Fundamental Research On Solid-state Graphitization In Spherical Graphite Iron By Pulse Current

To the spherical graphite iron pipe made by hydrocooling metallic centrifugal casting, the high-temperature annealing is employed to remove the cemenite of the as-cast pipes, which leads to high energy consumption and the pipe distortion. In the paper, the pulse current treating technique was designed to accelerate the solid-state graphitization of spherical graphite iron. Firstly, the solid-state graphitization of spherical graphite iron by electropulsing was studied by the way of theory. The vality of theoretical research results was verified by the way of experimental technique. The mechanism of solid-state graphitization of spherical graphite iron under the effect of pulse current was formulated systematically.The theoretical research results show that the free energy of cementite increases by the pulse current due to the dislocation pile-up around the cementite, which makes the stability of the cementite reduced. The driving force for the graphitization of cementite by electropulsing increase, which create favorable thermodynamic condition for the quick graphitization of cementite. In the paper, the theoretical relationship between the dissolution rate of cementite, nucleation rate of graphite and the pulse current density was established. With the increase of pulse current density, the graphitization of cementite was accelerated. The dissolution rate of cementite was increased by increasing the unstability coefficient and time exponent, decreasing the diffusion activation energy. The nucleation rate of the graphite is increased by reducing the potential barrier of the graphite nucleation, shortening the incubation time of the graphite nucleation and increasing the nucleation site of the graphite. The diffusion coefficient of carbon increases by increasing diffusion preposition coefficient and reducing diffusion activation energy by the pulse current. The increased graphites shorten the diffusion distance of carbon in austenite and the carbon atom transform to the graphite quickly. So the graphitization of cementite is accelerated by the pulse current.The experimental results show that the content of cementite is decreased by 11% and the content of neonatal graphite is increased by 2% when the pulse current with the parameter of 300V,15Hz,3min was employed at 920°C compared with the unelectropulsing treating. The neonatal graphite nucleates at the austenite grain boundary. The content of cementite is decreased by 12% and the content of neonatal graphite is increased by 3% when the pulse current with j_max≈1.67kAmm^-2 was employed at room-temperature compared with the unelectropulsing treating. The neonatal graphite nucleates near the cementite. These results showed that the graphitization of cementite is accelerated no matter the pulse current is employed at high temperature or at room temperature, which proves the theoretical analysis result that the pulse current can accelerate the solid state graphitization in spherical graphite iron. The electron probe analysis showed that the distribution of carbon becomes more homogeneous in the austenite after electropulsing treating, which declares that the pulse current can accelerate the carbon diffusion ability in the austenite. The experimental results agree well with the theoretical analysis results.The solid-state graphitization by the pulse current with different parameter was analysed theoretically. The results showed that secondary action of the pulse current to accelerate the graphitization of cementite can be displayed when the temperature is high enough. When the high density pulse current was applied at room-temperature and low density pulse current was employed at high temperature, with the increase of pulse current density, pulse current frequency and pulse current treating time, the graphitization of cementite is accelerated. The quick graphitization of cementite can be realized when the sample was heated to 1100℃in 13s by the pulse current with 3000V,800μF. The graphitization of cementite was finished basically when the pulse current with 800V,20Hz was employed at 920℃for 3 min. Compared with the normal graphitization for removing cementite completely, the time of high temperature graphitization was shortened about 7min, the temperature was decreased about 60℃.In the paper, the matrix structure transition of spherical graphite iron by the pulse current was studied. The results showed that the content of ferrite was increased, the content of perlite was decreased and the lamellar distance of perlite was decreased when the pulse current was employed during high temperature graphitization.The research work creates theoretical and experimental evidence for using electropulsing to accelerate solid state phase graphitization of spherical graphite iron.