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Marino D.,Georgia Institute of Technology | Kim J.-Y.,Georgia Institute of Technology | Ruiz A.,Institute Investigacion en Metalurgia y Materiales | Joo Y.-S.,Korea Atomic Energy Research Institute | And 2 more authors.
NDT and E International | Year: 2016

This research investigates second harmonic generation in Rayleigh surface waves propagating in 9%Cr ferritic martensitic steel. Previous experimental results show that nonlinear ultrasound is sensitive to certain microstructural changes in materials such as those due to thermal embrittlement and precipitation hardening. This research measures the ultrasonic nonlinearity parameter as an indicator of microstructural changes due to thermal aging in 9%Cr ferritic martensitic steel specimens. The specimens are isothermally aged for different holding periods to induce progressive changes in the microstructure and to obtain different levels of thermal damage. As thermal aging progresses, the existing dislocations are annihilated in the beginning and precipitates are formed; these microstructural evolutions lead to large changes in the measured nonlinearity parameter, β. Nonlinear ultrasonic experiments are conducted for each specimen using a wedge transducer for generation and an air-coupled transducer for detection of Rayleigh surface waves. The amplitudes of the first and second order harmonics are measured as a function of propagation distance, and these amplitudes are used to obtain the relative nonlinearity parameter at different aging stages. A possible scenario for the microstructural evolution during thermal aging is proposed based on the results from the nonlinear ultrasonic measurements, scanning electron microscopy (SEM), and Rockwell HRC hardness. These results indicate a clear trend that the measured nonlinearity parameter is sensitive to variations in dislocation and precipitate density, and thus can be useful in tracking microstructural changes in this material during thermal aging. © 2015 Elsevier Ltd. All rights reserved. Source


Flores R.B.,Institute Investigacion en Metalurgia y Materiales | Flores R.B.,Instituto Tecnologico Superior Of Uruapan | Ruiz A.,Institute Investigacion en Metalurgia y Materiales | Lopez V.H.,Institute Investigacion en Metalurgia y Materiales | And 3 more authors.
Materials Characterization | Year: 2016

In this study, plates of AL-6XN and 316L stainless steels were gas metal arc welded with an ER-NiCrMo3 filler wire and the effect of two different heat inputs and different accumulated fatigue damage is evaluated in terms of microstructure and mechanical behavior. Microstructural characterization of the as-welded samples revealed differences in dendrite size and interdendritic spacing in the weld metal with respect to the side of the dissimilar weld and to the heat input that results in weld metal regions with strong texture. Mo-Nb rich particles were found in the weld metal for both welding conditions and high heat input yielded very wide unmixed zones. Failure of the as-welded samples, under tensile stress, consistently occurred in the 316L base metal irrespective of the heat input. However, it was found that for a high heat input (1.6 kJ/mm) accumulated fatigue damage changed the mechanical response of the welded dissimilar joints so that fracture took place in the weld metal. Ultrasonic attenuation measurements were used to characterize the welded joint and results showed regions of high attenuation that are related to dendritic structures. © 2015 Elsevier Inc. All rights reserved. Source

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