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LEAL, PEDRO ; DE ASSIS, KIOSHY S. ; MATTOS, OSCAR R. ; CAMERINI, CESAR G. ; PEREIRA, GABRIELA R. (2015).
Resumo: The development of techniques to detect deleterious phases in duplex and super duplex stainless steel has a great technical-scientific appeal. However, establishing a quality control of these steels using a unique technique of inspection requires an attentive analysis about its limits and sensibility of detections. Thus, the current work compares magnetic techniques based on eddy current methods and electrochemical potentiokinetic reactivation with double loop to detect deleterious phases in duplex stainless steel. The limitations and advantages of each technique are discussed mainly concerning the viability to detect sigma phase and chromium nitride.
LAGE, M.A. ; ASSIS, K.S. ; Mattos, O.R. (2015).
Resumo: The super duplex stainless steels have a microstructure composed by two phases, ferrite (α) and austenite (γ). This dual microstructure improves simultaneously the mechanical and corrosion resistance properties. However, the welding of these steels is often a critical operation. The present work evaluated the fracture toughness through critical tip open displacement (CTOD) tests of welded joints, with two different heat input, 1.1 kJ mm−1 and 2.0 kJ mm−1. The steel used was a super duplex stainless steel (UNS S32750) in presence of hydrogen. The CTOD tests (according BS 7448-1 and BS 7448-2 standards) were performed in air and under different times of hydrogenation. The procedure of hydrogenation has been performed using cathodic potential of −1400 mVSCE by 96 and 360 h. The microstructural analysis allowed to determine relevant aspects (α/γ balance, inter austenitic spacing and γ morphology) and to compare with CTOD results. The results showed strong evidence that the reductions of CTOD values is related to differences in the γ2 morphologies. Another important result was the high statistic dispersion in the measures of austenitic spacing, according DNV RP F112-08, which implies in low reliability of using this standard in presence of high anisotropy. The paper also aims to discuss and evaluate which is the best approach to hydrogenated duplex stainless steels: linear elastic fracture mechanics or elasto plastic fracture mechanics.
LEITE, N. A. ; BARCIA, O. E. ; MATTOS, O. R. ; ORAZEM, M. E. ; TRIBOLLET, B. (2015).
Resumo: A mathematical model for electrohydrodymanic impedance was developed in which hydrodynamic and convective diffusion equations were coupled by a relationship between viscosity and concentration. This procedure allows better understanding the physical problem separating the mass transport and the hydrodynamic effects and the model fit to previously reported experimental results revealed a larger viscosity gradient than previously inferred. These results are consistent with the hypothesized formation of a colloidal suspension of ferrous sulfate particles.
CAMERINI, C.; SACRAMENTO, R. ; AREIZA, M.C. ; ROCHA, A. ; SANTOS, R. ; REBELLO, J.M. ; PEREIRA, G. (2015).
Resumo: Super duplex stainless steel (SDSS) is a two-phase material where the microstructure consists of grains of ferrite (δ) and austenite (γ). SDSS exhibit an attractive combination of properties, such as: strength, toughness and stress corrosion cracking resistance. Nevertheless, SDSS attain these properties after a controlled solution heat treatment, leading to a similar volumetric fraction of δ and γ. Any further heat treatment, welding operation for example, can change the balance of the original phases, or may also lead to precipitation of a deleterious phase, such as sigma (σ). For these situations, the material corrosion resistance is severely impaired. In the present study, several SDSS samples with low σ phase content and non-balanced microstructure were intentionally obtained by thermally treating SDSS specimens. Electromagnetic techniques, conventional Eddy Current Testing (ECT) and Saturated Low Frequency Eddy Current (SLOFEC), were employed to characterize the SDSS samples. The results showed that ECT and SLOFEC are reliable techniques to evaluate σ phase presence in SDSS and can provide an estimation of the δ content.
