Abstract
Super duplex stainless steel (SDSS) combines corrosion resistance and mechanical properties, interesting to applications in the oil extraction industry. In this process, wear between the tools friction and rocky materials occurs with SDSS pipes. Due to the low tribological properties, plasma nitriding process is applied to provide a surface layer with excellent wear resistance and without significant losses in corrosion resistance. Here, conventional plasma nitriding (CPN) and cathodic cage plasma nitriding (CCPN) techniques were compared to modify SDSS surface. Before and after passing by these two nitriding processes, welded joints (WJ) were subjected to a microstructural and corrosion resistance evaluation. Aiming to characterize the final product obtained, it was used characterization techniques, such as scanning electron microscopy (SEM) and X-ray diffraction (XRD). Results show that using the CPN technique, at 400°C, was responsible for providing greater resistance to corrosion, when analyzing the result of potentiodynamic polarization — jpassivation = 2.60x10-5 A.cm-2 and Epite = 0.83 V than the CCPN technique when applied to the SSDS, and this was attributed to the formed diffusion layer, which is composed of the expanded austenite and ferrite phases.
Highlights
Super duplex stainless steel (SDSS) is a category of stainless steel that exhibit austenitic and ferritic microstructural phases in proportions of approximately 50:50 each, due to their chemical composition and heat treatment[1,2]
High corrosion resistance is a trademark of SDSS steels; they wear out when subjected to localized attacks, as in the case of corrosion, like pitting, crevice and tribocorrosion[5,6]
In the micrograph obtained by scanning electron microscopy (SEM) with a magnification of 1000x, Figure 2b, it is possible to observe the presence of elongated austenitic islands (γ) in the lamination direction, the characteristic microstructure appears in the first plane regarding the ferritic matrix, similar to the results presented in Senatore et al.[36]
Summary
Super duplex stainless steel (SDSS) is a category of stainless steel that exhibit austenitic and ferritic microstructural phases in proportions of approximately 50:50 each, due to their chemical composition and heat treatment[1,2] This material has a high chromium content (24% - 27%), and a low carbon content (less than 0.03 wt.%), containing alloy elements such as molybdenum (2.5% - 5.0%), tungsten (0.5% - 2.5%), copper (0.2% - 2.5%) and nitrogen (0.10% - 0.35%). It has several applications mainly in the petrochemical and chemical industries, in the manufacture of tubes, pumps, pressure vessels, separators and heat exchangers[3,4].
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