Etude des proprietes mecaniques statiques et dynamiques de pieces d'acier elaborees par metallurgie des poudres

The main objective of this thesis ais to gain a better understanding of the effects of processing variables on the porosity and microstructure of powder metallurgy (P/M) steel and their effect on mechanical properties, both static and dynamic. The effects of processing variables on porosity and microstructure were evaluated by advanced microstructural characterization using both optical and electron microscopies. Then the impacts of porosity and microstructure on both static and dynamic properties were measured. Static mechanical properties were characterized by hardness measurements and by tensile tests while fatigue testing was used for evaluating dynamic properties. Fractographic observations were made on tensile and fatigue fracture samples to correlate the microstructural features to the mechanical performance of P/M steels. The experimental work covered in this Ph.D. thesis enables to further understand the mechanisms by which process variables affect the microstructure and the mechanical properties of samples.;In the second paper, the improvement of performance that can be achieved by copper infiltration was quantified. Tensile and fatigue properties of a Fe-2.0Cu-0.7C PM steel were compared to the same alloy infiltrated with 8 wt-% copper. Microstructural characterization, using optical and electron microscopies, was carried out to understand the effect of copper infiltration on mechanical properties. Copper infiltration improves the ultimate tensile strength by 40% by increasing the load bearing section, decreasing the stress concentrations associated to open porosity and increasing the hardness of the steel matrix. Fractographic observations show the evidence of stress transmission from the sinternecks to the steel particles due to infiltrated copper. The beneficial effect of copper infiltration is less pronounced for fatigue properties as the endurance limit is increased by only 10%. This lower improvement is explained by crack initiation at the copper/steel matrix interface.;In the last paper, the microstructural characterization of nickel rich areas and their influence on the endurance limit of a P/M steel was investigated. The addition of nickel powder to a P/M steel increases hardenability, enabling the formation of martensite directly after the sintering process. However, the slow diffusion rate of nickel into iron leads to the formation of nickel rich areas (NRA). Two steel alloys were studied, the first one is a Fe-6.4Ni-0.7Mo-0.7C with standard size nickel powder additions and the second one is a Fe-2.4-0.7Mo-0.7C where a finer size nickel powder was added. The influence of the size of the nickel powder used on hardenability and on the presence of NRA has been investigated by optical microscopy. A complete identification of the microstructural constituents and the effect of nickel concentration on their presence were achieved using X-ray energy dispersive spectrometry and electron diffraction in the transmission electron microscope. Results obtained by these techniques show the presence of austenite and martensite. Fatigue tests were carried out on P/M steel parts with and without austenitic areas to study their impact on the endurance limit. The analysis of the results showed that these areas are not a governing factor for the endurance limit of sintered steels. (Abstract shortened by UMI.);In the first paper, the effect of prealloying MnS up to 1.0 wt pct on the microstructural features of non-metallic inclusions and their impact on tensile and fatigue properties of a 7.0 g/cm3 P/M steel (Fe-2.0Cu-0.7C) have been investigated. As the MnS content increases, larger, more irregular and more closely spaced inclusions are obtained. Whilst no significant impact on both static and dynamic properties was observed when prealloying up to 0.65 wt pct MnS, a decrease of more than 15 pct of the ultimate tensile strength and of the endurance limit was found when the MnS content reaches 1.0 wt pct. The decrease in the ultimate tensile strength is attributed to a lower ductility of the sinternecks, as void initiation and void growth were promoted at lower stress levels by larger inclusions. The larger size of the MnS particles and the lower mean free path between non-metallic inclusions also favor microcrack initiation and their coalescence into cracks, leading to premature fatigue fracture.