Thomas Clarke

Modeling and Design Optimization of an Eddy Current Sensor for Superficial and Subsuperficial Crack Detection in Inconel Claddings

Inconel 625 alloys are currently being extensively used as a cladding material for carbon steel pipes. A common fabrication process for this is weld overlay, which can produce defects such as cracks, porosity, inclusions, and lack of fusion. Inspection of the cladded layers is, therefore, important for quality control and structural integrity evaluations. In this paper, finite-element models were used as a tool for eddy current sensor design to optimize their geometry and operating frequency for detection of common, expected defects in these layers. The sensor with the most interesting geometry was then built and tested to validate the results of the model. A good agreement was found between simulated and experimental results, showing that this is a robust strategy for special sensor design.

Effect of microstructure on electrical conductivity of Inconel 718 alloys

Abstract The microstructural evolution of Inconel 718 during aging processes has been studied through a combination of eddy current testing, X-ray diffraction analysis, and metallography and hardness measurements. Measurements were carried out in samples subjected to eight different heat treatment cycles, between 620 and 1035°C for 1–18 h. Different amounts of secondary precipitates were achieved, reaching 18% of delta phase for samples overaged at 900°C for 18 h. Results show that the different microstructures of Inconel 718 obtained have a distinguishable effect on electrical conductivity when this is measured through an appropriately sensitive technique (i.e. eddy current testing). The lowest conductivity values were observed for under aged samples (1·44% IACS). A clear increase in conductivity values was seen for all aged or overaged conditions, reaching a maximum of 1·63% IACS, when coarsening of intra granular precipitates, associated with an increase in density of globular precipitates at grain boundaries, was identified. The influence of microstructure on conductivity could be shown to be due to the competition between two effects on the scattering of electrons: matrix purification and precipitation characteristics. A combination of hardness values and electrical properties proved to be a fast and practical way of determining the stage of aging of the alloy.

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.

Evaluation of the Fatigue Life of High-Strength Low-Alloy Steel Girth Welds in Aqueous Saline Environments with Varying Carbon Dioxide Partial Pressures

High-strength low-alloy steel girth weld specimens were subjected to fatigue tests in saline environments saturated with different carbon dioxide partial pressures. As expected, results show that increases in gas concentration initially affect fatigue life adversely, but when higher partial pressures are associated with low stresses, a reduction in the negative impact of environmental conditions is seen. This may be related to a competition between corrosion rates and mechanisms of crack initiation and propagation. Data is presented with the aim of contributing toward the establishment of a database of results in literature which may lead to better understanding of the phenomena involved through association of these with ongoing research.

Friction Hydro-Pillar Processing of a High Carbon Steel: Joint Structure and Properties

A coupled experimental and theoretical study is reported here on friction hydro-pillar processing of AISI 4140 steel, which is a novel solid-state joining technique to repair and fill crack holes in thick-walled components by an external stud. The stud is rotated and forced to fill a crack hole by plastic flow. During the process, frictional heating occurs along the interface of the stud and the wall of crack hole leading to thermal softening of the stud that eases its plastic deformation. The effect of the stud force, its rotational speed and the total processing time on the rate of heat generation and resulting transient temperature field is therefore examined to correlate the processing variables with the joint structure and properties in a systematic and quantitative manner, which is currently scarce in the published literature. The results show that a gentler stud force rate and greater processing time can promote proper filling of the crack hole and facilitate a defect-free joint between the stud and original component.

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.

An investigation on friction hydro-pillar processing

ABSTRACT Friction hydro pillar processing (FHPP) is a novel technique to fill in crack-holes in thick-walled metal structures by an external stud and forming a solid-state bond between the stud and the metal substrate. During the process, the stud is rotated against the crack-wall to facilitate friction heating and flow of plasticized material for proper filling of the crack-hole. We present here a coupled experimental and numerical study on FHPP of ASTM A36 steel to understand the effect of processing conditions on the joint structure and properties. An axi-symmetric heat transfer analysis is carried out to compute the temperature field. The computed thermal cycles are used to estimate the hardness distribution across the joint. The estimated thermal cycles and hardness distribution are tested with the corresponding experimentally measured results.

Validation Methodology of Crack Growth Measurement Using Potential Drop Method on SENB Specimens

The Brazilian pre-salt oil and gas discoveries brought technical challenges as impressive as the reserves themselves. Besides the concerns with exploration, the oil contamination with CO2, H2S and chloride enriched seawater combined with critical cyclic loads due to the relative movement of the production vessel and high water depths imposes an environment chemically and mechanically aggressive. Suitable materials to work on such harsh conditions are few and one should consider the use of special materials, such as supermartensitic and superduplex stainless steels. Although the corrosion and mechanical properties of these materials are well established, still additional crack growth data in specific environments should be provide to the subsea equipment designer. Indeed, due to the combination of cyclic loading and corrosive ambient the corrosion fatigue phenomenon is a major concern. In order to evaluate the effect of oil contaminants on the corrosion fatigue resistance of candidate materials, one should provide methods for crack growth measurement other than the use of crack gauges since those can not be used in chemically aggressive solutions. The present work aims to validate the potential drop crack growth measurement method comparing the results obtained by this technique with those produced by crack gauges on SEN(B) (Single Edge Notch Bending) specimens in air. This validation effort is essential because the ASTM E647 standard only consider the use of C(T) (Compact Tension) specimens which actually does not represents the real cracks propagation path in crucial subsea equipment, such as risers, drill pipes et cetera, that is through the wall thickness. The results produced by the two tested methodologies have an excellent agreement which makes reliable the use of the potential drop method as an alternative to monitor and measure crack growth in corrosive media.Copyright

Preliminary evaluation of fatigue in carburized, conventionally and intensively quenched steels

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.