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1. A Comparison of Sigma Phase Formation in Solubilized Hyper Duplex Stainless Steel and Super Duplex Stainless Steel Filler Metals

   https://doi.org/10.1007/S11661-024-07442-4  

   Acuna, Andres; Riffel, Kaue Correa; Ramirez, Antonio (August 2024, Metallurgical and Materials Transactions A)

   This study focuses on the kinetic analysis of sigma phase formation in filler metal wires on Super Duplex Stainless Steel (SDSS) and Hyper Duplex Stainless Steel (HDSS). Precipitation data reveal that in the solubilized microstructure, sigma phase kinetics are more prominent in SDSS. This increased susceptibility is attributed to the greater number of nucleation sites, which is facilitated by the larger interface area/volume and the higher chromium content in the ferrite. The difference in interface area/volume is significantly more influential in determining kinetics than the composition difference, with nucleation sites playing a central role. The sigma phase transformation in both materials was modeled using the JMAK kinetic law. The JMAK plots exhibit a transition in kinetic mechanisms, evolving from discontinuous precipitation to diffusion-controlled growth. In SDSS, the JMAK values indicate “grain boundary nucleation after saturation,” followed by “thickening of large plates.” In contrast, HDSS values point to “grain edge nucleation after saturation,” followed by “thickening of large needles.” The higher kinetics in SDSS are characterized by a smaller nucleation activation energy of 56.4 kJ/mol, in contrast to HDSS's 490.0 kJ/mol. CALPHAD-based data support the JMAK results, aligning with the maximum kinetics temperature of SDSS (875 °C to 925 °C) and HDSS (900 °C to 925 °C). Therefore, the JMAK sigma phase kinetics effectively describe the experimental data and its dual kinetics behavior, even though CALPHAD-based TTT calculations often overestimate sigma formation. 

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2. Sigma phase formation kinetics in hyper duplex stainless steel welding filler metal

   https://doi.org/10.1016/j.matchar.2023.112832  

   Acuna, Andres; Ramirez, Antonio J. (June 2023, Materials Characterization)
3. Using kinetics to avoid sigma phase formation on hyper duplex stainless weld cladding

   https://doi.org/10.1080/13621718.2023.2246719  

   Acuna, Andres; Ramirez, Antonio; Riffel, Kaue Correa (December 2023, Science and Technology of Welding and Joining)

   The Hyper Duplex Stainless Steel HDSS enhanced corrosion resistance and toughness relies upon high alloying to obtain a balanced ferrite and austenite volume and pitting resistance equivalent number PREn. However, during welding, sigma phase precipitates might form, hindering corrosion and mechanical performance. Therefore, a kinetics model is developed to avoid the sigma phase's formation during welding and validated using physical simulation, finite element analysis (FEA), welding, and SEM characterisation. The sigma phase kinetics model produced calculated and validated temperature-time-transformation (TTT) and continuous-cooling-transformation (CCT) curves from which a 4°C/s cooling rate was found as a cooling rate threshold for sigma phase formation in this new material. Three-layered gas tungsten arc welding GTAW cladded mockup with 53 beads produced 24°C/s minimum cooling rate. Moreover, microscopy, mechanical, and corrosion testing attested it as a sigma-free weld. 

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4. Corrosion resistance and microstructure analysis of additively manufactured 22% chromium duplex stainless steel by laser metal deposition with wire

   https://doi.org/10.1016/j.jmrt.2023.09.037  

   Baghdadchi, Amir; Cary, Claire; Sridhar, Narasi; Valiente Bermejo, Maria Asuncion; Fink, Carolin; Andersson, Joel (September 2023, Journal of Materials Research and Technology)
5. Radiation-resistant nanotwinned austenitic stainless steel

   https://doi.org/10.1016/j.scriptamat.2018.09.030  

   Meric de Bellefon, G.; Robertson, I.M.; Allen, T.R.; van Duysen, J.-C.; Sridharan, K. (January 2019, Scripta Materialia)