Materials Science Forum Vols. 591-593

Abstract: The machinery used in coal thermoelectrical plants usually is submitted to erosive wear. The erosive wear occurs mainly in the metallic pipe set of heat exchangers due the flow of hot gases carrying erosive particles. Jorge Lacerda’s thermoelectrical complex at Capivari de Baixo city holds seven power units, where two units use approximately 20 000 ASTM A178 heat pipes. The set is submitted to a semester maintenance schedule (preventive and corrective) where the damaged pipes are changed. So, in this work a set of erosive wear accelerated tests according ASTM G76 were performed in order to develop and specify materials and methods to diminish the erosive action caused by the combustion gases over the heat pipes. Specimens were coated with WC12Co and Cr3C2-25NiCr alloys using the HVOF technique and the coated specimens were tested at 450°C, the heat pipes working temperature. Silica was used as abrasive material at 30° and 45° impact angles, simulating a harder erosive condition than the real condition. The best performance coating at laboratory scale was later used in field condition. The results showed the coated specimen performance is better than the ASTM A178 alloy. The erosion resistance of the Cr3C2-25NiCr and WC12Co coatings is eight times higher than the uncoated alloy, and the coatings also presented a better corrosion resistance. This feature is important, because despite the erosive action the circulating gases also present a large amount of sulfur in their composition. Sulfur at lower temperatures forms H2SO4, causing intense corrosion of the pipes located at the heat exchangers colder parts. Based on the results and considering the coating costs the Cr3C2-25NiCr alloy was selected to coat a set of pipes mounted at the region of the heat exchanger with the most intense erosive wear. At the moment these coated tubes are in field operation and under observation regarding their performance in comparison with the uncoated pipes located at the same heat exchanger. The real operation conditions of the coated pipes will be estimated from the field life cycle analysis, and after all the cost-benefit of the studied coating.

Abstract: In this work, porous titanium samples processed by powder metallurgy and coated with biomimetic coatings, obtained during different periods of immersion in a simulated body fluid (SBF), were tested for corrosion resistance in a phosphate buffer solution (PBS). Uncoated samples were also tested for comparison. The corrosion resistance of both types of titanium samples was evaluated by electrochemical impedance spectroscopy and potentiodynamic polarisation curves. The electrochemical results indicated the formation of a surface film on the porous Ti samples with immersion in the SBF solution and this biomimetic film increased their corrosion resistance. This film helps osteointegration besides increasing corrosion resistance.

Abstract: The purpose of this paper is the evaluation of the use of a low power CO2 laser beam (50W) to promote the thermal treatment of AISI M2 high-speed steels and evaluate the tribological properties of the graphite coating that takes place on the steel surface after laser irradiation. In order to minimize the steel surface reflection at approximately 90% of the incident radiation wavelength (10.5 μm), an incident radiation absorber layer was applied to the steel surface sample. This coating aims to absorber the incident heat and transfer part of heat to the steel surface. As results the surface presented high hardness and a thin coating of graphite on the surface. The micro-structural changes, occurred on the steels surface, resulting from heat transfer from the absorbing layer, were evaluated through optical microscopy (MO), scanning electron microscopy (MEV), micro-hardness essays (MH), X-ray analysis (XRD), Raman spectroscopy and tribological testers on the graphite coating. MO analysis showed on the transversal section of the heated affected zone a brighter layer than the sample’s core, 30 micrometers thickness, in the surface sample a black coat 10 micrometers thickness. By MH analysis this brighter layer presented hardness approximately 30% superior than the regions without treatment, and by Raman spectroscopy it was evaluated the graphite coating. The XRD analysis on the surface sample reveals an increasing of the martensite and iron-carbide phase. The experimental results of pin-on-disk tests on the graphite coatings reveal a reduced coefficient friction as compared to the original surface.

Abstract: The purpose of this study is to evaluate the effect of the stages and tempering temperatures in the microstructure, tenacity and hardness of the vacuum sintered high speed steel AISI T15. The material was uniaxial pressing at 700 MPa and sintered in a vacuum furnace at 1275°C. After that, different samples of the materials were submitted to the annealing treatment at 870°C, quenching at 1235°C and tempering (single, double and triple) at 540, 550 and 560°C. Concluded the treatments, Rockwell C measurements of hardness were accomplished and, to evaluate the toughness of the material, TRS (Transverse Rupture Strength) tests were done. Later on, the materials were submitted to the metallographic preparation for microstructure analysis in optical microscopy, SEM, EDX and X-ray diffraction. Finally, each property analyzed was evaluated and correlated with the different tempering stages and temperatures accomplished.

Abstract: The sintering of Sm2Fe17 compound prior to the nitrogenation process is studied as an alternative process to produce dense Sm2Fe17N3 hard magnets with higher maximum energy products (BH)max than conventional polymer bonded magnets. In order to optimize sintering and nitrogenation processes, powders made from alloys, with different compositions, in the as-cast as well in the as-homogenized state were used. It could be shown that the amount of α-Fe, formed in the casting process, was reduced during sintering and that Sm selective loss is restricted to the surface up to a depth of 100 +m. Moreover the density of the sintered samples was not much affected by the initial composition or by the prior homogenization of the alloys. Although the nitrogenation process was successful in producing the Sm2Fe17N3 as proved by the mass gain, magnetic properties measurements and X-Ray diffraction, the hard magnetic properties were much lower than expected, indicating that other variables, which are discussed in the text, must be considered.

Abstract: SmCo5 sintered magnets are produced according the following main processing: milling until single crystal particle size, compaction and sintering. It is necessary high density to maximize remanence, but small grain size to maximize coercivity. A sintering model able to incorporate both, the densification rate and the grain growth rate, is described. This makes easier to find the better sintering conditions for optimization of the magnetic properties (coercivity and remanence). The presented model represents a refinement of previous sintering models, because it takes into account the coupled effects of grain size evolution and shrinkage.

Abstract: The known process as Metal Injection Molding is derived from the conventional powder metallurgy (M/P) being an alternative for production of parts with complex geometry, great dimensional precision and freedom of chemical composition. The present work has the objective to evaluate the processing of the Fe3Si alloy sintering in the vacuum furnace using as raw materials iron powder carbonyl and prealloyed powder Fe45Si with D90<10-m. Properties of microhardness, density, coercivity, magnetic permeability, and chemical composition was evaluated. The obtained results were compared with what is presented in the literature for parts processed by conventional ways and with parts processed by M/P. A density of 7,620 kg/m3, a coercive field (Hc) of 101.14 A/m, a relative maximum permeability of 5,484 and a residual induction of 1.1 T was achieved by MIM. Comparing with conventional processes (where 100% of densification is reached), the MIM process results were worse, however they were better than P/M.

Abstract: Ni-Fe based soft-magnetic alloys, processed via Metal Injection Molding (MIM), were investigated regarding the influence of processing route on final magnetic properties and compared to fully dense cast materials. The process variations included high and low temperature debinding, different sintering routes and the application of hot isostatic pressing (HIP). The different densities resulting from the process variations were related to maximum magnetic permeability. Results have shown that density, in the range between 7,5g/cm³ and 8,0g/cm³, does not have significant influence on the maximum permeability, allowing cost-effective process routes. It was also verified that fullydense cast alloys still exhibits superior properties, with lower coercive fields and higher permeability, but results achieved after HIP process overcame even the values of these commercial grade alloys.

Abstract: The hydrogenation decrepitation process was employed to produce sintered magnets using a mixture of two alloys. The effect of niobium and boron content on the magnetic properties of Pr14FebalCo16BxNby-type sintered magnets was studied. Niobium and boron have a significant effect on the magnetic behavior of these permanent magnets. The optimum amount of boron was 6 at% and niobium should be kept below to 0.50 at%. The squareness factor (0.90) has been improved considerably and good overall magnetic properties (Br=1320 mT, iHc=700 kAm-1 and (BH)max=315 kJm-3) have been achieved for the sintered magnets prepared from the Pr14Fe63.85Co16B6Nb0.15 alloy.

Abstract: This study reports the results of investigations carried out to determine the Curie temperature (Tc) of various homogenized praseodymium-based alloys represented by the formula: Pr14Fe79.9-xCo16B6Nbx (where x = 0, 0.01, 0.05, 0.1, 0.15, 0.50). The influence of niobium content on the microstructure of these alloys has investigated. The Curie temperature decreased about 3 °C for x = 0.5 at. %. It has been observed a remarkable grain refinement on the microstructure of the Pr14Fe79.8Co16B6Nb0.1 alloy when compared to the niobium-free alloy (x = 0 at. %).