Papers by Author: A.B.C. Arnt

Abstract: One of the problems encountered in heat exchangers working at high temperatures is the degradation caused by erosive processes, especially in coal-fired power plants. One of the factors that contribute to the degradation of the material is the microstructural changes, like the spheroidization of the carbides. This reduces the mechanical strength of the material and consequently accelerates its wear process. The objective of this work was to evaluate the effect of spheroidization under erosive wear conditions on ASTM A106 steel. The samples were submitted to different treatment times and temperatures, aiming to generate spheroidization in the samples, later submitted to erosive wear tests. Loss of mass, microstructures and mechanical strength were evaluated from the microhardness, with the evolution of the spheroidization process. The samples with higher level of spheroidization, obtained in the temperatures of 730 ° C in times above 50 hours presented greater reduction of hardness that resulted in greater losses of mass. In general, with the increase of spheroidization, there is reduction of mechanical resistance and erosive erosion, but with a gradual reduction in wear rate. This is possibly due to the increased ductility / toughness of the matrix, which delays the process of debris formation on the ASTM A106 steel surface during the hot erosive process.

Abstract: In this study it was evaluated the performance of coatings based on Cr₃C₂-25 (80Ni-20Cr) and CrC-30NiCr. The coatings were deposited by high velocity oxygen fuel (HVOF), with an average thickness of layer equal to 7.8μm. Samples were subjected to adhesive wear test (according ASTM G99) with a pin Ø 6 mm (SAE 52100). In the test was applied normal force equal to 50 N and tangential speed equal to 0.5 m/s. The test time was 30 minutes at room temperature, without lubrification. The wear surfaces were characterized by optical microscopy, scanning electron microscopy and X-ray diffraction. The microhardness of the coatings was also evaluated. The results showed that the coating based on Cr₃C₂-25(80Ni-20Cr) presented a performance ten times higher in wear resistance when compared to coating CrC-30NiCr.

Abstract: In this work, microstructural changes in coatings deposited by hypersonic thermal spraying technique (HVOF) were subjected to adhesive wear tests. Materials subjected to wear lose efficiency, increase costs and direct or indirect way can stop large production systems. The mechanisms involved in this type of wear can be minimized by the microstructural changes that in many cases reduces or stops the loss of material. To perform this assessment coatings compounds of Cr₃C₂25 (80 Ni-20 Cr) e Al₂O₃.TiO₂ (87-13) investigated had similar hardness values (1000 HV) and chemical characteristics. Were subjected to the ASTM G99 using commercial hard metal pin 2 mm, load 50N force and tangential velocity of 0.5 m/s. Worn surfaces and debris were analyzed by optical microscopy, scanning electron microscope and mass loss. The results show high resistance sprayed coatings worn surfaces and debris were fragile nature with little loss of material during the tests.

Abstract: This study aims to evaluate the behavior of thermally sprayed coatings with the HVOF technique (High Velocity Oxygen Fuel), subject to tribological testing of reciprocal movement with the SAE 52100 flat steel pin, in accordance with ASTM G 133. The carbon with Cr2C3-25NiCr and WC-12Co were evaluated under conditions of intense surface fatigue. The microstructure and surface modifications were analyzed by optical microscopy and scanning electron microscopy. By using these techniques were possible to compare the modifications of the coatings under repeated loading conditions, and to expand the knowledge about wear mechanisms, mainly the adhesive one, present in several mechanical components, particularly in bearing and internal combustion engines. The coatings tested showed lower wear rate when compared to SAE52100 steel and equivalent behavior among the different loading conditions applied.

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.