Elineudo Pinho de Moura

Ultrasonic sensor signals and optimum path forest classifier for the microstructural characterization of thermally-aged inconel 625 alloy.

Secondary phases, such as laves and carbides, are formed during the final solidification stages of nickel-based superalloy coatings deposited during the gas tungsten arc welding cold wire process. However, when aged at high temperatures, other phases can precipitate in the microstructure, like the γ” and δ phases. This work presents an evaluation of the powerful optimum path forest (OPF) classifier configured with six distance functions to classify background echo and backscattered ultrasonic signals from samples of the inconel 625 superalloy thermally aged at 650 and 950 °C for 10, 100 and 200 h. The background echo and backscattered ultrasonic signals were acquired using transducers with frequencies of 4 and 5 MHz. The potentiality of ultrasonic sensor signals combined with the OPF to characterize the microstructures of an inconel 625 thermally aged and in the as-welded condition were confirmed by the results. The experimental results revealed that the OPF classifier is sufficiently fast (classification total time of 0.316 ms) and accurate (accuracy of 88.75% and harmonic mean of 89.52) for the application proposed.

CHARACTERIZATION OF MICROSTRUCTURAL CHANGES IN COARSE FERRITIC‐PEARLITIC STAINLESS STEEL THROUGH THE STATISTICAL FLUCTUATION AND FRACTAL ANALYSES OF BARKHAUSEN NOISE

This work aims to identify the changes in the microstructure of ferritic‐pearlitic stainless steel, through the statistical fluctuation and fractal analyses of Barkhausen noise. The samples studied were obtained from pipes of steam pressure vessels, and presented coarse ferritic‐pearlitic phases before degradation. Due to temperature effects, two different microstructures were obtained from pearlite that has partially and completely transformed to spheroidite. The statistical fluctuations of the Barkhausen signals are obtained by means of Hurst and detrended‐fluctuation analyses, and the fractal analyses are carried out by applying the minimal cover technique to the signals. The curves obtained for the statistical fluctuations and fractal analyses, as functions of the time window, were processed by using pattern classification techniques such as principal‐component analysis and Karhunen‐Loeve expansion. Approximately a 100% success rate has been reached for the classification of the different microstructu...

Microstructure identification via detrended fluctuation analysis of ultrasound signals.

We describe an algorithm for simulating ultrasound propagation in random one-dimensional media, mimicking different microstructures by choosing physical properties such as domain sizes and mass densities from probability distributions. By combining a detrended fluctuation analysis (DFA) of the simulated ultrasound signals with tools from the pattern-recognition literature, we build a Gaussian classifier which is able to associate each ultrasound signal with its corresponding microstructure with a very high success rate. Furthermore, we also show that DFA data can be used to train a multilayer perceptron which estimates numerical values of physical properties associated with distinct microstructures.

Evaluation of proportion of phases in joints welded from duplex stainless steel pipes by means of non-destructive testing

Duplex stainless steels (DSSs) combine excellent corrosion resistance and high mechanical strength, due to the fine two-phase microstructure composed of similar amounts of ferrite (δ) and austenite (γ). These alloys are used in piping and industrial equipment for which a high strength-to-weight ratio is required, especially in building construction and off-shore assembly. However, welding operating conditions can promote a significant microstructural imbalance of these phases, resulting in a decrease of the properties cited above. Evaluation by feritscope is normally used in these activities. During evaluation with a feritscope, the welded joint may be rejected when the weld metal is found to have ferrite values outside of the range established by project specifications. Thus, analysis of these joints is important, along with complementary techniques such as the use of metallographic replicas. This study aimed to evaluate the proportion of phases in relatively thin spools of DSS, welded in the field, comparing the non-destructive techniques described. The results show similar values, yet the result may be influenced by the shape and surface conditions of the welded joint.

Avaliação da proporção de fases em juntas soldadas de tubulações de aço inoxidável duplex mediante aplicação de ensaios não destrutivos

Duplex stainless steels are high strength and corrosion resistant alloys, whose properties are devoted to the fine microstructure composed by similar amounts of ferrite and austenite and also to the high concentrations of Cr, Mo and N in solid solution. Not for coincidence, duplex steels are extensively used in chemical and petrochemical industries. However, welding operations conditions can promote the unbalance of the ferrite/austenite proportions mainly in the welding metal, with decrease of the properties mentioned. For this reason, non destructive measurements of ferrite content, by ferritoscope, is used after welding pipes. Ferritoscope is a commercial instrument which uses the initial magnetic permeability to analyze the ferrite content in duplex and austenitic stainless steels. The inspection with ferritoscope is a procedure commonly used in building and assembly of spools of duplex and superduplex stainless steels. The objective of this work was to evaluate the amounts of phases in the welding metal for different spools welded in the field by use of ferritoscope and metallographic replica techniques. The results showed that the two techniques measured similar values of ferrite and austenite, but ferritoscope analysis can be influenced by shape and surface conditions of the welded joint.

EVOLUTION OF RECRYSTALLIZATION BY CHANGES IN MAGNETIC HYSTERESIS LOOP IN A NON‐ORIENTED ELECTRIC STEEL COLD ROLLED

Non‐oriented steels, with low carbon, are widely used in the fabrication of electrical motor nucleus. The performance of these motors is affected by the level of recrystallization. These steels can come from the steel plant in two different conditions: totally processed or semi‐processed. The semi‐processed steels have a partially deformed structure and are submitted to the final annealing process after reaching the end shape. An adequate annealing heat treatment is important to get an appropriate magnetic property. In the present study, samples of an electric steel, with the composition (0.05 wt% C, 1.28wt% Si, 0.29wt% Mn), cold rolled 50% in thickness, were withdrawn during the industrial heat treatment at temperatures of 575, 580, 600, 620 and 730 °C with the objective of evaluating the evolution of recrystalization with temperature. Magnetic properties were measured at room temperature in a vibrating sample magnetometer. Although the changes in magnetic hysteresis loop with temperature are difficult t...