| Iron base powder sintering structural steels is one of the most important application of P/M materials.However, more than 10% pores retain in powder metallurgy parts after traditional compaction and sintering,therefor properties of materials are reduced. Nowadays it is difficult that properties of P/M sintering structural steels approach or reach one of wrought steels. The mostly iron base P/M materials are manufactured into low load parts such as automobile gearwheel etc, and cannot meet demands of automobile industry. To extend applied field of P/M material, it is necessary that improve properties of P/M materials. Now P/M technology develops to multi-alloying and full density sintering, especially, for powder sintering structural steels, good structure and high density are foundation of which obtain high performance.In this thesis, the primary goal is obtaining sintering steel with high density,high-tensile,and high wear resistance.On the basis of theory of alloying,densification,enhanced,and friction and wear, sintering structural steel with low cost and higher performance are designed and produced by the use of hot-forging process and deformation enhancing process.These works lay a foundation of theory and process for insteading of wrought and cast steel in the automobile and other field.Two types of low alloying structural steel.i.e. Fe-Ni-Mo-Cr-C and Fe-Ni-Mo-Cu-Cr-C system alloys are designed and manufactured by the use of multi-alloying way.Chromium element is added to prealloyed Fe-2.0Ni-0.6Mo and Fe-0.4Ni-0.85Mo-2.0Cu powders,respectively,the influence of chromium content on alloying are investigated. Results show that oxidation of chromium can be effective avoided. Under 1150℃for 1h condition, FeCr powders(partical size﹤20μm), prealloying powder, and graphite,etc allying element can homogenous diffused, and homogenized sintering structure are obtained. Moreover,grains size are refined with chromium content increasing, refined sorbite-pearlite structure are obtained after cold pressing, and fine martensite are formed after hot-forging deformation quenching.The chromium content on green density and sintered density have effect,and existed an optimal value.In this testing,the green density and sintered density of Fe-Ni-Mo-Cr-C and Fe-Ni-Mo-Cu-Cr-C system alloy with 0.6% Cr addition are better.To combine sintering process with hot-forging deformation-enhanced process,the influence of hot-forging deformation on microstructure, sintered density,and mechanical properties are discussed. The results show that the tempered stability of microstructure is improved with simultaneous density increasing under 20% deformation condition.The substructure of Fe-Ni-Mo-Cr-C system alloy and Fe-Ni-Mo-Cu-Cr-C system alloy exhibit that density of dislocation increase in lath martensite, therefore,tempered stability increase and tempered sorbite retain martensite form,a greater amount of fine carbide distributed in ferritic matrix after hot-forging deformation quenching tempered at 560℃.It can be discovered that sintered density and homogenized of microstructure are improved after by hot-forging sintering. The net pore distribute in boundaries between the original powder particals and inside grains after cold pressing sintering. Small and rounded pore locaed at grains,and bigger irregular pore distributed in boundaries, porosity is 7.1～11.5%. After hot-forging sintering,bigger pores disappear,and only exist small interface pores which near rounded, porosity is 0.5～4.3%. The highest sintered density of 7.76 g/cm3 and porosity of 0.5%( near full density structure) are obtained for Fe-2.0Ni-0.6Mo-0.5C-0.6Cr alloy.The mechanical properties of P/M materials are considerably influenced by the density and microstructure. Comparing with cold pressing sintering structure, mechanical properties are enhanced after hot-forging deformation quenching and 560℃tempered for both of Fe-Ni-Mo-Cr-C and Fe-Ni-Mo-Cu-Cr-C system alloy. Hardness value of HRC40～48 for hot-forging tempered structure,tensile strength value of 750～1320MPa,and elongation value of 2.7～6.06% are obtained. Fe-2.0Ni-0.6Mo-0.5C-0.6Cr alloy has the best properties with tensile strength value of 1320MPa and elongation value of 6.06%.Studies of fracture surfaces display that a great of lacuna and impurities exist in interface, specially, many pores which exist in interface reduce strength of interface and easily produce the area of sliping and plastic flowing, and thereby the fracture surfaces of cold pressing sintering samples with high porosity appear along with pore between initial particles, percentage of through crystal fracture is few.However,after hot-foring sintering,the pores decrease or deform and vary small, the elongation which highly sensitive to structure of pore is improved. In addition, the mass of mixed dislocation are formed during hot-forging deformation,the result show that resistance to crack expanding are increased and greater strength and ductility are obtained,therefore, the fracture surfaces of hot-forging tempered samples with low porosity appear more dimples and area of shear deformation. The pores and impurities are mainly fracture origin for cold pressing samples and hot-forging samples, the characteristic of microstructural features of failure surface can be observed that cold pressing samples are brittle surface and hot-forging samples are mixture brittle-ductile fracture.Tribology characteristic are systemic studied by dry sliding wear.Wear resistance of the specimens are conducted on a pin-on- roller wear tester of M-200 type at room temperature for 60min, in the test the speed of roller is 6.6m per second and the load is 50N,100N,150N ,200N,and 250N,respectively. Results show that porosity, microstructure,alloying element, and wear parameter have mostly effect on wear rate of sintered structure.Since hot-forging tempered samples exhibit higher sintered density,higher strength,hardness,and dispersion of carbide precipitates grains, wear resistance are higher than hot-forging quenching samples and cold pressing sintering samples.In present test, wear resistance of Fe-2.0Ni-0.6 Mo- 0.6Cr -0.5C alloy is the best.Wear testing proved that abrasive wear, adhesive wear, oxidation wear and delamination wear are mainly wear mechanism in cold pressing sintering and hot-forging sintering condition.For cold pressing samples with high porosity, abrasive wear and oxidation wear and are dominant wear mechanism,and for hot-forging sample with low porosity, oxidation wear and delamination wear are dominant wear mechanism.Based on the above test, The optimal component design of Fe-Ni-Mo-Cr-C system and Fe-Ni-Mo-Cu-Cr-C system alloy are Fe-2.0Ni-0.6Mo-0.6Cr-0.5C and Fe-0.4Ni-0.85Mo-2.0Cu-0.6Cr-0.5C,respectively . The optimal process is sintering at 1150℃for 1h and then 930℃hot-forging deformation quenching (20% deformation) and tempering at 560℃.
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