The Calculation And Analysis Of Carbon Diffusion Coefficient In Ductile Iron

As one kind of important casting metal materials, ductile cast iron is widely used in machinery manufacturing, mining metallurgy, petrochemical industry and national defense. It gradually replaces the steel castings. When ductile cast iron experiences spheroidization and inoculation, its mechanical properties such as strength, ductility, toughness improved a lot to the level of steel since spherical graphite reduces the fragmentation effect to the organization. Although ductile cast iron is thriving, something which couldn’t be ignored appears in the production process. Graphite shape and quantity in ductile cast iron is the important factor affecting the performance of castings. It is a point of concern to ensure the graphite balls in castings present a good condition and sufficient quantities. Graphite ball is formed by diffusion of carbon. It is a crucial issue to control the size and shape of the graphite through the study of carbon diffusion effect and get a good casting.This paper studies the factors which influence spherical graphite morphology in ductile cast iron from3aspects. They are the cooling rate, the carbon content and the silicon, nickel, chromium content respectively.In this paper, the purpose of controlling cooling rate is realized by pouring wedge castings. Casting temperature change is recorded with the S-type galvanic couple and paperless temperature recorder. Cooling curve of metallographic sample is simulated by casting simulation software, and then cooling time and eutectic temperature are acquired. The average diameter of the spherical graphite is obtained by using metallographic analysis software to deal with metallographic photos. The diffusion coefficient of carbon in austenite in ductile cast iron is calculated according to the spherical graphite growth model. The result is roughly consistent with values in related articles, so as to verify the effectiveness of the selected parameters in calculation process. It also provides calculating basis for the solidification simulation of ductile cast iron. And the cause of the graphite distortion is analyzed from the perspective of cooling rate. Castings of different carbon content are poured according to ingredients of different carbon content. Spherical graphite growth parameters can be obtained with the same method. The diffusion coefficient of the carbon element in austenite of ductile cast iron is calculated. By fitting the relationship between diffusion coefficient and temperature, we can find that they meet the Arrhenius relationship. The effect of carbon content on diffusion coefficient is different under different temperatures. The higher the temperature is, the greater the effect is. When the temperature is very low, the effect is almost negligible. And graphite distortion phenomenon is analyzed from the perspective of the carbon content.As to the respectively small content of silicon, nickel, chromium in ductile cast iron, controlling its added content can realize the effect of changing composition. Casting temperature change is recorded with the S-type galvanic couple and paperless temperature recorder. Then cooling curve of different ingredients is obtained. The effect of elements content on spherical graphite morphology and size is studied by analyzing metallographic photos. At the same time the effect of cooling rate on spherical graphite morphology and size is verified.