| Aluminide coatings, which have excellent high-temperature oxidation and corrosion resistance, are widely used in aeronautics&astronautics, fire powered stations, petrochemical equipments, et.al. In the engineering field, the processes of preparation of aluminde coatings need to be carried out at low temperatures (under the tempering temperatures of steels) to avoid deterioration of substate materials’ mechanical properties. Though there are many reports indicate that aluminizing at low temperatures are feasible, the prepared coatings are usually composed of brittle phases like FeAl3, Fe2Al5and Fe14Al86. And these brittle iron aluminide coatings are restricted in applying since they are tend to peel or easy to crack. Therefore the point is the preparation of tensile FeAl coatings under low temperatures.In this thesis, by means of the technology of electrodeposition Al at room temperature, we proposed two methods of aluminizing at low temperatures, which are Al coating aluminizing and Cr/Al composite coating aluminizing. The effects of treating temperature and time to the microstructure of formed Iron aluminde coating by these two methods were investigated. The metallurgical features and phase structures of treated samples were studied by optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD).Firstly, we investigated the two methods’thermo-feasibility at low temperatures through heat treatment at440℃～650℃in8h. Results showed that Al coating aluminizing method can obtain an iron aluminde coating consisted of Fe4Al13phase with the thickness of3μm and trace of FeAl6phase at440℃, while Cr/Al composite coating aluminizing method can successfully prepare iron aluminide coatings over the temperature range of440℃～480℃. For Cr/Al composite coating aluminizing method, Al coating aluminizing method seemed easier to prepare FeAl coatings at the same conditions.Secondly, the reaction diffusion process at540℃between Al coatings and two substrates, which were1Cr17and pure Fe, were investigated in order to understand the phase transformation patterns between different Fe-Al phases and their correspond growth behavior. In the time range of5min～100h, FeAl6, FeAl3, Fe2Al5and FeAl phases were formed respectively at the surface of1Cr17substrate, while the FeAl6 phase can not be observed at the surface of Fe substrate. The FeAl6phase layer on the1Cr17subatrate can grow into about7μm of thickness in25min. The maximum thicknesses of FeAl3phase and Fe2Al5phase are about9μm and16μm in both substrates. While the thicknesses of FeAl phase observed at the surfaces of two substrates are quite different, there are4μm at the surface of Fe sample and the thickness of FeAl layer at the substrate of1Cr17can not be measured under optical microscopy. The relationships between the thicknesses of these Fe-Al phase layers and treating time in both substrates showed that the growth behaviors of FeAl6, FeAl3and FeAl phases are controlled by volume diffusion over grain boundry diffusion, while the growth behavior of Fe2Al5phase is controlled by grain boundry diffusion. The great amount of Cr atoms in1Cr17subatrate will hinder the transformation between theses Fe-Al phases and lower the growth rate of tensile FeAl phase layer greatly.Finally, the Al coating aluminizing method was applied on the oil case P110grade steel to prepare the tensile FeAl coating at low temperature. Outcome indicated that the rate of aluminizing. was notably increased when the diffusion temperature is over590℃, while the mechanical property of substrate material can be kept under treat temperature of620℃. The tensile FeAl coating about8μm thickess was obtained by electrodeposition of6μm Al coating followed by heat treatment at620℃for24h. |
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