Research On Solidification Features And Microstructural Formation Of High Nickel Ductile Iron

High nickel ductile iron is one of the advanced materials to manufacture high-performance automobile exhaust manifold and turbocharger housing.It has excellent high-temperature structure stability,good resistance of corrosion and oxidation,good resistance of high temperature fatigue performance,and good casting properties.However,molten iron of high nickel ductile iron that often results in lower nodularity and intergranular carbide for smaller cooling rates or the effect of impurity elements,which can lead to the decline of high temperature properties.Because of the lack of basic theory and data support,it will be full of blindness and probability in production.The article studies the solidification process and formation law of microstructure of high nickel ductile iron using the color etching teclhnique to reproduce the evolvement rule of solidification microstructures of high nickel ductile iron,the solidification sequence of all phases are analyzed by liquid quenching.Based on this,the impacts of cooling rate on microstructure and properties of high nickel ductile iron are analyzed,in order to provide theoretical support for broaden the engineering application of high nickel ductile iron.The results of research are indicated that:High temperature solidification structure of high nickel ductile iron can be clearly showed using pyrosulfite reagents under the condition of temperature 24-26? and etching time 45?50s.Un,der the condition of non-equilibrium solidification,the first precipitate phase of high nickel ductile iron is austeniten and consequently eutectic transformation of graphite and austenite is occurred.Solidification structure of high nickel ductile iron is composed of austenite dendrite,spherical graphite/austenite eutectic and intergranular carbide.High nickel ductile iron has the endogenous paste solidification way and sponge-paste solidification way,it has halo dendrite and chilling dendrite accordingly.These microstructures that distribute along austenitic matrix edge are Mo-Cr-rich carbide in bulk,strip and dendritic,and Ti-V rich carbide in tiny polygonal.At the same time there are also intermetallic compound FeNi3 and complex solid solution with(NiFe)3Si as matrix that closely connected with carbides.Compared with normal ductile irons,the segregation of Mn and Si are opposite in high nickel ductile iron.Ni and Mn elements are negative segregation and Si,Cr and Mo elements are positive segregation.Coolin,g rate has a significant impact on microstructure and mechanical properties.With the decrease of the cooling rate of high nickel ductile iron,on the one hand,the number of dendrite are reducing,the dendrite grows slowly and develops thick,the morphology tend to be developed.On the other hand,the area of LTE is expanded and easy to farm m icro-shinkage,the quantity of carbide increases slightly.Meanwhile,when the cooling rate is decreasing,the number of graphite and nodularization and nodularity reduces,graphite diameter increases,and explosive graphite and chunky graphite are appeared,which lead to comprehensive mechanical properties decrease.