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

Victor De Albuquerque,João Tavares,Elineudo Moura,Cleisson Barbosa,Cleiton Silva,Pedro Filho,João Papa

doi:10.3390/s150612474

Abstract

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.

Highlights

Nb-bearing nickel-based superalloys, in particular inconel 625, has greater applicability, especially in highly corrosive environments, such as the ones in the oil and gas industry, than many other nickel (Ni)-based alloys
For a further assessment of the distance functions’ performance, the results obtained were very satisfactory in terms of accuracy rate, train and test times, confusion matrix and harmonic mean between specificity and sensitivity, which makes the results presented and discussed of noteworthy value
We propose the use of a harmonic average between sensitivity and specificity, that is the harmonic mean (HM ): Se × Sp

Summary

Introduction

Nb-bearing nickel-based superalloys, in particular inconel 625, has greater applicability, especially in highly corrosive environments, such as the ones in the oil and gas industry, than many other nickel (Ni)-based alloys. During the welding of an inconel 625 alloy, there is an intensive microsegregation of some elements, such as niobium (Nb) and molybdenum (Mo), within the interdendritic regions, causing the supersaturation of the liquid metal with these chemical elements in its final stage of solidification, which results in the precipitation of the Nb-rich laves phase and MCprimary carbides of type NbC [1,2]. This segregation and precipitation of the secondary phases can change the mechanical properties of the alloy and decrease its resistance to corrosion [3].

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Conclusion