Microstructure Evolution And Properties Of（Fe,Cr）₇C₃/Fe Surface Composite Produced By In Situ

Many studies show that the main reasons for failure occuring on the material surface are fatigue fracture, excessive wear or corrosion occurs. Thus a new type of surface composite with high hardness and wear resistance to improve the service life of machine is great significance. At present, metal matrix composites（MMCs） reinforced with particles become one of the hot topics in the study. In this work, the（Fe,Cr）₇C₃/Fe surface gradient composite was prepared by in situ method plus subsequent heat treatment with pure chromium plate and gray cast iron. The phase, microstructure and microhardness of the surface gradient composite were characterized by X-ray diffraction（XRD）, scanning electron microscope（SEM） and microhardness tester. The microstructural evolution, reaction layer growth dynamics and wear performance with mechanisms of（Fe,Cr）₇C₃/Fe surface gradient composite were also investigated. The main results obtained showed as follows:（1）The Gibbs free energy of chromium carbide will decrease with the increase of temperature in the range of 1223¹473K. The Gibbs free energy of carbide were negative, which explained that the Cr₃C₂, Cr₂₃C₆, and Cr₇C₃ phases could be exist stably at the studied temperature range. The stability of carbides can be placed in the order:Cr₂₃C₆ > Cr₇C₃> Cr₃C₂. Base on the isothermal section of Fe-Cr-C system, it can be deduced that the Cr₃C₂ phase could not be formed in this study.（2） The reaction layer of surface composite prepared by in situ method plus subsequent heat treatment presented a gradient distribution. The four reaction layers were: dense ceramic layer（Fe,Cr）₇C₃（zone I）,（Fe,Cr）₇C₃ granular ceramic layer（zoneⅡ）,（Fe,Cr）₇C₃ lath ceramic layer（zoneⅢ） and non-graphite layer（zoneⅣ）.（3）The growth rate constants and activation energies of different reaction zoneswere achieved through the growth dynamics study of the Fe-Cr-C ternary system.Further, the relation between the thickness of reaction layer and holding parameter has been concluded. The formation mechanisms of different reaction zones were also studied according to the microstructural evolution along with holding time.（4） The microhardness of（Fe,Cr）₇C₃/Fe surface gradient composites had been increased obviously. The microhardness of dense ceramic layer（1478HV0.1） is about 7times than the matrix（214 HV0.1）. The relative wear resistance of zone I,Ⅱ,Ⅲ, andⅣ of the sample prepared by in situ method plus subsequent heat treatment were found to be 7.1, 5.3, 5.1 and 1.4, respectively. According to the wear morphologies of different reaction zone, the wear mechanisms of（Fe,Cr）₇C₃/Fe surface gradient composites was analyzed with the physical model of wear.