Engenharia de Materiais
URI permanente desta comunidadehttps://repositorio.fei.edu.br/handle/FEI/17
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3 resultados
Resultados da Pesquisa
- Kinetics of sigma phase formation in a duplex stainless steel(2009) Magnabosco R.This work determines the kinetics of sigma phase formation in UNS S31803 Duplex Stainless Steel (DSS), describing the phase transformations that occur in isothermal aging between 700 and 900 °C for time periods up to 1032 hours, allowing the determination of the Time-Temperature-Precipitation (TTP) diagram for sigma phase and proposing a model to predict the kinetics of sigma phase formation using a Johnson-Mehl-Avrami (JMA) type expression. The higher kinetics of sigma phase formation occurs at 850 °C. However, isothermal aging between 700 and 900 °C for time periods up to 1032 hours are not sufficient to the establishment of thermodynamic equilibrium. Activation energy for both nucleation and growth of sigma phase is determined (185 kJ.mol-1) and its value is equivalent to the activation energy for Cr diffusion in ferrite, indicating that diffusion of Cr is probably the major thermally activated process involved in sigma phase formation. The determined JMA type expression presents good fit with experimental data between 700 and 850 °C.
- Pit morphology and its relation to microstructure of 850°C aged duplex stainless steel(2005) Magnabosco R.; Alonso-Falleiros N.The relation between pit morphology and microstructure formed during 850°C isothermal aging of UNS S31803 duplex stainless steel (DSS) could be reached through the explanation of pit nucleation and growth during potentiodynamic polarization in 3.5 wt% sodium chloride (NaCl) aqueous solution. Aging times up to 10 min at 850°C led to sigma formation by direct precipitation from ferrite, resulting in Cr- and Mo-depleted ferrite, or secondary ferrite. Between 30 min and 5 h of aging, the ferrite suffered eutectoid decomposition, resulting in sigma phase and secondary austenite, also impoverished in Cr and Mo. Those Cr- and Mo-depleted zones around the sigma phase were preferential sites for the nucleation of pits. At those sites, the passive film breakdown was easier, as shown by the low pitting potential values found in aged samples. Pit growth occurred as selective corrosion of the metallic matrix surrounding the sigma phase, as a consequence of the lower corrosion resistance of the secondary ferrite and austenite phases found. The low passivation potential values indicated the difficulty in repassivation of the pits formed, probably related to the Cr and Mo depletion of the regions surrounding the sigma phase. © 2005, NACE International.
- Sigma phase formation and polarization response of UNS S31803 in sulfuric acid(2005) Magnabosco R.; Alonso-Falleiros N.For a better understanding of the relationship between the microstructure of UNS S31803 duplex stainless steel (DSS) and the shape of the polarization curves, this study evaluated the influence of the microstructure on the potentiodynamic polarization of the 850°C isothermal-aged UNS S31803 DSS in 0.5 M sulfuric acid (H2SO4). In the transpassive region, selective corrosion of chromium- and molybdenum-rich phases occurred. In the solution-treated sample, ferrite was selectively corroded, and in all aged samples, the sigma phase was the selectively corroded phase. Five current density maxima in the passive region were found during potentiodynamic polarization, and they can be related to the microstructures formed. The current density maximum at 564 mV us. saturated calomel electrode (SCE) can be related to secondary ferrite, impoverished in chromium and molybdenum, that was formed during direct precipitation of sigma phase from the original ferrite. Secondary austenite, impoverished in chromium and molybdenum and formed together with the sigma phase during eutectoid decomposition of the original ferrite, can be related to the current density maxima at -85 mVSCE and -40 mVSCE. The austenite phase, present in all heat-treatment conditions, can be related to the current density maxima at -155 mVSCE and 111 mVSCE. © 2005, NACE International.