Research On Composite Processing Methods And Their Microstructure And Properties Of The High Strength Fe-based P/M Materials

With the entrance to 21st century, the resource and environment problems have become the principle problems in the sustainable development of human beings. Therefore in the long term, the trends of the industrial development all over the world can be reduced to the saving of energy and resource, and what is more, the pursuit of natural production. Powder metallurgy (PM) parts, which have many excellent and unique properties, such as net shape, economize on material and energy, cost-effective, unpolluted and material with good integrated properties; therefore, increasing attention has been paid to powder metallurgy in many industrial applications. Consequently, it is considered that powder metallurgy parts are the most worthy and exploitative "natural engineering materials" with greatly potential application in this century.This paper is mainly aimed at the preparation of high strength PM with the methods of carbusintering (CBS) and hot forging of sintered compacts by laser irradiation (HFSCLI) and at the current processing route of the sintering for further textual research, and then present a novel and more feasible approach of activation carbusintering as a result of the combination of internal reaction attribution of material system and the feature of the carbusintering technique, followed with a deep and systematic study to test and realize it. The high performance PM parts are prepared by HFSCLI, and then laser sintering process and microstructure evolvement of HFSCLI are further studied for the first time.To begin with carbusintering powder compacts represented with Fe-C,Fe-Ni,Fe-Ni-Cu,Fe-Ni-Cr, we discussed the effects of carbusintering on the Fe-based materials of porosity structure, sintering behavior and the densification behavior. Establishing the carbusintering process: sintering temperature 1120℃~1150℃and sintering time 60 min was carried out. In the aspect of alloy elements distribution in carbusintering, the sintering model of Cu/Fe and Ni/Fe powder is designed,and homogenizing parameter ( F ) is calculated in theory. The experimental results of carbusintering process further indicated that the method of CBS can decrease activity energies of iron, accelerate the spheroidizing process of the pores, facilitate the diffusion of alloy elements, and promote the sintering and densification. The reasons were as follows: the carbusintering accelerated the diffusion velocity ofγ-Fe and the increase of carbon content can facilitate the grain fine of austenite, so it provided more boundary area for the diffusion, densification and sintering.A reasonable mathematical model of the carbon concentration distribution after carbusintering of the Fe-based powder compacts was set up and given a mathematical analysis and an experimental verification. The results show that the deduced analytical formula is accurate and obvious in the expression of physical meanings; on the other hand, it has been numerically solved by the MathCAD computer program. And the finite elemental analysis result coincides well with that of the experiment. Besides, the integrated influences of green density, sintering temperature, sintering time and the carbon potential in the carburizing box on the carbon concentration distribution were discussed detailed in the paper. The above model can be fully used to guide the design and production, selection of material systems and optimization of processing parameters for the carbusintering.The diffusion coefficient of carbon and activity energies under 5 different densities were calculated by diffusion equation and the values were: 295 kJ/mol (6.0 g/cm3), 21 kJ/mol (6.2 g/cm3), 35 kJ/mol (6.8 g/cm3), 82 kJ/mol (7.0 g/cm3), 184 kJ/mol (7.3 g/cm3). It can be seen that the activity energies of the carbon diffusion had a consanguineous contact with the porosities of green compacts. When the porosity is above 20%, the pores are almost open interconnected pores and the carbon atoms that dissolve in the Fe crystal lattice decrease due to the fluxion of carburizing gas in the open pores, i.e., the migratory atoms dissolved in the clearance of Fe crystal lattice which can change their positions and fluctuate decrease. So it induces the increase of the carbon diffusion activity energies. With the decrease of the porosities (from 20% to 10%), the fluxion capability of carburizing gas in the green compacts weakens, and the carbon atoms that dissolve in the Fe crystal lattice increase. At this time, it has enough energy and the atoms that can change their positions and fluctuate increase, so the activity energies of carbon diffusion decrease.The effects of the post-treatment process on Fe-based microstructure and properties during carbusintering were thoroughly investigated. The microstructures and properties of the carbusintering materials were also studied in detail with SEM, EDS, TEM, MTS810, MM-200 type wear testing machine measurement. It was found that the physical and mechanical properties of carbusintering materials were better that of the conventional sintering: the fracture strength of carbusintering materials reached 471 MPa (withρof 6.75g/cm3), and after heat treatment it was up to 530 MPa (withρ? of 6.79g/cm3); and the tensile strength improved by 23~25% compared with that of the conventional materials. The wear property of carbusintering materials was also better than that of the conventionally sintered materials. The TEM micrographs of the carbusinterig materials after heat treatment showed that the surface layer was mainly composed of typical needle-like martensite and 3 mm beneath the surface layer was lathing martensite. The optimal heat treatment parameters from the data of carbusintering materials were: quenching temperature about 870℃, heat preservation time 30 min and tempering temperature about 250℃.The microstructure evolvement and properties of the high-powered materials prepared by HFSCLI were studied. The results showed that the HFSCLI not only consisted of laser sintering, plastic deformation and heat treatment, but also it could make sintered steel toughening by deformation strengthen and heat treatment strengthen. HFSCLI can produce high strength PM parts with: the density of 7.5 g/cm3, the radial crushing strength of 900 MPa after 500℃tempering and hardness beyond 100 HRB, which achieved the performance of 45# steel. The laser sintering forging can facilitate the grain fine of austenite, smash of lathing martensite and precipitation ofε?carbide. The amount of lathing martensite increased after laser sintering forging and quenching in the high-carbon steel, so it improved the properties of the sintering steel.The PM productions, produced by CBS and HFSCLI, have been certificated by correlative department and have been manufactured by corporations. And it has brought good economical and social benefits for the corporations.In a word, based on the systematic study of the correlative theories and processes about the CBS and HFSCLI, a basic frame of CBS and the HFSCLI preparation of high strength Fe-based powder materials has been established. It is considered that some work in this paper is not only for the improvement on the current PM process, but also the important supplement of the conventional sintering process and its related theories. Also, it provides a referable processing route and ideology for the current research of sintering process. Therefore, the work in this paper is of rather available values in theory and practical application.