Engenharia de Materiais
URI permanente desta comunidadehttps://repositorio.fei.edu.br/handle/FEI/17
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10 resultados
Resultados da Pesquisa
- Kinetic Study to Predict Sigma Phase Formation in Duplex Stainless Steels(2016) dos Santos D.C.; Magnabosco R.© 2016, The Minerals, Metals & Materials Society and ASM International.This work presents an improved kinetic study of sigma phase formation during isothermal aging between 973 K and 1223 K (700 °C and 950 °C), based on Kolmogorov-Johnson-Mehl-Avrami (K-J-M-A) model, established from volume fraction of sigma phase determined in backscattered electron images over polished surfaces of aged samples. The kinetic study shows a change in the main mechanism of sigma formation between 973 K and 1173 K (700 °C and 900 °C), from a nucleation-governed stage to a diffusion-controlled growth-coarsening stage, confirmed by a double inclination in K-J-M-A plots and microstructural observations. A single inclination in K-J-M-A plots was observed for the 1223 K (950 °C) aging temperature, showing that kinetic behavior in this temperature is only related to diffusion-controlled growth of sigma phase. The estimated activation energies for the nucleation of sigma phase are close to the molybdenum diffusion in ferrite, probably the controlling mechanism of sigma phase nucleation. The proposed time-temperature-transformation (TTT) diagram shows a “double c curve” configuration, probably associated to the presence of chi-phase formed between 973 K and 1073 K (700 °C and 800 °C), which acts as heterogeneous nuclei for sigma phase formation in low aging temperatures.
- Influence of the Heterogeneous Nucleation Sites on the Kinetics of Intermetallic Phase Formation in Aged Duplex Stainless Steel(2017) Melo E.A.; Magnabosco R.© 2017, The Minerals, Metals & Materials Society and ASM International.The aim of this work is to study the influence of the heterogeneous nucleation site quantity, observed in different ferrite and austenite grain size samples, on the phase transformations that result in intermetallic phases in a UNS S31803 duplex stainless steel (DSS). Solution treatment was conducted for 1, 24, 96, or 192 hours at 1373 K (1100 °C) to obtain different ferrite and austenite grain sizes. After solution treatment, isothermal aging treatments for 5, 8, 10, 20, 30, or 60 minutes at 1123 K (850 °C) were performed to verify the influence of different amounts of heterogeneous nucleation sites in the kinetics of intermetallic phase formation. The sample solution treated for 1 hour, with the highest surface area between matrix phases, was the one that presented, after 60 minutes at 1123 K (850 °C), the smaller volume fraction of ferrite (indicative of greater intermetallic phase formation), higher volume of sigma (that was present in coral-like and compact morphologies), and chi phase. It was not possible to identify which was the first nucleated phase, sigma or chi. It was also observed that the phase formation kinetics is higher for the sample solution treated for 1 hour. It was evidenced that, from a certain moment on, the chi phase begins to be consumed due to the sigma phase formation, and the austenite/ferrite interface presents higher SV for all solution treatment times. It was also observed that intermetallic phases form preferably in austenite-ferrite interfaces, although the higher occupation rate occurs at triple junction ferrite-ferrite-ferrite. It was verified that there was no saturation of nucleation sites in any interface type nor triple junction, and the equilibrium after 1 hour of aging at 1123 K (850 °C) was not achieved. It was then concluded that sigma phase formation is possibly controlled by diffusional processes, without saturation of nucleation sites.
- Use of composition profiles near sigma phase for assessment of localized corrosion resistance in a duplex stainless steel(2019) Magnabosco R.; da Costa Morais L.; dos Santos D.C.© 2018 Elsevier LtdPitting corrosion potential of a UNS S31803 duplex stainless steel (DSS) heat treated between 750 °C and 850 °C at different aging times was related to the extent of the Cr- and Mo-depleted areas at sigma-ferrite and sigma-austenite interfaces, estimated using DICTRA® simulations. It is possible to describe the sigma formation kinetics using a model that assumes a spherical region of austenite, with radius equivalent to the mean intercept of austenite islands, surrounded by ferrite. Composition profiles at sigma-ferrite and sigma-austenite interfaces were calculated, allowing the evaluation of the degree of depletion in Cr and Mo at sigma interfaces. There is a reduction of pitting potential with increase of the degree of depletion, characterized as the weighted sum of depleted areas in Cr and Mo composition profiles, making possible the assessment of the localized corrosion resistance of aged DSS through simulation of sigma phase formation.
- Evaluation of microstructural effects on the degree of sensitization (DOS) of a UNS S31803 duplex stainless steel aged at 475°C(2015) De Melo E.B.; Magnabosco R.© 2015, NACE International.In this study, microstructural and electrochemical behavior of a UNS S31803 duplex stainless steel aged at 475°C were investigated. Hardness values increased with aging at 475°C and evidence of alpha prime (α′) phase formation was confirmed by transmission electron microscopy analysis. Double loop electrochemical potentiokinetic reactivation measurements in a 2 M H2SO4 + 0.5 M NaCl + 0.01 M KSCN solution showed an increase in the degree of sensitization values, mainly for the sample aged for 360 h at 475°C. Moreover, the results suggest that α′ phase formation occurred via spinodal decomposition mechanism.
- Experimental investigations and DICTRA® simulation of sigma phase formation in a duplex stainless steel(2017) Morais L.D.C.; Magnabosco R.© 2017 Elsevier LtdSigma phase formation in an UNS S31803 duplex stainless steel aged at 940 °C was evaluated by computational simulation in DICTRA® software, using MOB2 diffusion database and TCFE8 thermodynamic database. Simulation results were compared to experimental tests. Two models were tested in DICTRA® software: in model 1 sigma phase are placed between ferrite and austenite, and in model 2 sigma is placed at one side of ferrite region, and austenite on the other. The volume fraction of sigma and ferrite phases obtained in model 1 showed adherence to the experimental results up to 7200 s (2 h) of simulation, indicating the ability of the model in the description of early stages of sigma formation. Model 2 showed good agreement with experimental data up to 86,400 s (24 h) of simulation. The composition profile obtained by the simulation of the model 1 represented better the impoverishment in Cr and Mo in ferrite/sigma and austenite/sigma interfaces, while the profiles obtained by the simulation of model 2 described better the partition of the chemical elements between austenite and ferrite during sigma formation.
- Influence of sigma phase formation on pitting corrosion of an aged uns s31803 duplex stainless steel(2013) Dos Santos D.C.; Magnabosco R.; De Moura-Neto C.The main purpose of this contribution was the study of the relation between sigma formation and pitting corrosion resistance of a UNS S31803 duplex stainless steel aged at 850°C. Solution-treated samples were isothermally aged at 850°C for up to 360 h. The corrosion resistance of aged UNS S31803 steel was analyzed through cyclic potentiodynamic polarization tests in 0.6 M sodium chloride (NaCl) solution. The main mechanism of sigma phase formation was nucleation and growth from ferrite, in agreement to the Johnson-Mehl-Avrami- Kolmogorov kinetics model. This model suggests a change on the mechanisms of sigma phase formation from nucleation and growth from ferrite to the growth from austenite. Pitting morphology was related to the material microstructure. The pitting growth mechanism occurred by selective corrosion of the chromium-depleted areas of the specimens. Both pitting and repassivation potentials decreased with an increase of sigma volume fraction. Recovery of the corrosion resistance was observed in the early stages of sigma formation, probably related to chromium redistribution to depleted areas. © 2013, NACE International.
- Influence of the microstructure on the degree of sensitization of a duplex stainless steel UNS S31803 aged at 650°C(2013) De Melo E.B.; Magnabosco R.; De Moura Neto C.This work evaluates the phase transformations during aging of an UNS S31803 DSS at 650°C and its influence on the DOS. The material was solution treated at 1175°C and then aged at 650°C for times up to 360 h. SEM-BSC images indicate the formation of Cr2N, σ and ÷ phases in the samples aged at 650°C. The analysis of DL-EPR curves, obtained in a 1 M H2SO4 + 0.25 M NaCl + 0.01 M KSCN solution, shows an increase in DOS values for samples aged at 650°C. Probably, this increase observed in DOS values is mainly related to the presence of Cr-and/or Mo-depleted á, as a result of σ phase, ÷ phase and/or Cr2N formation at 650°C. Moreover, a possible healing up of Cr-and/ or Mo-depleted areas is observed between 1 h and 4 h of aging at 650°C.
- 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.