Study On The Process, Structures And Properties Of Sintering-Boronizing Of Fe-based Powder Metallurgy Materials

With the development of morden industry and technologies, the requirement on the performance of Powder Metallurgy (PM) part increases. Due to their usage as constructral materials, PM parts are requested to have better wearing resistantce and surface hardness, making surface mordification of PM parts a very important field in material science. The surface chemcal heat treatment, which is used to alter the composition, structure and morphology by atoms diffusion and chemical reactions, has been widely used in PM parts.Boronization, as a common surface chemical heat treatment, can increase the surface hardness to 1200-2000HV. It can improve the parts'wearing resistance comparing with other chemical heat treatment such as carbonization, nitriding, carbonitriding and percolations. Besides, boronizing layer has very good corrosion resistance and hot hardness. Presently, boronizations are generally carried out after the sintering of PM parts. The energy comsumption can be reduced and the distortion of PM parts induced by second heating can be eliminated, if the sintering and boronizing can be combinded.In this thesis, a new sintering-boronizing process is developed and Fe-based PM parts with comparable surface properties as traditional boronizing process are achieved. Process parameters, resultant boronizing layer and properties are stuied. HRTEM is employed to study the microstructures as resultant boronizing layer. Phase structures, positions of iron and boron atoms are determined.The growth disciplinary of boride is also observed. Based on above experiments, a mechanism of sintering-boronizing process is proposed.Due to the shortage of boronization such as brittle and shedding, rare earth element or solid carburizing agent is added to produce a rare earth element boronizing or a carbonious boronizing atmosphere. Surface hardened Fe-pased PM part with rare earth element containing or carbon containing boronizing layers are achieved. The effect on boronizing structures and properties are studied.The effects of sintering temperature, heating duration and compact density on PM parts properties are first studied. And the results indicate that sintering temperature has a very big influence on the thickness of boroning layer, followed by heating duration. At a lower compact density, the resultant higher porosity is benefit for increasing the thickness of boronizing layer but the quality of boronizing layer is reduced when the porosity is too big. The optimized boronizing parameters are 1100°C and 4 hours, with a resultant boronizing thickness of 145μm.Studies on traditional boronization structures show that boronization structures are mainly composed of Fe2B phase and small amount of FeB phase. The micro hardness of boronizing layer can reach 1000¹600HV0.3 while that of substrate is only 190 HV0.3. Quenching process can eliminate the grads of hardness transitions from boroning layer to substrate and increase the interaction between boronizing layer and substrate. The three-point bending experiments indicate that the bearing capacityof boronizing layer is bigger against a compressive stress than against a tensile stress. The wearing resistance of sintering-boronizing layer is better than quenched carburizing speciments and direct sintering specimens. The wearing rate of sintering-boronizing specimens is only 5¹0% of that of direct sintering specimens. The sintering-boronizing specimens also have a better corrosion resistance. The corrosion rate is reduced to 50% by replacing direct sintering with sintering-boronizing process.The effect of rare earth element content on sintering-boronizing process is also studied. The results demonstrate that rare earth element has a positive effect on sintering-boronizing in a certain range and excessive rare earth element can reduce the thickness of boronizing layer. The EDX observation show that the solid solution content of rare earth element in boronizing layer is 0.44 at.% and that in transition region is 0.68at.%. Rare earth element tends to exist inside the defects. Rare earth element bearing boronizing layer is also mostly composed of Fe2B phase with little FeB phase. The FeB in boronizing layers decreases as the increase of rare earth element content. Results of three-point bending experiment show that the bending strength of 6wt.% rare earth element content boronizing specimens can reach as 606.7 MPa, which is higher then that of derect sintering specimens (510.6 MPa) and that of none rare earth element boronizing specimens (566.1 MPa). Quenching process can obviously increase the bending strength of boronizing specimens. Rare earth element containing boronizing layer have a higher microhardness, improved wearing resistence, reduced friction coefficient and enhanced corrosion resistence. Studies on the effect of carburizing atmosphere on the microstructure and properties of sintering-boronizing showed that the carbon atoms hinder the borozing to some extent. The boronizing layer decreases as the increase of the concentration of carbon atoms. At 1120°C, due to the infiltration of carbon atoms, the microstructure of boronizing layer is changed from original Fe2B phase to eutectic boride, ferrite and pearlite. X-Ray diffraction analysis at this temperature show that Fe2B,α-Fe and Fe3(C,B) domain in the boronizing layer. Fe2B phase reduces and Fe3(C,B) increases as the increase of carbon content in boronizing agent. The result of three-point bending experiments shows that sintering boronizing specimens have a higher bending strength and improved tenacity. Bending strength of carbon containing boronizing specimen can be further improved by quenching process. It can reach 1258.3 MPa with 30% carbon atoms, which is significantly higher than that of carburized specimen (899.3MPa) and quenched boronizing specimen (922.1MPa), indicating the carbon content of carbon boride layer greatly improve the bending properties. Due to the existence of eutectic structure in carburous boronizing layer, the hardness of boronzing layer decrease to 680HV0.3. However, the wear resistence is comparable with that of boronizing specimen, which shows that the carburous boronizing specimen has good wear resistance. Corrosion tests show that the same carbon boride samples has good corrosion resistance.TEM samples are prepared. HRTEM was employed to analysis the microstructures of boronizing layer and Fe2B phase. The results are listed below. The boronizing layer is found to be Fe2B phase based on the calculations on diffraction patterns and measurements from HRTEM images from different directions i.e. B= [1(2|-)0], B= [1(3|-)0]and B= [1(4|-)0]. Elliptical Cu-rich precipitates are observed in boronizing layer and substrates. The phase relationship of Cu-rich precipitate and Fe2B is that {111}_Cu//{002}_Fe2B and that of Cu-rich precipitate andα-Fe is {111}_Cu// {110}_α-Fe. HRTEM observations show that Fe2B grows along the <002> direction. Based on the analysis of growth mechanism of powder under different conditions and the precipitation of elliptical Cu-rich phase, Mechanism model of the formation of sintering-boronizing process is established. From this model, it is believed that sintering and boronizing process simultaneously but the preicipitaion of Cu-rich pahse is before the formation of boronizing layer. Additionally, boride may nucleate and grow inside the holes in the materials.In this thesis, the sintering boronizing process of iron based PM material was further improved, and the understanding of the formation of boride and related theories are also presented. In addition, a reference for future studies of iron based PM materials is provided. Then this thesis bring both practical guidance and values for the preparation of high performance PM materials.