameter and spheroidization rate increase,the number of spheres and pearlite decreases,and the number of ferrite increases.When the ductile iron composition is 1.6%Si0.2%Ni,the average impact energy at-60℃ is12.7J,the tensile strength is 404 MPa,the elongation is 23.4%,and the hardness is 145 HBW,which satisfies the low temperature and high toughness of ductile iron.Work requirements in cold conditions.