Formation Mechanism Of Graphite Phase And Vermicularizing Quality Control Technology For Vermicular Graphite Cast Iron

Diesel engine is a main power supplier in transportation,engineering machinery and national defense equipments.With the development of society,environmental awareness and market requirement,diesel engines are developing towards low fuel consumption,low emissions,and high reliability,whose mechanical load and thermal load increase sharply,and traditional gray iron diesel engine can’t meet the requirement.Compared with gray iron and ductile iron,the tensile strength and thermal fatigue of compacted graphite iron(CGI)is closed to that of ductile iron,and the thermal conductivity is closed to that of gray iron.As a result,CGI has an excellent combination property,which has become the first choice of high end diesel engine’s cylinder blocks and cylinder heads.At present,all the CGI productions are used to take advantage of spheroidizing agent to treat the iron,where the spheroidzing elements were controlled within a very narrow range,and Mg was usually controlled in 0.007%-0.015%.During practical production,there are many factors affecting the forming of graphite phase,such as alloying elements contained in liquid iron and cooling rate in casting molding process,which makes it difficult to control the batch stability of CGI materials used in high-end diesel engine body and cylinder head.In order to explore the influence mechanism of various elements in liquid iron on CGI’s graphite morphology,and then achieve the technology of stable controlling vermicularity.In this paper,the first principles and Wulff cluster model were used to study the liquid structure of raw material and inoculated liquid iron at the atomic level.On this basis,the interaction between graphite clusters and 10 common alloying elements was explored,the parameters of spheroidizing ability were defined,and the distinction and quantification of spheroidizing elements and despheroidizing elements were realized.A new type of vermicular agent had been developed,a two-chamber thermal analysis sample cup had been developed,and a quality measurement and control system for liquid iron vermicular had been established,forming a new paradigm from theory to application.The main contents of the paper are as follows:First,the cluster structure of Fe-C-Si melt was studied based on first principles and Wulff cluster model.It was composed of pure iron cluster.Fe3C cluster and FeSi cluster.Through the simulation of the interaction between Fe3C clusters and Si atoms,it revealed that the distribution of C atoms in liquid iron evolved from Fe3C→freed C atoms→graphite clusters after the addition of Si inoculant.It confirmed that the graphite clusters in liquid iron after Si inoculation were characterized by graphene nanoparticles,which laid a foundation for the study of the effect of addition elements on the growth of graphite.Second,the interaction between 10 common added elements in liquid iron and graphene nanoparticles was calculated using the first principles,and the concept of spherodizing ability parameter was innovatively proposed,that is,the ratio of the difference between the adsorption of the same element on the top and side of graphite tto the top adsorption:According to value of the spheroidizing ability parameters.the added elements were divided into strong nodularizing elements,weak nodularizing elements,weak anti-nodularizing elements and strong anti-nodularizing elements,which was basically the same order as the previous experiments.Consequently,the quantification of the spheroidizing ability of elements was realized,so as to identify the influence of elements on the growth of graphite.Based on this,the mechanism of graphite growth was proposed that the competitive mechanism of top and side adsorption of added elements affected the growth mode of graphite.Third,a method was established to characterize the vermiculizer level of the element by the nodularizing ability and the proportion of the element(calculated by the molar number and mass percentage),and the total vermiculizer level was formed by the sum of the vermiculizer levels of all elements in the liquid iron.Considering the total amount of graphite precipitated during the eutectic reaction,the promoting effect of Si amount on graphite precipitated,the antinodularizing effect of that S consumed Mg,omitting elements with nodularizing ability in the neutral interval and elements with very low atomic numbers,a model for calculating the vermiculizer level of liquid iron was established.Through the production practice,the prediction accuracy was as high as 98%,realizing the goal of directly predicting the form of graphite through the composition of lliquid iron.Fourth,based on the quantification of elements’ nodularizing ability,it was found that rare earth elements have a stabilizing effect on the growth of vermicular graphite.Based on this,a vermicular agent with Mg and RE weight ratio of 1.1:1 was developed to maximize the role of rare earth in purifying molten iron,participating in inoculation,and expanding the vermicular range.A dual-chamber sand mold thermal analysis cup with the optimal structure was designed,and the additive inside the cup was determined to be CaSi with a particle size of 200 mesh and a dosage of 0.42wt.%.Combining the characteristic points of the thermal analysis curves and artificial neural network technology,an evaluation model of vermiculation index,an evaluation model of inoculation index,a prediction model of vermicularity,and a prediction model of mechanical properties were established by experiments,respectively.By integrating into wirefeeding treatment,thermal analysis detection,and vermicular quality evaluation and mechanical properties prediction as one,a measurement and control system for vermicular quality of CGI was developed.Combining with the calculation model of element vermicular level in the iron liquid,the real-time feedback and control of the metallurgical state of liquid iron can be achieved.