Duplex Stainless Steels (DSSs) are widely used in the oil and gas industry. When the steel is subjected to a thermal gradient, as in welding may occur precipitation of intermetallic phases. Among these phases, there is the sigma phase, which even in small quantities reduces drastically the mechanical and corrosion resistance properties. According to some studies in the literature, the limit amount of sigma phase present in steel in welded joints must be in a maximum volume fraction of 2,5%. In search to improve the detection sensitivity of the sigma phase, this paper presents results obtained by Linear Sweep Voltammetry (LSV) in Duplex Stainless Steel UNS S31803 as a Nondestructive Testing (NDT) model. The innovation in the application of this test is to use a microcell-based system that allows a reduced area of analysis obtaining density of currents in the order of microamperes and in the use of selective electrolytes to the elements of the oxides present in the intermetallic phases. With the use of these systems and through the optimization of control parameters it was possible to detect very low amounts of deleterious phases such as the sigma and chi phases.

Resumo A precipitação de fases intermetálicas nos aços inoxidáveis duplex constitui um problema no que diz respeito à utilização destes materiais em determinadas faixas de temperatura, desta forma, fazem-se necessários métodos de quantificar, de forma não destrutiva, a presença de alterações microestruturais decorrentes de aporte térmico. Neste trabalho, ensaios eletroquímicos de voltametria linear, de tração e de microdureza Vickers foram realizados no aço inoxidável duplex UNS S31803 para avaliação da presença da fase intermetálica sigma na microestrutura do material. Amostras deste aço foram envelhecidas isotermicamente a 870 °C, em tempos de até 3600 segundos para induzir a precipitação de diferentes teores desta fase. Os ensaios de voltametria linear foram conduzidos em solução eletrolítica de 5% KOH, em uma célula eletroquímica de 3,3.10–6 m2 de área, com eletrodo de referência do tipo Calomelano Saturado e eletrodo auxiliar de fio de platina. Os resultados desta técnica apresentaram sensibilidade para detectar teores de fase sigma maior que 1% no material viabilizando sua aplicação como um Ensaio Não Destrutivo (END).

Abstract Precipitation of intermetallic phases in duplex stainless steels constitute a problem regarding the use of such materials in certain temperature ranges, in this way, are necessary methods to quantify, non-destructively, the presence of microstructural changes due to thermal contribution. In this work, electrochemical tests of linear voltammetry, tensile and Vickers hardness were performed in duplex stainless steel UNS S31803 for evaluation of the intermetallic sigma phase presence in the microstructure of the material. Samples of this steel were isothermally aged at 870 °C for times up to 3600 seconds to induce the precipitation of different grades of this phase. Linear voltametry tests were conducted in 5% KOH electrolyte solution in an electrochemical cell 3.3.10-6 m2, with a reference electrode of the type Saturated Calomel and auxiliary electrode of platinum wire. The results of this technique showed sensitivity to detect sigma phase content greater than 1% in the material enabling its application as a Non-Destructive Testing (NDT).

In this article the delamination phenomena, which occurs in the X70 steel during fracture process from the Charpy impact tests, was investigated. Microstructure, microtexture and Taylor factor map analyses were performed by scanning electron microscopy, light microscopy and electron backscatter diffraction, respectively. In a cross-section of two fractured surface regions namely, in the perpendicular and parallel regions to the propagation fracture direction where delamination occurred, it was possible to notice that delamination showed a key role in the anisotropy of impact toughness between the L-T and T-L orientations. The results also revealed that the cause of the delamination can be attributed to the presence of microstructural banding and elongated ferrite grains aligned in a rolling direction. The presence of (100)[011] and (111)[110] crystallographic orientations, with Taylor factors close to 2.7 and 4.5, respectively, were identified in the delamination region. They contribute to the occurrence of cleavage delamination during the fracture process.