Study On Modification Mechanism Of （Cr,Fe）₇C₃ Carbides In Hypereutectic High Chromium Cast Iron Under The Electric Current Pulse Treatment

M₇C₃ carbide is an important hard phase in hypereutectic high chromium cast iron（HEHCCI）.HEHCCI has poor toughness because the size of primary carbides is relatively large,which limits the improvement of the wear resistance of alloy materials.The refinement of M₇C₃ carbides is usually carried out by inoculation and rapid solidification.Although good results have been achieved,there are still some limitations,such as inoculation fade and limited casting size.The electric current pulse（ECP）processing technology has been extensively studied and verified in the refinement of solid solution crystal grains;there is still a lack of systematic mechanism research in the refinement of compound crystal grains.Earlier research found that the ECP has a different inoculation and refinement of M₇C₃ crystal grains from solid solution crystal grains,However,it is currently unclear about the mechanism and influencing factors,as well as the competition and coordination mechanism of the different effects of the ECP in the nucleation and growth of M₇C₃ compounds.Based on the previous research,the ECP of different density was applied to the cooling stage above liquidus,the initial stage of solidification,the middle stage of solidification and the whole course of solidification.The competition and coordination mechanism of the free energy effect of pulse current and the impact effect of stirring on melt was studied by comparing the size of primary and eutectic（Cr,Fe）₇C₃ carbides in HEHCCI,The inoculation mechanism of the ECP on carbides in the cooling stage above liquid phase and the refinement mechanism of carbides in nucleation stage are expounded.Combined with SEM,TEM,XRD,EPMA,microhardness,nanoindentation and other techniques,the influence law and mechanism of current density and action time on the Cr/Fe ratio,microstructure and properties of carbide were analyzed.Finally,the influence of the ECP on the strength and wear properties of HEHCCI was investigated.The results show that when the current density is 196.6 A/cm²,low processing temperature（melt superheat is 5～10℃）,the primary carbide is refined,the average size is reduced by about 30.5%and the quantity is increased by about 78.3%.The free energy effect of the ECP is the main reason for the inoculation of primary carbides by the ECP,which increases the number of like-primary phase clusters,and improve the nucleation rate during subsequent solidification.However,the inoculation effect is also affected by the ECP processing temperature,current density and internal binding force of like-primary phase clusters.If the ECP processing temperature is too high,the current density is too large or too small,and the internal binding force is weak,the inoculation effect of the ECP will be weakened.At the initial stage of solidification（primary carbides nucleation stage）,the size of primary carbide decreases first and then increases with the increase of current,and the maximum decrease is 26.5%.At this stage,the free energy effect is competitive with the electromagnetic impact effect:the nucleation proliferation caused by the free effect can play an immediate role when the ECP is applied;The effect of suppresses nucleation can play a significant role when the electromagnetic impact force needs to be increased enough to overcome the cohesion of carbide-like clusters.In the middle stage of solidification（primary carbides growth stage）,the size of the primary carbide decreases first and then remains unchanged with the increase of current.The maximum size of carbide decreases by about 32.1%.The free energy effect and electromagnetic stirring effect increases the growth anisotropy of carbides,making the primary carbide rods more slender,which is easy to be broken by electromagnetic impact and stirring effect,and the grain refinement is caused by the increase of carbides.The three effects are a collaborative relationship.The eutectic colony diameter decreases with an increasing number of primary carbides;the thickness of the eutectic carbide sheet is almost unchanged when the sample is treated by the ECP in the melt cooled above liquidus and solidified stage.The ratio of Cr/Fe in the primary carbide decreases with the increase of current density when the sample was processed in the melt cooling stage above the liquidus;The corresponding Cr/Fe ratio of primary carbide and eutectic carbide when samples were processed at different stages of solidification.On the one hand,the free energy effect,electromagnetic impact effect and electromagnetic stirring effect of the ECP reduce the effective distribution coefficient of Cr by electro-induced undercooling,reducing Cr activity and increasing hot undercooling;on the other hand,the electromagnetic stirring effect reduces the thickness of solid-liquid interface diffusion layer and improves the effective distribution coefficient of Cr.The change of the Cr/Fe ratio is the result of competition between two aspects.The stacking fault density increases from about 20%to about 50%and the stacking fault density decreases to about 30%when the current density increases to 491.5 A/cm²when the current density is 196.6 A/cm²≤J≤393.2 A/cm² during carbide growth.The phenomenon is related to the magnitude of the energy W acting on a single atom by the ECP.When the W is small,it is beneficial to the transition of liquid phase atoms to the solid phase,to increase the growth rate and dislocation probability,and to increase the stacking dislocation density.When the W is too large,the formation of stacking faults and the stacking fault density is reduced because the ECP promotes the transition of solid-liquid atoms at the same time.When the Cr content is equal,the hardness of carbides with high-density faults is 100～200 HV_0.3.higher than that of carbides with low-density faults.Compared with samples without the ECP,the ability to resist external indentation and breakage decreased after primary carbide refinement,and the hardness and wear properties of the materials decreased by 4.5%and 46.7%,respectively.After the primary carbide is refined,the stress concentration can be reduced and the crack propagation is hindered.The compressive strength and impact wear resistance of the sample are increased by 32.8%and 40.7%,respectively.when the size of the primary carbide is the same,the carbides treated at the middle stage of solidification can enable the sample to exhibit higher hardness,strength,and three-body and impact wear resistance.