Sigma Phase in Superduplex Stainless Steel: Formation, Kinetics and Microstructural Path

Abstract

The superduplex stainless steels (SDSS) are widely used in chemical, oil and gas industries, to pipelines and storage material facilities. In welding process or working in temperature elevated, secondary phases may appear in the form of precipitates, as the sigma phase (σ) which is an intermetallic compound. This compound is harmful to the properties of steel, deteriorating its mechanical properties, such as decreasing corrosion resistance and toughness. In this paper it is analyzed the formation, kinetics and microstructural evolution of sigma phase in SDSS UNS S32750 after isothermal aging at 700ºC, 750ºC and 800ºC. In this work sigma phase kinetics is studied by JMAK theory and by two microstructural path descriptors, S V , interfacial area per unit of volume between sigma phase and austenite, and <λ>, mean chord length of sigma, both in function of the V V , volumetric fraction of sigma, known in the literature as microstructural partial path (MP). The MP formulation is common in recrystallization studies, but so far has not been used in the sigma phase precipitation studies, being applied here for the first time. The results indicated that the sigma phase nucleates by site saturation with anisotropic linear impingement. This means that sigma phase nucleates on edges.

Microstructural, Mechanical, and Electrochemical Analysis of Duplex and Superduplex Stainless Steels Welded with the Autogenous TIG Process Using Different Heat Input

“…The maximum cooling rates are about 5 • C/s during a period of around 60 s within the range of deleterious phase formations, such as sigma and the chromium nitrate. Therefore, the residence time that could allow the formation of these intermetallic phases and precipitates are much higher [1]. Thus, we can conclude that the amount of phase that could be formed would be negligible, which justifies the non-detection on these phases on the microstructural analysis carried out in this work.…”

“…The electrode used was a 1/8′′ diameter tungsten and thorium alloy (red tip). Table 2 shows the heat input calculated with the aid of Equation (1) [7], and the parameters obtained during the welding of the plates (1) where η = Efficiency of heat transfer. In this case η = 0.80 [15]; E = Volts; I = Amperes; and V = Velocity of the welding heat source (mm/s).…”

“…With this etching for 1 min, ferrite, austenite, and sigma phase, if it is present, can be distinguished. This procedure was presented in [1]. …”

“…Applied to oil extraction structures, pipes, and pressure vessels, these steels combine good mechanical properties with an excellent ability to withstand the corrosive media to which they are exposed. These characteristics are directly linked to the biphasic structure formed, composed basically of ferrite and austenite in approximately equal proportions [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. For the application of these steels, welding processes are usually involved, and weldability is another positive point, as DSS and SDSS are weldable by almost all processes available in the market.…”

“…These phases are also undesirable, since their nucleation and growth are detrimental to the mechanical properties and corrosion resistance. Numerous articles on the subject can be found in the literature [1][2][3][4][8][9][10][11][12][13][14]. Mohammed GR and co-authors [7] also concluded that the analysis of the effects of heat input on different types of duplex stainless steels welded by different techniques still needs further studies for a deeper understanding on the subject.…”