Research On Microstructure And Properties Of Spherical/vermicular Graphite Layered Composite Cast Iron

The metal molds used in the industry,such as glass molds,steel ingot molds,aluminum ingot molds,etc.,are characterized by repeated scouring of the inner wall with high temperature melt,and the temperature difference between the inner and outer mold walls is large,resulting in large thermal stress,which requires the material has good oxidation resistance,thermal fatigue resistance and good thermal conductivity.The traditional homogeneous cast iron is difficult to meet the above requirements.Therefore,the microstructure and properties of cast iron with different graphite forms are studied first.The layered composite structure cast iron with spheroidal graphite and worm-like graphite inside was proposed.The effects of different process factors on the thickness of the surface spheroidal layer were studied and the mechanism was analyzed.The ball/vermicular layer composite was successfully applied to the glass mold,and the ball/vermicular/flake layer composite mold was prototyped.The paper has obtained the following research results:(1)The main cause of transverse cracks in the D-type graphite glass mold: the gas eye itself is a gap in the material;the E-type graphite is present in the inner surface structure;the carbide formed during the casting process is not completely eliminated during the annealing process.The reason why the middle surface of the vermicular cast iron glass mold is easily oxidized is due to the high vermicular rate of the working surface and the highest temperature in the middle part when the mold is in service.(2)In ductile iron,compacted graphite iron and D-type graphite cast iron with vermicular rate of 50% and 80%,80% vermicular rate of compacted graphite iron has the best thermal conductivity;ductile iron has better oxidation resistance and thermal fatigue performance than other cast iron,therefore,the ideal mold material should be spherical graphite on the inner surface and the other is a high vermicular rate compacted graphite iron.(3)When the thickness of cold iron increases,the thickness of the surface chill layer increases;when there is no cold iron,the magnesium content in the coating increases,which can increase the thickness of the surface spheroidal layer.When there is cold iron,it will inhibit the diffusion of magnesium into the interior.Therefore,the effect of the magnesium-containing coating is offset;when the residual magnesium content in the molten iron is increased,the graphite form changes in a flake?vermicular?spherical manner,and in the case of a cold iron thickness of 40 mm,when Mg residual=0.0083%,the surface layer is spherical graphite,the core is in the form of a flake,and when the residual Mg content of the molten iron is further increased,the thickness of the surface of the chill layer increases.The reason for the above phenomenon is that the graphite form is affected by the cooling rate and the content of Mg element.The increase of the degree of supercooling and the increase of the Mg content can increase the speed of graphite growth along the screw dislocation step and promote the growth of the graphite into a spherical shape.(4)The preparation of the spherical graphite layered composite glass mold was carried out.When designing the pouring system,the runner should be avoided in the middle part of the mold to reduce the scouring of the middle part of the cold iron during the filling process of the molten iron;A suitable inoculant is added in an amount of 0.2% when the packet is inverted.The spherical/vermicular/flake layer composite mold has been successfully developed.Under the premise of ensuring that the molten iron is in the critical state of flakevermicular transition,the casting temperature should be appropriately lowered to improve the cooling rate of the molten iron crystal,thereby improving the surface spheroidal graphite thickness.