Study On Siliconizing Behaviors And Properties Of Electromagnetic Pure Iron And Silicon Steel Produced By Surface Nanocrystallization

It is known that the electromagnetic pure iron and silicon steel have superior comprehensive soft magnetic properties,such as high permeability,low core loss and magnetostriction.It is difficult to use traditional rolling technology to produce the high silicon steel,because with the increase of Si content,the soft magnetic properties become well,however,the plasticity and brittleness become poor.Fe3 Si,acted as a ferromagnetic alloy with high hardness,good corrosion resistance and low conductivity,might be the ideal coating of silicon steel substituted the non-ferromagnetic separant.At present the methods of siliconizing include chemical vapor deposition,pack siliconizing,molten salt electrochemical reduction method and electroless siliconizing,which the first three methods need high reaction temperature,high energy consumption and complex process with the poor surface quality,so this paper chooses the more simple siliconizing method at low temperature without complex device electrolysis,protective gas required.It can be effective in low temperature of 650~900? for siliconizing.The surface mechanical attrition treatment is one of methods of metal surface nanocrystallization.The gradually refined grains and gradient structure surface are obtained due to the super plastic deformation,which can promote the subsequent siliconizing process.Higher thermodynamic driving force for the formation of compounds is provided through the high energy boundary on the surface of SMAT samples accelerated the surface chemical reaction.The surface nanocrystalline grain with high volume fraction could be used to provide many channels for atomic reaction dynamics and enhance the diffusion process.Two kinds of soft magnetic material,electromagnetic pure iron and silicon steel,were selected in this paper to deal with SMAT and siliconizing.The micro structure and the composition of phase were tested by metallographic microscope,X-ray diffraction,scanning electron microscopy and transmission electron microscopy.The microhardness tester,electrochemical workstation and vibrating sample magnetometer were used to test hardness,corrosion resistance and magnetic.The relationship or enhancement between siliconizing behavior and nanostructured surface layer induced by the super plastic deformation was studied in this paper.The formation,composition and thickness of coating were discussed and the reaction thermodynamics and diffusion kinetics were analyzed.The main results are summarized as follows.1.After SMAT,the grain size of the deformed layer is about 21 nm and the thickness of the deformation layer is 30~100?m.The grain size and the thickness of the deformed layer would not change with the increase of processing time.SMAT would increase the surface roughness of samples,which is conducive to promote the surface diffusion at the initial stage of the coating layer,so that the thickness of the layer is increased.2.Single-phase Fe3 Si is the only compound after siliconizing.The reaction diffusion coefficient k value is obtained by the analysis the thickness of coating layer which can draw SMAT on the promotion effect of siliconizing behavior.SMAT can also reduce the reaction activation energy of diffusion significantly so as to promote the interdiffusion reaction of Fe-Si.The enhancement of siliconizing behaviors will reduce or even disappear when the siliconizing temperature rise after SMAT.3.The mechanism of SMAT for promoting siliconizing behavior are summarized as follows.(1)The deformed layer can provide short path for diffusion so that the diffusion rate and the thickness of the coating layer both can be increased;(2)The deformed layer can provide enough driving force for diffusion due to the stored energy in SMAT.It can reduce the diffusion activation energy effectively.4.With the increase of Si content in the matrix,analyzed the diffusion rate,diffusion activation energy,vacancy formation energy and micro migration of siliconizing,the siliconizing behaviors are enhanced.5.The hardness of the samples reduced gradually from the deformed layer to the matrix after SMAT.And with the increase of SMAT time,the variation distance of hardness increased.The influence of SMAT time has on the hardness of the siliconizing layer is slight.The higher the siliconizing temperature is,the smaller the maximum hardness of the layer,but the greater the variation distance of hardness is.6.The microcracks,pores and other defects on the SMAT surface seriously affect the corrosion resistance of the material.The corrosion resistance of the matrix can be effectively improved by the formation of Fe3 Si layer.The porosity of the surface can be reduced by the increase of the siliconizing time,and the corrosion resistance of the material can be greatly improved.7.The coercivity and hysteresis loss of the samples are not changed,but the saturation magnetization increased with the increase of the siliconizing time and temperature.