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The weight loss test was done according to ASTM A Test Method evaluated using DLEPR test according to ASTM G standard . For metallographic examination, they were etched in NaOH solution as described in ASTM A . The solution for etching was prepared. Epub Feb 12, . a commercial penetrant liquid, in accordance with the manufacturer's instructions and the ASTM standard, .. the sodium hydroxide etch test (ASTM A ), used to check for deleterious secondary phases (Test.
The FSWed plate was etched to identify the different zones, then small samples were taken from the top surface of each zone. A voltage of 1. The specimens analyzed using electrochemical tests were mounted in bakelite, leaving an area of 0. The relatively small area exposed to the electrolyte was due to the width of the TMAZ. Consequently, the same exposure area was used for all the tested zones.
In the literature [15,23,29] , this is circumvented by assuming N saturation in ferrite nearly 0. The remaining N content in the alloy was ascribed to austenite, according to the calculated phase fraction of each zone. These assumptions are required for estimating the PREN of each phase. Electrochemical methods were used for characterization of pitting susceptibility.
The polarization tests were performed in 0. The tests were initiated after s of immersion in the solution. The localized corrosion resistance in chloride-containing media was investigated by cyclic potentiodynamic polarization tests and by determining the critical pitting temperature CPT. An electrochemical cell composed of three electrodes was used in both tests. A platinum wire and a saturated calomel electrode SCE were used as the counter and reference electrodes, respectively.
The critical pitting temperature CPT was evaluated by applying a fixed potential of 0. The corresponding temperature is the critical pitting temperature. This method has been used to classify materials according to their pitting corrosion resistance [30,31]. For all zones evaluated, at least three experiments were carried out to check the reproducibility of the results. FSW is asymmetrical with respect to the centerline of the joint due to the different relative velocities at both sides of the tool.
The side in which the tool tangential and translational velocities are on the same direction is called the advancing side AS , while the side where the tool tangential and translational velocities are opposite with slower relative speed is called the retreating side RS .
The literature reports that no significant microstructural changes exist on the duplex stainless steels, even at high temperatures [5,17]. Optical micrographs of the four zones taken from the top of the welded joint are also shown in Fig. In these images, austenite is the white phase, and ferrite is the dark one.
Intermetallic phases were not observed in the optical micrographs. Microstructural changes due to the FSW were observed in terms of grain morphology and size. A significant reduction in grain size was noticed in the SZ compared to the BM, due to dynamic recrystallisation.
In the TMAZ, the effect of deformation is seen in the morphology of the elongated and deformed grains following tool rotation and displacement. The ferrite is the white phase and the austenite is the dark one.
XRD analysis only detected ferrite and austenite in all zones. The presence of deleterious phases, such as sigma, commonly found in fusion-welded duplex stainless steels, was not identified.
If sigma was present at all, it was at a lower volume than the detection limit of 5vol. The results in Fig. In order to calculate the PREN of each phase, the alloying elements in ferrite and austenite in each analyzed zone were quantified using X-ray energy dispersive spectroscopy EDS , except for nitrogen. The results are shown in Fig. Larger amount of Cr and Mo was found in ferrite compared to austenite in the same zone, while Ni was preferentially found in austenite. Distribution of alloying elements in the ferritic and austenitic phases of the UNS S steel used in this study.
Transmission electron microscopy TEM of the various zones was also carried out to characterize the effect of FSW on the steel microstructure, and the results are shown in Fig. In Fig. Figure 1 represents the microstructure of solution-annealed parent alloy in which white phase is austenite and the dark phase is ferrite. The volume fractions of and were 0. The microstructure does not reveal any visible precipitates of other phases. The nucleation rate [ 17 , 18 ] and the growth kinetics of this reaction have been studied extensively [ 19 ].
It is well known that the controlling step is the nucleation rate. The growth of sigma phase causes a decrease in chromium and molybdenum content of the neighboring ferrite which becomes unstable and transforms into new austenite. This austenite, in turn, is rich in chromium and molybdenum and has a lower content of nickel if compared with the neighboring primary austenite. These changes in chemical composition promote the formation of an additional sigma phase.
The overall result is the coprecipitation of secondary austenite and sigma. For ageing from to min, the first tiny precipitates of sigma phase appear at and boundaries. Incoherent twin boundaries and dislocations inside the ferrite matrix may also be the nucleation sites for precipitation [ 20 ]. The precipitations of -phase were first encountered after ageing for min and the size and amount of phase increased with ageing time.
Nucleation and growth of -phase are thermally activated processes involving diffusion. Therefore, temperature will have a significant effect on the kinetics of the transformation. After min, a lot of sigma precipitates have developed at , , and inside the ferrite phase as shown in Figures 2 c and 2 f.
Longer ageing treatment leads to the increase and coarsening of the phase in an irregular shape. Phase Volume Fraction by Light Optical Microscopy Microanalysis reveals that the solution-annealed material consists of ferrite and austenite phases. The main microstructural change during ageing is the formation of -phase and secondary austenite from the ferrite phase due to the eutectoid reaction.
The amounts of phases were estimated by measuring the fractions of colored area on polished and etched specimens by light optical microscopy LOM. The measured volume fractions of ferrite, austenite, and sigma phases for different ageing times are given in Table 2 and Figure 3. Since the ferrite transforms into and , the ferrite phase decreases as the volume percentages of sigma and secondary austenite increase with ageing time until the whole ferrite is totally consumed.
Ferrite continuously decreases throughout each of the isothermal holds and reaches zero at the end of the min. Table 2: The phase volume percentages obtained from digital image analysis. During the early stages of the transformation, sigma forms preferentially at the and grain boundaries and inside the -grain and grows into the ferrite phase via a transformation mechanism involving diffusion. Because both nucleation and diffusional growth of are thermally activated processes, the ageing time and temperature will have a significant effect on the kinetics of the transformation.
Weight Loss Test Results The degree of sensitization is also given by the loss of weight due to the dissolution of chromium-depleted areas and is expressed as the rate of weight loss in mdd. The results of standard weight loss immersion test are given in Table 3 and plotted in Figure 4.
Moreover, the lower chromium and molybdenum content is not the only factor responsible for the increase in the rate of corrosion. The neighborhood of more noble phases sigma phase will enhance the anodic dissolution of secondary austenite extensively.
Figure 4: The change in weight loss with ageing time. The surfaces of the corroded specimens were examined using LOM. The localized attack in black points is initiated in the secondary austenite phase adjacent to the sigma phase as shown in Figure 5.
The austenite and the sigma phase resist the ferric chloride solution more than the ferrite and the secondary austenite. Once a pit is formed it rapidly propagates within the initial ferrite region. As a result, it is observed that the sigma and austenite phases are almost intact, while the secondary austenite and some iron rich ferrite phases are attacked. Figure 5: Dissolution of the secondary austenite in weight loss test black: attacked region in the secondary austenite phase adjacent to the sigma phase, gray: austenite, and white: sigma.
EDS Analysis of Phases The precipitation of the intermetallic sigma phase is accomplished by the formation of secondary austenite. As the sigma phase grows, chromium and molybdenum are enriched in these precipitates, and, simultaneously, nickel diffuses into the ferrite.
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