Study On Vermicularizing Process Characteristics Of Low Lanthanum Content Magnesium Ferrosilicon Alloy

Vermicular graphite cast iron has good combined mechanical properties and is more and more widespread application in the cylinder head as the best material for a new generation of high performance engines.However,the bottle neck which restricts the application of vermicular graphite cast iron is that the proper addition amounts range is considerably narrow in actual production,which results in that the controlling process of actual production becomes very difficult.In this paper,low lanthanum content magnesium ferrosilicon alloy was used to perform the vermicularizing process.With the help of color etching technique,thermal analyzer(TA),scanning electron micrograph(SEM),and energy dispersion spectrum(EDS),the effects of the addition amount of this vermicularizing alloy on microstructure and alloy properties were researched.The solid/liquid phase transformation characteristics and thermodynamics parameters were investigated as well.Besides,the solidification mechanism of vermicular graphite cast iron was summarized.The results indicate that:When the sulphur content of industrial base iron is around 0.017%～0.018%,the precondition of which vermicular graphite cast iron with a vermicularity of 50%-85%were produced is that addition amounts of the vermicularizing alloy is between 0.4%and 0.9%.Accordingly,the equilibrium concentrations amoung magnesium residue,lanthanum residue and sulphur residue is 0.011%～0.018%,0.007%～0.009%and 0.013%～0.015%respectively.With the increase of the addition amount of vermicularizing alloy,the residual Mg and La contents increase,while the residual S content decreases.And the graphite morphology transforms from flake to vermicular and spherical graphite.The addition amounts of vermicularizing alloy which converts flake graphite to vermicular graphite is in a narrow range of 0.35%～0.4%.In addition,the fracture morphologies on the wedge specimen show a color transformation of dark gray→silvery gray,dark spots and light fringes→silvery gray and light fringes→bright white.Under the precondition that the addition amount of vermicularizing alloy exceeds 0.4%,with the increase of the addition amount of vermicularizing alloy,the VG percent tends to decrease,and the morphology changes of vermicular graphite is small→long and thick→short and thick.Meanwhile the quantity of pearlite and alloy properties both increase.The base iron undercooling(△T)decreases with the increase of the silicon content on the precondition that the other element contents are constant.After the vermicularizing process,the undercooling of the molten iron is affected by both the chemical components of base iron and the addition amount of the vermicularizing alloy.A small amount of vermicularizing alloy will not enlarge the undercooling of the molten iron dramatically.But when the addition amount of the vermicularizing alloy increases to a certain value,the molten iron undercooling will increase continuously and obviously follow the quantity increment of the vermicularizing alloy,and the amount of austenite shows a jump,and the size of the eutectic cells drops.The solidification process of vermicular cast iron is divided into three stages according to the precipitation order of graphite.Firstly,primary solidification stage.Nodular or agglomerate primary graphite precipitate preferentially.And the austenitic shell is formed around graphite,and continuously thickened outwards.The austenite dendrite grow from the austenitic shell as well due to the thermal undercooling and composition undercooling.Then,the earlier stage of eutectic.Eutectic graphites nucleate preferentially at interdendritic region in which fluid is rich in carbon and grow along the austenite dendrite arm gaps,while the graphite is surrounded laterally by austenite.Besides,the graphite head is always in contact with the undercooling molten iron.Gradually,eutectic austenites and austenites dendrites formed in advance grow into one,that is vermicular graphite eutectic cells.Lastly,the later stage of eutectic.The smaller and independent vermicular graphites precipitated from the residual liquid grow in the model of divorced graphite austenite eutectic,and then solidification is completed.The segregation characteristics of alloy elements in vermicular graphite cast iron during the solidification are as follows.On the one hand,the farther away from graphite,the lower the silicon content in matrix microstructure.On the other hand,the silicon content in the austenite on both sides of the graphite gradually decreases along the graphite growth direction.In addition,the positive segregation elements such as manganese,phosphorus and titanium are segregated in the final solidification region,which gives rise to carbides,phosphorus eutectic and non-metallic inclusions.