Effects Of Alloying Elements And Vermicular Agent Content On Solidification Behavior Of Compacted Graphite Cast Iron

Vermicular graphite cast iron has been widely applied in high power density diesel engine cylinder head because of its excellent comprehensive performance.However,with the continuous improvement of engine power,the performance index of vermicular graphite cast iron must be further improved.The properties of the material depend on its microstructure,and the microstructure transformation depends on the solidification process,so it is necessary to gain a deeper understanding of the solidification process of compacted graphite cast iron in order to improve its properties better.Many scholars have studied the solidification process of compacted graphite cast iron,however,almost all of the research results are qualitatively described,and there is rare quantitative description.On the basis of vermicular graphite cast iron used in high power density diesel engine cylinder head,the crystal structure of the compacted graphite cast iron during the solidification process was obtained by means of liquid quenching method,and its microstructure transformation process was analyzed with thermal analysis together,so that the effect of alloying elements and the amount of vermicular agent on the solidification process of compacted graphite cast iron was analyzed,and the rule of microstructure transformation of compacted graphite cast iron was summarized,the vermicularizing mechanism was investigated as well.Besides,the 3D morphology transformation process of vermicular graphite was investigated by high resolution X-ray 3D scanning imaging technology,and the nucleation mechanism of vermicular graphite was analyzed by SEM.And the rule of vermicular graphite nucleation and growth was summarized by DT2000 professional metallographic analysis software,so that the dynamics of vermicular graphite nucleation and growth was studied.The results are as follows:?The alloying elements Cu,Mo,Sn have no obvious effect on the characteristic value of solidification curve,the eutectic structure transformation process and the length and content of graphite.?The effect of the amount of vermicular agent on the eutectic microstructure transformation of vermicular graphite cast iron is very large.With the addition of 0.2%,the growth rate of eutectic austenite is slower than that of the distorted graphite,so the distorted graphite is formed into a flake,and connected in a wide range.The connectivity of the lamellar graphite is about 88.87%at the end of the eutectic reaction.With the addition of 0.4%,the growth rate of eutectic austenite is faster than that of the distorted graphite,which makes the distorted graphite can not be connected on a large scale,and ultimately grows into vermicular graphite.With the addition of 0.6%,the growth rate of eutectic austenite is faster than that of the 0.4%addition,which leads to the formation of spherical graphite which is completely wrapped and can not be distorted,so the vermiculating rate drops.?The initial morphology of graphite in the compacted graphite cast iron is spherical.After the beginning of the eutectic reaction,the content of oxygen and sulfur in the molten iron is enriched to a certain extent due to the precipitation of eutectic austenite,which leads to the distortion of the spherical graphite,and then the distorted graphite is connected with each other.The eutectic austenite gradually wraps the distorted graphite because the growth rate of the eutectic austenite is faster than that of the distorted graphite,which makes the distorted graphite can only be connected with each other in a small range,and finally grow into vermicular graphite,the connectivity rate of the vermicular graphite at room temperature is about 32.5%.?The nucleation core materials of vermicular graphite include CeS?MgS?CaS?La₂S₃?FeS?Al2O3?TiC,and the nucleation core of vermicular graphite consists of several components,which size is 1?2.5?m The graphite of hypereutectic vermicular graphite cast iron nucleates mainly in the primary graphite stage,and the graphite nucleation is also found at the beginning of eutectic reaction,besides,the graphite nucleation rate decreases.The relationship between nucleation number N and time t is N =11.1+1.1sin[?t + 5911.8??/11929.4].?The eutectic reaction stage is the main stage of graphite growth,the crystallization rate of graphite increases firstly and then decreases,and the crystallization rate of graphite is fastest at eutectic reaction 70s,at this time,the growth rate of graphite length is 1.33?m/s,and the growth rate of graphite content is 0.18%/s.The relationship between graphite length L and time t is L = 100.54-93.87/(1 + e^{?t-99 98?/16.98}).The relationship between graphite content C and time t is C = 9,02-8.64/(1 + e^{?t-101.74?19.54}).