Engenharia de Materiais

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Navegando Engenharia de Materiais por Autor "ALONSO-FALLEIROS, N."

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    Artigo
    Effect of aging heat treatment H950 and H1000 on mechanical and pitting corrosion properties of uns s46500 stainless steel
    (2018-01-05) BERALDO, C. H.; CALDERON-HERNANDEZ, J. W.; Rodrigo Magnabosco; ALONSO-FALLEIROS, N.
    © 2018 Universidade Federal de Sao Carlos. All rights reserved.The effect of aging temperature on mechanical and pitting corrosion properties of UNS S46500 was investigated. Tensile and Hardness tests were carried out and the microstructure was analyzed by optical microscopy, scanning electron microscopy and X-ray diffraction; Thermo-Calc simulations helped in the phase identification. Pitting corrosion properties were investigated in 0.6M NaCl electrolyte with sulfate additions by Potentiodynamic Polarization (PP). Hardness, tensile and yield strength of the UNS S46500 steel after lower aging temperature, 510ºC (H950), are higher than the ones found in the 538ºC (H1000) aged steel.This result is explained by microstructure, X-ray diffraction and Thermo-Calc analysis, which indicated the presence of austenite, chi phase and probably Ni3Ti precipitates finely and uniform distributed throughout the martensite matrix. Pitting corrosion resistance is equivalent in both aging temperatures. The sulfate inhibitor effect on UNS S46500 steel was enhanced for 538ºC condition when the electrolyte reaches 1Cl-:1SO4 2- ratio, which is explained by Ni sulfate adsorption and the amount of interfaces in the microstructure resulting in smaller amount of adsorption sites, such as coarsened Ni3Ti precipitates, smaller fraction of chi phase and recovery of dislocations in martensite structure.
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    Artigo
    Investigation of the effect of the interstitial content on the degree of sensitization at 600°C in ferritic stainless steels Investigação do efeito do teor de intersticiais sobre o grau de sensitização em aços inoxidáveis ferríticos a 600°C
    (2007-01-05) SERNA-GIRALDO, C. A.; IDE, C. R.; Rodrigo Magnabosco; KUNIOSHI, C. T.; ALONSO-FALLEIROS, N.
    The purpose of this work was to investigate the effect of carbon and nitrogen in 17%Cr ferritic stainless steels (16%Cr-0.04%C-0.032%N; 17.2%Cr-0.02%C-0.0062%N; 18.5%Cr-0.008%C-0.0213%N) on the degree of sensitization (DOS). These steels were solution annealed at 1200°C and quenched in water. Isothermal treatments were carried out at 600°C between 5 minutes and 16 hours. The Double Loop Electrochemical Potentiokinetic Reactivation test (DL-EPR) was carried out in a 0.5M H2SO4 solution at (25 ± 2)°C. The DOS was evaluated using a ratio between maximum anodic current density in reversion, ir and activation, ia. The microstructural examination of specimens was carried out using optical and scanning electron microscopy after metallographic etching with Vilella's reagent. The results showed a DOS variation with time. Maximum values of DOS were obtained for each steel, followed by its reduction. The intensity of DOS and its kinetics vary in function of the carbon and nitrogen content in steels.
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    Artigo de evento
    Pit morphology and its microstructure relation in 850°C aged UNS S31803 (SAP 2205) duplex stainless steel
    (2003-07-24) Rodrigo Magnabosco; ALONSO-FALLEIROS, N.
    This work described the relationship between microstructure of UNS S 31803 (SAP 2205) aged at 850°C and pitting morphology after cyclic polarization in chloride aqueous solution. The initial material, solution treated for 30 minutes at 1120°C and water quenched, was aged at 850°C for periods up to 100 hours. Cyclic polarization in 3,5% NaCl aqueous solution was conducted on #600 grounded sample surfaces. After polarization, the samples were sectioned transversal and perpendicular to polarized surface to allow the inner view of pitting corrosion using scanning electron microscope. It was found that sigma phase formation after 850°C aging treatment reduced pitting potential, and pitting corrosion occurred as selective corrosion of chromium and molybdenum impoverished regions, like interfaces between sigma phase and metallic matrix.
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    Artigo de evento
    Pitting corrosion of UNS S41000 and UNS S42000 stainless steels
    (2009-09-06) BORGES, A. O.; ALONSO-FALLEIROS, N.; Rodrigo Magnabosco
    The aim of this work is to evaluate the pitting corrosion resistance, through potentiodynamic polarization tests, in two commercial martensitic stainless steels, named UNS S41000 and UNS S42000, by using four electrolytes of 0.10M Na2SO4, with different concentration of chloride: 0.01M NaCl; 0.05M NaCl; 0.10M NaCl; 0.60M NaCl (3.5%). The received samples showed a ferritic microstructure with spheroidized carbides (annealed), whereas it was observed a microstructure of tempered martensite after heat treatment. Considering the same condition - annealed or quenched and tempered - the hardness of steel UNS S42000 was always higher than steel UNS S41000. In potentiodynamic tests, the potential scan rate was defined as 1mV/s after exploratory tests which showed this parameter influence on the curve shape. From then on, it was possible to obtain polarization curves with scanning rate of 1mV/s, starting at open-circuit potential after a five minute immersion and 600 grit surface finish. The results showed that the higher chloride ion concentration, the lower the pitting potential. For a given electrolyte, pitting potential measured for annealed steel UNS S41000 is slightly higher than the same steel on quenched and tempered condition. The intense chromium carbide precipitation at the grain boundaries of former prior austenite, a phenomenon that can lead to sensitization, may be a reason for it. Through immersion tests, it was found out that pitting potential of UNS S41000, after heat treatment, is lower than corrosion potential in the electrolyte composed of (3.5% NaCl + 0.10M Na2SO4). Otherwise, steel UNS S42000 presented higher pitting potential after heat treatment. In this case, intergranular carbide precipitation was not observed and the quench and temper treatment, at first, generates a higher content of chromium in solid solution if compared with the annealing treatment, leading to a greater corrosion resistance. Pitting density and their sizes are chloride concentration dependent: the higher ion chloride concentration, the lower is pit density and the larger is pit width. According to the studied conditions, the most suitable electrolyte for pitting potential determination is composed of (0.10M NaCl + 0.10M Na2SO4). The shape of the obtained potentiodynamic polarization curves with this electrolyte allowed the identification and accurate determination of the pitting potential.