Study On The Microstructure Evolution Of Hypereutectic High Chromium Cast Iron Under The Action Of Pulse Current

The hypereutectic high chromium cast iron has a potential application prospect in wear-resistant material fields,because it has a high hardness.This attractive behavior is mainly associated with a large amount of M7C3 type primary carbides and eutectic carbides in their microstructure.However,the M7C3 type primary carbides are coarse in conventional as-cast condition,resulting in the continuity of matrix broken and the toughness decreased,which leads to the practical application of hypereutectic high chromium cast iron be limited.In order to improve the practical application value of hypereutectic high chromium cast iron,the size and shape of primary carbides have to be controled.Refining the primary carbides can improve the continuity of matrix and increase the toughness of material,without sacrificing wear resistance.In this work,the electric current pulse(ECP)is applied to hypereutectic high chromium cast iron during the solidification process and heat treatment process in order to improve the microstructrue.The microstructrues of samples in different experiment conditions are researched and analyzed by all manner of ways,such as optical microscope(OM),scanning electron microscope(SEM),energy dispersive spectrometer(EDS),transmission electron microscope(TEM),and X Ray Diffraction(XRD).The mechanisms of pulse current effect on microstructrue are discussed,and the performance of high chromium cast iron in conventional cast condition and electric current pulse(ECP)process are compared.The main contents and conclusions are as follows:It has a remarkable effects to refine primary carbides,reduce the primary carbide defects,and increase the density of primary carbides,applying the pulse current during the process of solidification.The equivalent diameter of primary carbides reduces from about 150?m to 65?m,the cracks and holes in primary carbides obviously decrease,and the microhardness of primary carbides increases from HV 1398-1420 to HV 1485-1501.The pulse current can not change the hexagonal prisms morphology of primary carbides,but can reduce the segmentation effect of matrix by primary carbides,and it is helpful to improve the usability of hypereutectic high chromium cast iron.The mechanism of the pulse current refining primary carbides is that the pulse current plays a role of magnetostrictive effect,which results in the pressure in the melt increasing,and then the nucleation rate of primary carbides in the melt markedly increasing;Meanwhile,the pulse current also has electromigration effects on the melt,which results in the ability of carbon atoms diffusion into primary carbides significantly enhancing.The pulse current increases carbon content of primary carbides,promotes the carbon atoms solid solution into tetrahedral interstice of hexagonal close-packed,and increases the lattice distortion of carbides,improves the solution strengthening and density of primary carbides.The pulse current can not change the growth process of primary carbides,but can maintain the sustained growth in the later stage of carbides growth,so that the center holes in primary carbides be reduce.In the process of austenitization from 700 ? to 1000?,the size and shape of primary carbides and eutectic carbides remained the same as before,with or without applying pulse current.But,the pulse current can reduce the temperature demands of solid state phase transformation in the high chromium cast iron,and increase the degree of phase transformation.Under the cohventional and ECP technology,the matrix phase both is translated into austenite during the process of austenitization at 1100?,and the eutectic carbides both is dissolved and coarsened.Nevertheless,the degree of primary carbides dissolution in the conventional austenitization process is far lower than that in the ECP process at 1100?,and the primary carbides preferably remained the original morphology.The macrohardness of sample and the microhardness of primary carbides get their peak value by the ECP process at 700?,to HV 958 and HRC 82 respectively.As the temperature of ECP process increasing from 700 ? t0 800 ?,the macrohardness of sample and the microhardness of primary carbides both reduce,and reach a plateau in in 800??1000? range,then fell again to HV 390 and HRC 71 respectively at 1100?.