J. A. Moreto

Desenvolvimento de dispositivo e estudo do comportamento ao microdesgaste abrasivo do aço AISI 420 temperado e revenido

O ensaio de micro desgaste abrasivo tem sido objeto de uma quantidade significativa de pesquisas nos ultimos anos, quando o seu potencial para avaliar o comportamento de recobrimentos finos se tornou relevante. O objetivo deste trabalho e a construcao de um dispositivo para a realizacao de ensaios de micro desgaste abrasivo por esfera fixa. O contato abrasivo e realizado por uma esfera de aco AISI 52100, a qual e presa num eixo que gira com grande precisao, com ajuda de um motor, em relacao a uma amostra plana que se mantera estacionaria. A carga normal de ensaio e aplicada atraves de ajustes de peso no suporte pivotado do corpo de prova, a qual pode ser variada a cada ensaio realizado. Este dispositivo foi utilizado para a determinacao da resistencia ao desgaste abrasivo e da eficacia de tratamentos de superficie utilizados em pecas que sofrem este tipo de solicitacao em servico, alem de utiliza-lo para estudar novos produtos desenvolvidos para o aumento de resistencia ao desgaste de superficies. Foi avaliada a resistencia ao microdesgaste abrasivo de aco AISI 420 temperado e revenido. Observou-se que o aumento da dureza do material tratado aumentou a resistencia ao desgaste abrasivo quando comparado com o material sem tratamento termico.

Environmentally-assisted Fatigue Crack Growth in AA7050-T73511 Al Alloy and AA2050-T84 Al-Cu-Li Alloy

The aim of this study was to evaluate and compare the effect of low temperature and saline environment on the fatigue crack growth behavior of the AA7050-T7451 Al alloy and the recently developed AA2050-T Al-Cu-Li alloy. Fatigue at room and low temperature and corrosion-fatigue tests were carried out using an applied stress ratio (R) of 0.1, 15 Hz frequency (air at RT and –54 °C) and 1 Hz frequency (seawater fog) using a sinusoidal wave form. In the near-threshold region, in air and at RT it was found a ΔKth = 2.9 MPa.m1/2 for AA2050-T84, in saline environment this value increased to ΔKth = 4.9 MPa.m1/2, due to closure effect through wedge effect by the corrosion products. At the beginning of the Paris-Erdogan region, the crack closure effect was not present for the AA7050-T7451, but persisted for the AA2050 Al-li alloy. It was observed that both alloys were equally affected by temperature reduction. When the saline environment is considered it was observed that the AA7050-T7451 presents lower m value (2.6) than the one for AA2050-T84 (3.4), meaning a lower FCG rate variation with ΔK, however it presented the highest C value, as a consequence the worst FCG behavior.

Surface Contact Fatigue Failure of a Case Hardened Pinion Shaft

An investigation was made to determine the causes of surface contact fatigue failure of a case hardened driver pinion located in the intermediate shaft of a reducer gearbox used in a sugar and alcohol mill. The examination of the component revealed the presence of a cemented layer substantially thicker than that generally specified for pinions devised for this application. This, associated with the massive presence of brittle threadlike carbon-rich cementite phase (Fe 3 C) in prior austenite grain boundaries of the pinion teeth, favored surface crack nucleation and propagation during cyclic loading, leading to spallation of the contact surface with the counterpart gear, which impaired the systems operation. Poor carburization practice was discovered as the root cause of the mechanical failure, thus demanding the implementation of a new manufacturing route to avoid problems in similar load-bearing rotating components.

Effect of salt-water fog on fatigue crack nucleation of Al and Al-Li alloys

Fatigue and corrosion-fatigue tests were performed to quantify the fatigue properties of AA2524-T3 and AA2198-T851 Al alloys. High cycle axial fatigue tests were carried out under air and salt-water fog conditions. In air, the specimens were fatigue tested at a frequency of 50 Hz, using specimens with and without preconditioning in a salt spray chamber, and for the corrosion fatigue condition, the tests took place at a frequency of 30 Hz in a salt-water fog condition. In all cases it was used a sinusoidal waveform and a stress ratio (R) of 0.1. The results indicate that the saline environment had a deleterious effect on the fatigue life of the two aluminum alloys. AA2524-T3 exhibited a better fatigue strength than AA2198-T851 when fatigue tested in air. However, considering the corrosion fatigue test in a saline fog environment an inverse behavior was observed with the AA2198-T851 exhibiting higher fatigue strength.

Study on friction and wear behavior of SAE 1045 steel, reinforced nylon 6.6 and NBR rubber used in clutch disks

The friction and wear properties of AISI 1045 steel, nylon 6.6 composites with different types and percentages of reinforcement and a nitrile rubber, Alpha 66, were submitted to sliding wear tests. The five types of nylon 6.6 composites tested generated matching, low values for the coefficient of friction. This may be due to the greater influence of the nylon 6.6 matrix on the coefficient of friction. However, the wear in nylon composites showed wide variation, determined by the type of reinforcement added to the composite. The other two materials tested, nitrile rubber and 1045 steel, showed behavior different from that of the composites. The NBR (signifying a norm established by the Brazilian Association of Technical Norms) rubber showed a superior coefficient of friction and greater wear. On the other hand, the steel showed a high coefficient of friction and low wear, suggesting that the coefficient of friction has no direct connection to the wear resistance of the materials studied here.

Thermomechanical and Isothermal Fatigue Behavior of Gray Cast Iron for Automotive Brake Discs

At the end of the 19th century, in the wake of railway transportation and the beginning of automotive vehicle production, new technology-based materials became necessary for the manufacture of brake systems to provide safer and more effective braking of vehicles transporting heavy loads at higher speeds. These devices serve to decelerate vehicles by friction, transforming most of the kinetic energy into thermal energy, which is dissipated by the brake system during the braking process [IOMBRILLER, 2002]. Many parts contribute actively or passively to a vehicle’s satisfactory performance, but safety is closely linked to the efficiency of the brake system, which is subjected to relatively high thermal and mechanical stresses during regular braking action. Therefore, a crucial factor is the precision of the analysis and development of brake systems taking into account all the aspects involved in their thermal and dynamic behavior [MAZUR et al., 2005]. During severe deceleration by braking, the temperature of the brake system may reach up to 650oC and overheating of the brake discs may lead to serious consequences that reduce the vehicle’s safety [IOMBRILLER, 2002]. This temperature variation causes thermal shock and localized overheating points, changing the behavior of the metal involved due to metallurgical transformations, as well as crack nucleation in the disc in response to plastic flow of the surface metal and inducing stresses after cooling [MAZUR et al., 2005]. Even disregarding the presence of thermal shock, a few braking cycles with abrupt deceleration still suffice to produce small cracks in the usable part of brake discs. The study of the mechanical behavior and fracture mechanisms of these materials is essential to allow for the design and rational use of these components. Figure 1 illustrates the failure in front brake rotor in a disc submitted to penetrating liquid inspection to reveal cracks. The cyclic stresses resulting from the continuous use of vehicles can cause fatigue, propagate cracks and fracture of the brake component [IOMBRILLER, 2002]. This mechanism may cause crack nucleation and growth in the material when subjected to cyclic strain. As cyclic loading conditions in brake discs are induced mainly by temperature gradients, thus essentially strain-controlled tests were planned for this study. In this way, it

Micro Abrasive Wear Behaviour Study of Carburization and Ion Plasma Nitriding of P20 Steel

P20 steel, which is mainly used in plastic molds, was thermochemically treated by ion plasma nitriding and solid and gas carburization. In this work, the solid and gas carburization were performed at 925 oC for four different durations. The ion plasma nitriding was performed at 520 oC for four different durations. The thermochemical treatment increased the micro abrasive wear resistance of the studied material. The gas carburizing treatment resulted in a greater surface microhardness. Longer treatment times increased the effectiveness of the thickness layer in all cases. The solid carburization produced a larger thickness layer than the gas carburization and ion nitriding.

Preparation and characterization of a homemade Josephson junction prepared from a thin film sintered in a domestic microwave oven

A homemade Josephson junction was successfully obtained using a superconductor thin film of the BSCCO system. The film was deposited on a lanthanum aluminate, produced from a commercial powder with a nominal composition Bi1.8Pb0.4Sr2CaCu2Ox, was thermally treated by a domestic microwave oven. The XRD analysis of the film indicated the coexistence of Bi-2212 and Bi-2223 phases and SEM images revealed that a typical superconductor plate-like morphology was formed. From the electrical characterization, performed using DC four probes technique, it was observed an onset superconducting transition temperature measured around 81K. At the current-voltage characteristics curve, a step of electric current at zero-voltage could be observed, an indicative that the tunneling Josephson occurred.

Mechanical Properties of ASTM A283 Carbon Steel Subjected to the Sulphidation Process

This work presents a survey of the mechanical properties (hardness and modulus of elasticity), tribology (friction coefficient and wear rate), and metallography analysis of the ASTM A283 carbon steel subjected to the sulphidation process. The samples were submitted to the sulphidation process at an atmosphere with 0.2 % H2S (hydrogen sulphide) and 98.8 % H2 (hydrogen gas) for different time (5,10, 20, 60, and 120 min)at 300°C. After exposure to sulphidation environment the growth kinetics was analyzed. The results obtained with the sulphidation process indicate that the ASTM A283 carbon steel follows a parabolic law with a equilibrium constant in terms of pressure Kp = 1.47· × 10−12 g2 cm−4 s−1 for the material analyzed. The experimental results for the hardness and surface modulus of the samples show an increase in the values for the sulphide sample for 120 min.

Correlação de dados de tração a quente e fluência para a liga Kanthal A1

Ensaios de tracao a quente e fluencia foram realizados na liga Kanthal A1 na faixa de temperatura de 600o a 800oC. Cada um desses conjuntos de dados foram analisados separadamente de acordo com suas proprias metodologias, mas, neste trabalho, uma tentativa foi realizada para se estabelecer uma correlacao entre os mesmos. Um novo criterio proposto para converter dados de tracao para fluencia torna possivel a analise conjunta desses dois grupos de resultados de acordo com relacoes usuais de fluencia como: Norton, Monkman-Grant, Larson-Miller e outros. A notavel compatibilidade entre os dois conjuntos por esse procedimento sugere que dados de tracao a quente podem ser convertidos para dados de fluencia para a liga Kanthal A1, como verificado anteriormente para outros materiais metalicos.