Investigation of Ta-MX/Z-Phase and Laves Phase as Precipitation Hardening Particles in a 12 Pct Cr Heat-Resistant Steel
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A 12 pct Cr martensitic/ferritic steel was designed and produced to study Laves and Z-phase as precipitation hardening particles under creep conditions (650 °C). According to thermodynamic calculations, W and Cu additions were selected to ensure the precipitation of Laves after tempering. It is known that Z-phase formation does not follow the classical nucleation theory. Indeed, MX particles are transformed into Z-phase by Cr diffusion from the matrix to the precipitate. Therefore, to promote fast Z-phase formation, Ta, Co, and N additions were used to produce Ta-MX, which will be transformed into Z-phase. The main result achieved was the precipitation of Laves after tempering, with a particle size of 196 nm. As regards to Z-phase, the transformation of Ta-MX into Z-phase after tempering was confirmed by the formation of hybrid nanoparticles of 30 nm. Although W and Ta have a low diffusion in the martensitic/ferritic matrix, characterization of the precipitates after isothermal aging revealed that Laves and Z-phase have fast growth kinetics, reaching 400 and 143 nm, respectively, at 8760 hours. Consequently, creep test at 650 °C showed premature failures after few thousand hours. Therefore, it is expected that future research in the field of martensitic/ferritic steels will focus on the growth and coarsening behavior of Laves and Z-phase.
The authors would like to thank in particular: “DOCTORADO NACIONAL 21130630” for Doctoral Research Fellowship. Also, “FONDECYT de Iniciación 11110098” Project, “FONDECYT de Iniciación 11121384” Project, “Proyecto de inserción de capital humano avanzado 79112035” and “FONDECYT 1150457” Projects from the Chilean Government, for the financial support of this work. The authors are grateful for the support of the Create-Net Project (H2020-MSCA-RISE/644013) for research stays.
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