| Ferrous powder metallurgy parts are now widely used in automotive,machinery,appliances,military and many other applications.Fabrication of ferrous powder metallurgy parts has been one of the research hotspot in ferrous powder metallurgy area.Among the common elements of C,Cu,Cr,Ni,Mo used in ferrous PM for alloying,chromium owns excellent solution strengthening ability for iron matrix,can enhance the strength of the ferrous material and has no obvious negative influences on the compressibility of ferrous PM powders.Also,compared with Ni,Mo and some other alloying elements,chromium is abundant and inexpensive.Therefore,in this dissertation,ferrous parts with Cr as alloying element were prepared and characterized.Three different Cr-containing alloy powders were added into Fe-2Cu-0.8C raw materials to obtain Fe-2Cu-xCr-0.8C powder mixes for compaction by different mixing process.The powder mixes were subsequently pressed and sintered to obtain sintered ferrous parts.Density,microstructure,mechanical properties and corrosion resistance of the sintered samples were tested.The present works focus on the developments of ferrous powder metallurgical materials with optimized microstructure and high mechanical properties.In chapter 2 of this dissertation,Fe-2Cu-xCr-0.8C(x=0～5wt%)powder mixes were prepared by mechanically mixing reduced iron powder,copper powder,chromium powder,graphite powder and zinc stearate lubricant.Cr was introduced in the form of pure chromium.The powders mixes were pressed under 700MPa and sintered at 12000C for 1.5h in H2.Density,microstructure,mechanical properties,fracture morphology,microstructures and corrosion resistance of the sintered samples were investigated.The results show that with the increase of chromium addition,hardness and tensile strength of the sintered specimens first increase,and then decrease.The sintered samples have maximum hardness of 90HRB,and maximum tensile strength of 420MPa at Cr addition amount of 3%.In chapter 3 of this paper,Fe-2Cu-3Cr-0.8C powder mixture were prepared by adding pure Cr powder,low-carbon Cr-Fe powder and mechanically alloyed Cr-Fe powder into Fe-2Cu-0.8C raw materials and mixing,respectively.Sintered specimens were also prepared by using processing parameters as same as those in chapter 2.Microstructure and properties of the sintered materials were also investigated.It was found that sintered Fe-2Cu-0.8C materials have a maximum density of 7.07 g/cm3,and the sintered Fe-2Cu-3Cr-0.8C materials have the best performance with a maximum hardness and tensile strength of 448MPa and 95HRB,Furthermore,the sintered Fe-2Cu-3Cr-0.8C materials obtained from the mechanically alloyed Cr-Fe powders show homogeneous microstructure,less aggregation of Cr elements and reduction of the diffusion coefficient of carbon atoms.In chapter 4,a wet mixing process was employed to prepare Fe-2Cu-3Cr-0.8C powders mixes with addition of mechanical alloyed Cr-Fe powders,in which the raw powders together with zinc stearate,were added into ethyl alcohol,after mixing,powders mixes were obtained by drying the slurry and sieving.The powder mixes were also pressed and sintered to obtain sintered samples.Technical properties of the powder mixes were tested and compared.It was found that the wet mixing process obtained powder mixes with better flowability than dry mixing.Also the sintered samples from the wet mixed powders have better mechanical properties,evener microstructure and higher corrosion resistance.The sintered samples have maximum sintered density of 7.05 g/cm3,maximum hardness of 28HRC,and maximum tensile strength of 548MPa at Cr addition amount of 3%.The fracture morphology of the materials is mixed morphology,which consist of dimples and cleavage surfaces.Compared with dry mixing process,the sintered materials,which are prepared by wet mixing process,have more uniform microstructure and the better corrosion resistance.In chapter 5 of this dissertation,sintered Fe-(1-4)Mo-(0-6)Cr-3Cu-3Ni-1C materials were prepared by wet mixing mechanically alloyed Cr-Fe powders with 1300WB powders.The effects of Mo and Cr alloying elements,sintering temperature on the properties and microstructure of the sintered materials were investigated by orthogonal experiment.The results show that sintering temperature has the most significant influence on the properties of the sintered samples.The mechanical properties of samples sintered at 1050℃ were obviously lower than those of samples sintered at other temperature.After sintering temperature of 1200℃,the tensile strength of the Fe-4Mo-6Cr-3Cu-3Ni-1C samples reach 652MPa. |
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