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, SV, interfacial area per unit of volume between sigma phase and austenite, and , mean chord length of sigma, both in function of the VV, 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.
Highlights
The superduplex stainless steels (SDSS) are widely used in chemical industry, oil and gas, in the pulp industry, applied to pipelines and storage material, and are often used in petrochemical industry, in oil extraction offshore platforms and storage tanks of chemicals
It is well known that the chi phase can be a precursor of the sigma phase which is deleterious for the properties of SDSS
The microstructural partial path (MP) formulation is common in recrystallization studies, but so far has not been used in the sigma phase precipitation studies, and will be applied here for the first time
Summary
The superduplex stainless steels (SDSS) are widely used in chemical industry, oil and gas, in the pulp industry, applied to pipelines and storage material, and are often used in petrochemical industry, in oil extraction offshore platforms and storage tanks of chemicals. Summarizing, JMAK theory considers a random distribution of nuclei and a constant interface velocity In this case, is necessary measure or modeling only volume fraction of sigma phase, VV, in function of time. The samples were analyzed by optical microscope (OM), X-Ray Diffraction (XRD), and Microhardness Vickers (HV) These results help us to understand about nucleation and growth of sigma phase. Through OM, stereological measurements such as: VV, SV and were carried out In this case SV means area interfacial per unit of volume between sigma phase and austenite. These results is useful to understand about sigma phase kinetics with the aid of JMAK and nucleation mode with the aid of MP formulation. This result is important, since is experimental evidence of the mechanism and can be used for modeling and design of theses SDSS knowing the nucleation mode and growth of sigma phase with impingement
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