| In this thesis, improvement of the brittleness and microstructure of boride layer of 4Cr13 steel was studied by adding RE compound in boronizing medium and optimizing contents of main component KBF4, B4C, RE and process parameters by orthogonal tests, according to the disadvantages of 4Cr13 steel being high brittleness, easy spalling, thin boronizing layer after boronizing. Microstructure characteristics and depth and transition zone of various processes were observed and measured using metallographic microscope (OM). The phase structures of the surface and sub-surface of the boride layer were analyzed using X-ray diffraction (XRD). The high magnification morphology of boride layer, transition zone and surface were observed by means of scanning electron microscopy (SEM). Composition and quantity of all elements in a spot were tested and changes of B, Cr atom in boronizing layer were analyzed by using Energy-Dispersive Spectrometer (EDS). Hardness profiles of boronizing layer with RE and without RE addition were measured by using micro-hardness tester from surface to core. Wearing properties were carried out by the roll-slid wear between GCrlS standard sample and 4Cr13 steel samples which were treated by hardening, furnace cooling after pack boronizing treatment and hardening directly, and then the mechanism of excellent wear resistance after boronizing were discussed.The results of orthogonal tests showed that the content of KBF4 affects significantly to case depth, which decreases with the increase of KBF4, and the optimum content is 3% by weight. The influence of B4C on case depth is smaller than that of KBF4 and it's optimum content is 5% wt. The optimum content of RE, which has significant effect on microstructure of boride and a certain influence on case depth, is 0.5% wt. The processing parameters of boronizing of 4Cr13 steel have a significant influence on both case depth and morphology of boride, in which the temperature is largest. With the increase of holding temperature, the depth of boronizing layer ascends approximately linearly but the loose of boride is the stronger and stronger at the same time. Therefore 920℃ is temperature selected to be a optimum temperature. The influence of holding time is smaller than that of holding temperature, and 8 hour is selected.Observation by microscope showed that the layer is composed of white zones inwhich are boride and black zones. Boride with RE is addition dense and has a little loose and that without RE is loose greatly and has micro-crack, which indicates that the morphology and brittleness of boride are improved and the depth of boronizing layer is deeper by adding RE.The results of XRD detection on surface and subsurface showed that boride layer with RE addition consists mainly of FeaB, (Fe,Cr)2B phases which are more than that in without RE layer and a little FeB, and Fe4N,Fe3(BC) phases occur in sub-surface, CeOF, LaOCe phases occur in surface of boride. This verified RE atoms diffusion into surface layer and Cr element in the boride consists in the form of (Fe,Cr)2B.It was also showed that the porosity zones are made up of loose boride and have impurities in gaps by SEM. The results of EDS test showed that amounts of B atom in case are about 12~26%wt for with RE and 18~40%wt for without RE ,which proved that RE addition can promote the diffusion of boron atom to form mainiy Fe2B phases which can low brittleness. The quantity of alloy Cr element is 10.5~ 12.6%wt in the dense zones of boride which is lower slightly than in the matrix, and 9~10%wt Cr element in the loose boride which is lower greatly than in the matrix, and 10.5~38%wt Cr element in transition zones which is higher than in the matrix and distributes heterogeneous greatly, which proved that alloy Cr element diffuses toward matrix in the boronizing process and transition zones get strengthening and tightly adhere to boride.The results of micro-hardness test showed that hardness has a highest value in the boride, surface hardness is slightly low due to loosing, and there is...
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