Formation of hydrogen blisters during the solution treatment for aluminum alloys

The solution treatment of aluminum alloys can be restricted by the presence of porosity defects caused by the moisture present in the process or by the hydrated front on the material surface. Hydrogen blisters cause deleterious effects on mechanical properties and surface finish. However, the formation of bubbles in solid aluminum is not caused only by the known reaction 2Al+3H2 O=3H2 +Al2 O3 , as it does not explain the interaction of the aluminum oxide layer with the formation mechanisms. In addition, the literature approaches show that there is more than one mechanism for the formation of these defects, but no work has made an association between them. Thus, the objective of this work is to carry out extensive research on the state of the art of hydrogen blister formation in aluminum alloys during the solubilization heat treatment. Contemplating different proposed mechanisms of bubble formation on the surface and structure, the analysis of this association of approaches indicated that the mechanisms depend on both permeability, where the hydrated oxide front creates passage for the formation of blister in the sublayer, as well as diffusion and hydrogen solubility in the microstructure

A numerical study on the aerodynamic performance of building cross-sections using corner modifications

A numerical investigation is performed in this work in order to evaluate the aerodynamic performance of building cross-section configurations by using corner modifications. The CAARC tall building model is utilized here as reference geometry, which is reshaped considering chamfered and recessed corners. The numerical scheme adopted in this work is presented and simulations are carried-out to obtain the wind loads on the building structures by means of aerodynamic coefficients as well as the flow field conditions near the model’s location. The explicit two-step Taylor-Galerkin scheme is employed in the context of the finite element method, where eight-node hexahedral finite elements with one-point quadrature are used for spatial discretization. Turbulence is described using the LES methodology, with a dynamic sub-grid scale model. Predictions obtained here are compared with experimental and numerical investigations performed previously. Results show that the use of corner modifications can reduce significantly the aerodynamic forces on the building structures, improve flow conditions near the building locations and increase the Strouhal number, which may have an important influence on aeroelastic effects

The dispersion curve applied in guided wave propagation in prismatic rods

The early detection of failures in structures is a subject of great interest in engineering; several of these techniques are linked with the elastic wave propagation, using guided waves is one of these alternatives. Several structures of interest in engineering are laminar arrangements; the wave propagation in this type of structures depends not only on the material properties, but also on the geometric parameters, such as the plate thickness. Tubular structures, pressure vessels, tanks and also parts of ships hulls could be considered laminar. The elastic wave propagation in laminar structures could be considered as a sum of modal shapes that have its wave length and frequencies defined. These mode families are characteristics of each structure and could be represented through the dispersion curves. The definition of these dispersion curves is of crucial importance to understand the propagation of guided waves in the structure studied. In the present work the dispersion curves were generated using three different methodologies, specific for metallic rectangular stems that compound the strengthening armor in flexible riser duct. Each approach presented in the analysis were carried out using standard finite element commercial packages and an experimental verification, as well. The premise is to present the topics in the simplest way, not only to understand how the dispersion curves are built but also how these curves must be interpreted

Quantification of CTOD fracture toughness in welded joints to evaluate the efficacy of vibration stress relief compared to thermal stress relief

Resonant vibration residual stress relief (R-VSR) is an alternative method to post-weld heat treatments (PWHT) which is said to lead to less distortion and lower costs. The method of superposition of cyclic stresses and residual stresses, which can lead to localized yielding of the material, dislocation movement and subsequent stress relief. This article aims at investigating the efficiency of resonant vibration in the relief of residual stresses in welded joints of HSLA Domex700 steel sheets. Mechanical stress relief was compared to a common PWHT, and stress levels were then quantified through X-ray diffraction. Samples were also characterized by CTOD fracture toughness tests, fractographic analysis and tensile tests. Results indicate that the mechanical method was significantly less effective in relieving stresses in comparison with the PWHT, but led to apparent improvements in fracture toughness and in tensile tests. FWHM values indicated significant distortion for PWHT and less distortion for R-VSR

Model for wireless magnetoelastic strain sensors

This paper describes a magnetoelastic strain sensor based on the ∆E effect and discusses some materials used in its construction. A polycrystalline Fe–Al–B alloy with good quality magnetoelastic properties was used as the transducer and glued to the test object, either brass plates or rods of SAE 1010 steel. The strain-dependent magnetic field of the transducer changes the operating point of the resonator, a strip of field-annealed Metglas 2826MB3, resulting in a modification of its resonant frequency. A model was developed to simulate the strain-dependent magnetic field acting on the resonator and thus to calculate curves of resonant frequency vs. deformation. With the help of this model, differences in the shape of the frequency vs. strain curve can be understood. For a sensor with resonant frequency of 60.5 kHz glued to a rod of SAE 1010 steel, a total resonant frequency variation ∆f ~7 kHz was observed for a deformation of 1100 ppm. The geometry of this sensor is especially favorable for the remote monitoring of a steel surface, such as the wires of the tensile armor of a marine riser

The elastic wave propagation in rectangular waveguide structure determination of dispersion curves and their application in nondestructive techniques

The use of mechanic waves for assessing structural integrity is a well-known non-destructive technique (NDT). The ultrasound applied in the guided wave in particular requires significant effort in order to understand the complexities of the propagation so as to develop new methods in damage detection, in this case, knowing the interaction between the wave propagation and the geometry of the waveguides is mandatory. In the present work, the wave propagation in rectangular steel rod is presented. In this study, the section dimensions were fixed as 5 × 15 [mm], a typical element of the flexible riser structural amour commonly used in the offshore oil industry. The studies here presented were restricted to [0, 100 KHz] frequencies. This frequency interval is in the range of commercial waveguide equipment commonly applied in ducts in NDT applications. The computation of the dispersion curves is performed by using three different methodologies: (i) analytical solutions, (ii) a method that combines analytical approaches with finite element methods (SAFE), and (iii) experimental method that extracted information from the rod using laser vibrometers and piezoelectric actuators. Finally, two applications based on the dispersion curves determined in the rectangular waveguide are presented to illustrate the possibilities of the curve dispersion knowledge related to the specific geometry in the development and application linked to NDT. The first application consists on showing the possibilities of the techniques that use a fiber grating Bragg cell (FGB) to measure the wave displacement and the second application involves the simulation of pre-fissured prismatic waveguide aimed at searching to induce three characteristic acoustic events. The model was built combining the finite element method and a version of the discrete element method

Material flow during friction hydro-pillar processing

Friction hydro-pillar processing (FHPP) is a novel technique that involves solid-state joining of an external plug onto a substrate by plastic deformation. A systematic investigation on material flow during FHPP is required but rarely reported. The present work reports a coupled theoretical and a three-dimensional X-ray computer tomography-based experimental study using a Ti-alloy as a tracer material to realise the material flow during FHPP of a AISI 4140 steel substrate. The cumulative results showed that the central portion of the plug deformed in a series of layerwise shear planes. However, the plasticised material towards the outer area of the plug flowed through the clearance between the plug and the substrate with excess volume moving out as flash

Avaliação preliminar da fadiga em aços cementados e temperados de forma convencional e intensiva

Steels treated through carburizing thermochemical treatment and quenching and tempering thermal treatment are broadly used in components that need to have hardness and superficial mechanical resistance together with good toughness in the core of the component. Additionally, it is possible to produce surface compressive residual stresses that normally improve fatigue resistance. Relatively unknown, the intensive quenching thermal treatment is a method that presents some advantages, one of them being the possibility of avoiding cracking by distortion due to extreme cooling. Other advantages are the increase in mechanical resistance, the use of shorter carburizing cycles, improvement of fatigue performance, among others. Once the cooling rate is high, low carbon steels can be used instead of low alloy steel such as AISI 8620. This way, it is possible to use less costly steel and to obtain the advantages of intensive quenching.The present work aims to compare samples carburized during 6 hours at a temperature of 920oC and carbon potential of 0,9% both for AISI 1020 and AISI 8620 samples, through mechanical and metallurgical analyses, being the principal the production of Wohler curves together with fractographic analysis in low magnifying glass and scanning electron microscope. Results pointed out that the AISI 1020 steel presented grain size which is three times bigger than AISI 8620 steel grain size. The effect of optimizing intensive quenching when applied to AISI 1020 steel is practically covered by the fact that AISI 8620 steel presents a more refi ned structure, with smaller grain size comparatively and therefore better mechanical properties. This way, intensive quenching treatment can provide superior performance to non-alloy steels relatively to alloy steels only if grain size is equal or inferior.Key words: carburizing, intensive quenching, fatigue.Os aços tratados com o processo termoquímico de cementação e com posterior tratamento térmico de têmpera e revenido são amplamente utilizados em componentes que necessitam obter dureza e resistência mecânica superfi cial aliada a uma boa tenacidade no núcleo do componente. Adicionalmente, consegue-se produzir tensões residuais compressivas na superfície, o que normalmente eleva a vida em fadiga. Relativamente desconhecido, o tratamento térmico de têmpera intensiva é um método que apresenta uma gama de vantagens, sendo uma delas a possibilidade de evitar trincamento pela distorção devido ao extremo resfriamento. Outras vantagens são o aumento da resistência mecânica, a utilização de ciclos mais curtos de cementação, a melhoria no desempenho em fadiga, dentre outras. Uma vez que a taxa de resfriamento é grande, pode-se vislumbrar a utilização de aços de baixo carbono cementados ao invés de aços ligados, como o AISI 8620. Dessa forma, é possível utilizar um aço mais barato e obter os benefícios da têmpera intensiva. O presente trabalho objetiva comparar amostras cementadas por 6 horas a uma temperatura de 920ºC e potencial de carbono de 0,9%, das qualidades AISI 1020 e AISI 8620, através de análises mecânicas e metalúrgicas, sendo a principal a realização de curvas de Wohler e posterior análise fractográfica em lupa e microscópio eletrônico de varredura. Os resultados apontaram que o aço AISI 1020 apresentou um tamanho de grão três vezes superior ao aço AISI 8620. O efeito de otimização da têmpera intensiva quando aplicada ao AISI 1020 é praticamente encoberto pelo fato do AISI 8620 apresentar uma estrutura mais refi nada, com menor tamanho de grão comparativamente e, portanto, melhores propriedades mecânicas. Dessa forma, o tratamento de têmpera intensiva pode proporcionar um desempenho superior a aços não ligados frente aos ligados caso o tamanho de grão seja similar ou inferior.Palavras-chave: cementação, têmpera intensiva, fadiga

Evaluation of the residual stress state of 42crmo4 steel sheets in a production line

The residual stress state of a mechanical component is an important factor in its production planning and in estimates of its lifecycle since it can be responsible for geometric distortions and degradation of fatigue properties. Therefore, the development of reliable methods for non-destructively quantifying these stresses remains in the interest of most manufacturing industries; Barkhausen magnetic noise measurements have been investigated in several applications and remains a viable option. However, its effective implementation has occurred mostly in components with simple geometries and insignificant microstructural gradients; even in these cases, successful industrial adoption of the method depends on previous calibration with samples that are often difficult and costly to prepare and validate. This work aims at investigating the capability of the method of characterizing the residual stress state in a simple but generally useful application: samples of hot-rolled steel sheets collected at two different stages of processing in an industrial mechanical conformation and heat treatment plant. In this analysis Barkhausen noise measurements were compared to X-ray diffraction results, and statistical analysis tools were used to correlate the results

Investigation of the residual stress distribution in repairs in H13 steel by friction hydro pillar processing

The distribution of residual stresses (RS) in repairs generated by Friction Hydro-Pillar Processing (FHPP) in AISI H13 was investigated. Three axial force levels, with consequent different deposition rates, were applied to replicate possible repaired conditions. The contour (CM) and X-ray diffraction (XRD) methods were employed for RS analysis in samples that were also evaluated through metallography, microhardness analysis, micro-tensile and Charpy testing. CM produced 2D maps of the RS in the joints, showing symmetrical distributions around the welded rod for all welded conditions. Other common features for all conditions were the maximum level of compressive RS, which was found in the TMAZ of the rod, and the maximum tensile residual stress, which was found near the HAZ of the base block. There was good agreement between the RS measurement techniques. Mechanical tests show similar tensile resistance for all conditions and an apparent increase in toughness at higher force levels