Abstract

Influence of the microstructure on the degree of sensitization of a duplex stainless steel UNS S31803 aged at 650ºC

Evelin Barbosa de Mélo^I,* * e-mail: evy.barbosa@gmail.com ; Rodrigo Magnabosco^II; Carlos de Moura Neto^I

^IDepartment of Mechanical Engineering, Technological Institute of Aeronautics - ITA, Praça Marechal Eduardo Gomes, 50, CEP 12228-900, São José dos Campos, SP, Brazil

^IIDepartment of Materials Engineering, Ignatian Educational Foundation - FEI, Av. Humberto A. C. Branco, 3972, office K5-09, CEP 09850-901, São Bernardo do Campo, SP, Brazil

ABSTRACT

This work evaluates the phase transformations during aging of an UNS S31803 DSS at 650ºC and its influence on the DOS. The material was solution treated at 1175ºC and then aged at 650ºC for times up to 360 h. SEM-BSC images indicate the formation of Cr₂N, σ and χ phases in the samples aged at 650ºC. 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 650ºC. Probably, this increase observed in DOS values is mainly related to the presence of Cr- and/or Mo-depleted α, as a result of σ phase, χ phase and/or Cr₂N formation at 650ºC. Moreover, a possible healing up of Cr- and/ or Mo-depleted areas is observed between 1 h and 4 h of aging at 650ºC.

Keywords: duplex stainless steel, DL-EPR, degree of sensitization, sigma phase

1. Introduction

Duplex stainless steels (DSS) are those with a microstructure composed by two phases, ferrite (α) and austenite (γ), which appear in approximately equal volume fractions¹. Their favourable combination of properties makes this class of steels widely employed in oil and gas, petrochemical and pulp and paper industries².

Between -50ºC and 250ºC, a great combination of mechanical properties and corrosion resistance is offered by DSS. However, a large variety of secondary undesirable phases may form in the temperature range of 300ºC and 1000ºC, during isothermal aging or as a result of incorrect heat treatment procedures¹.

Between 300ºC and 1000ºC the most important secondary phases which can precipitate are alpha prime phase (α') and sigma (σ) phase. Alpha prime phase is chromium enriched and, depending on ferrite composition, its formation may happen by nucleation and growth of precipitates or by spinodal decomposition³. Moreover, α' phase formation is expected in temperatures below 500ºC¹, being mostly studied at 475ºC. Sigma phase is chromium and molybdenum enriched if compared to the matrix alloy⁴, and it is mainly formed between 650ºC and 900ºC⁵.

Results presented by Borba and Magnabosco⁶ reinforce the possibility of σ phase formation in temperatures below 700ºC, as it is observed the formation of this phase during aging of an UNS S31803 DSS at 600ºC and 650ºC, highly evidenced after 96 h of aging. Arikan and Doruk⁷, also working with an UNS S31803 DSS, observe the formation of σ phase from 1000 min (~16 h) of aging at 650ºC on, with an increase of σ phase amount with aging time.

Between 650ºC and 900ºC σ phase may form from ferrite phase by an eutectoid decomposition mechanism leading to secondary austenite (γ₂) formation (α→σ+γ₂)^1,7,8. However, at 850ºC, some authors^9,10 found that σ phase may form by different mechanisms depending on aging time. So, at 850ºC, for aging times up to 10 min, σ phase may form from direct precipitation from ferrite phase, resulting in Cr and Mo depleted α, also denominated secondary ferrite (α₂) phase (α→σ+α₂). Moreover, between 30 min and 5 h of aging at 850ºC the eutectoid decomposition of α, resulting in γ₂ (also depleted in Cr and Mo) is observed. Also, after total α phase consumption at 850ºC, σ phase fractions continue to rise due to γ phase consumption for its formation^9,10.

Besides σ phase formation, chi (χ) phase and chromium nitride (Cr₂N) formation may also result from isothermal heat treatments of DSS between 700ºC and 900ºC¹. Usually, Cr₂N precipitation occurs intergranularly at α/α grain boundaries or at α/γ interfaces^1,11, together to the formation of γ₂, causing chromium depleted zones¹¹. Chi phase formation generally occurs before σ phase formation and is gradually consumed by σ phase precipitation^11-13. As χ phase is enriched in Cr and Mo as well as σ phase (but with larger amounts of Mo than sigma phase^11-14), it is expected to promote depletion of the surrounding matrix in such elements¹¹. Besides some authors claim that χ phase formation occurs at α/γ interfaces¹³, other authors indicate that its formation starts at α/α grain boundaries¹².

The formation of Cr and Mo depleted phases such as α₂ and/or γ₂ have a strong influence in localized corrosion resistance and such influence can be measured by the degree of sensitization (DOS) through Double-Loop Electrochemical Potentiokinetic Reactivation (DL-EPR) tests.

Although DL-EPR tests were at first developed in order to evaluate the DOS of austenitic stainless steels¹⁵, recent studies use DL-EPR tests to evaluate the influence of isothermal aging on DSS and, consequently, of secondary phases precipitation, in the DOS of these steels^7,16-23.

As far as corrosion resistance is concerned, σ phase formation promotes an increase in the DOS^{7,16-18,20-23} probably as a result of etching to adjacent areas of formed σ phase, as these regions are likely to be depleted in chromium and/ or molybdenum. Some authors, working with UNS S31803 DSS aged at 650ºC, show that, after DL-EPR tests in a 2 M H₂SO₄ + 0.5 M NaCl + 0.01 M KSCN solution, the DOS increases from 0.036236 (samples aged for 16 h) to 0.824235 (samples aged for 527 h)⁷, while others show that, after DL-EPR tests in a 0.3 M HCl + 0.002 M Na₂S₂O₃ solution, the DOS increases from 0.075 (samples aged for 60 min) to 0.2574 (samples aged for 300 min)²².

Recent studies related to isothermal heat treatments of DSS at 650ºC are mainly based upon σ phase formation^6,7; however, the possibility of χ phase formation and even of Cr₂N formation may not be neglected, as there is lack of information about such phases as far as heat treatments below 700ºC are concerned.

Moreover, although a great number of recent works relate DSS phase transformations due to aging between 650ºC and 900ºC^{7,16-20,22-23} and DOS evolution, some present an incomplete microstructural analysis, mainly concerning the Cr and Mo depleted phases and/or areas, and fail to correlate DOS values and even DL-EPR curves features with the obtained microstructures.

Therefore, the aim of this work is to study phase transformations and its influence on the DOS of an UNS S31803 DSS aged at 650ºC.

2. Experimental Procedures

The studied material, UNS S31803 DSS, has the chemical composition given in Table 1, and was received as a 3 mm thick sheet, rolled at 1100ºC and cooled by forced water jet and air blast.

Solution heat treatment was conducted in the as received material for 30 min at 1175ºC, followed by water quenching. Then the samples were aged at 650ºC for 20 min, 1 h, 4 h or 360 h in a tubular electric furnace which maintained aging temperature 2ºC around the chosen temperature. All heat treatments were conducted in a 99.9% N₂ atmosphere to suppress oxidation of the samples surfaces. Following the heat treatments samples were water quenched.

After the heat treatments the specimens were ground using silicon carbide (SiC) papers to an 80-grit finish before mounting in thermosetting plastic, leaving an exposed surface area of approximately 0.4-0.6 cm², parallel to the rolling direction. The mounted samples were metallographic polished in a semi-automatic grinding and polishing machine, with final polishing provided by 1-µm diamond abrasive. Immediately after polishing, samples surfaces were washed with ethyl alcohol (C₂H₅OH) and dried with hot blown air.

X-Ray diffraction, using a Cu-Kα radiation, helped in phase identification. The scan was performed in the 2θ angular range of 30-120º with a scan speed of 1º/min and a step size of 0.04º.

Scanning electron microscopy (SEM), through backscattered (BSC) electron images, were also used for phase determination. BSC electron images were analysed considering that χ phase appears lighter than σ phase due to its higher molybdenum content^11-14 and that Cr₂N appear darker due to its nitrogen content²⁴.

Quantitative metallography of σ and χ phases (showed as a sum and denominated as intermetallic phases) was performed in six fields of each sample with an automated image analysis system attached to an optical microscope, using SEM-BSC images of polished samples. The evaluation of ferrite phase percentage was obtained by magnetic measurements using a ferritscope (FISCHER MP30) calibrated with standard samples. In each sample 20 measurements were made.

Taking into consideration only the presence of α, γ and intermetallic phases, γ content can be calculated using Equation 1:

DL-EPR tests were carried out in a 1 M H₂SO₄ + 0.25 M NaCl + 0.01 M KSCN solution, exposed to air, at a controlled temperature of 30ºC ± 2ºC, using a jacketed glass cell as test cell. The test temperature was controlled using a thermostatic circulating water bath attached to the test cell. The test cell had a saturated calomel electrode (SCE) as a reference electrode, a platinum wire as counter electrode and, as working electrode, samples polished until 1- m diamond abrasive finish were used.

Samples were immersed in the test solution for 5 minutes to ensure both the stabilization of the open circuit potential (OCP) and the testing temperature of the cell, and then the anodic polarization was conducted from the OCP, at a scan rate of 1.67 mV/s. The scan direction was reversed at the potential of +300 mV_SCE, and the tests were finished at the OCP previously measured. After DL-EPR tests, samples surfaces were washed with distilled water and dried with hot blown air to be examined by optical microscopy. The electrochemical tests were repeated three times for each heat-treatment condition.

The DOS was determined by the ratio between the maximum anodic current density during the reactivation (i_r) and the maximum anodic current density during the activation (i_a), as described by Equation 2:

3. Results and Discussion

Figure 1 presents the equilibrium phase content as a function of temperature, calculated using Thermo-Calc^® software and TC-Fe6 database. At 1175ºC (solution temperature) the equilibrium consists of α and γ phases with approximate volume fractions of 62% and 38% respectively. Ferrite phase volume fraction obtained experimentally (57.3 ± 2.3%) is also plotted in Figure 1 and it is almost identical to the calculated equilibrium value (62%), considering the standard deviation of the experimental measurements. This may indicate that the solution treatment performed at 1175ºC for 30 min is enough to reach equilibrium between α and γ phases and to develop a duplex microstructure. Still in Figure 1, at 650ºC, the calculated equilibrium phases are α, γ, σ and Cr₂N, with approximate volume fractions of 12%, 54%, 32% and 2%, respectively.

X-ray diffraction patterns for the UNS S31803 DSS studied are presented in Figure 2. For the solution treated sample (Figure 2a) and for the sample aged 4 h (Figure 2b) at 650ºC the X-ray diffraction patterns indicate only α and γ phases, while the X-ray diffraction pattern for the sample aged 360 h at 650ºC (Figure 2c) indicates Cr₂N, σ and χ phases, besides α and γ phases. However, as all Cr₂N and χ phase peaks appear combined with other phases peaks, this technique cannot be used to clear determine the formation of such phases.

In order to confirm the phases formed in the UNS S31803 DSS studied, SEM-BSC images were obtained (Figure 3). Solution treated sample (Figure 3a) indicates only α and γ phases. For samples aged 1 h (Figure 3b) and 4 h at 650ºC (Figure 3c), besides α and γ phases, Cr₂N (darker regions, due to nitrogen content²⁴) formation mainly occur at α/α phase boundaries. For the sample aged 360 h at 650ºC (Figure 3d), besides α and γ phases, it is also observed the presence of σ phase (light grey, due to chromium and molybdenum contents), χ phase (white, as it is enriched in molybdenum if compared to sigma phase^11-14) and Cr₂N (darker points, due to nitrogen content²⁴). Therefore, BSC images confirm the evidences of Cr₂N and χ phase formation previously pointed out by X-ray diffraction patterns (Figure 2c).

Comparing the microstructural observations of the present work with those of Arikan and Doruk⁷ it seems that, besides these authors make a brief comment on the possibility of χ phase or Cr₂N formation after aging an UNS S31803 DSS, they claim it was not possible to observe χ or Cr₂N in their optical micrographs. However, Arikan and Doruk⁷ would have probably observed such phases if they had not performed SEM analysis in their samples after electrolytic etching with oxalic acid, as they would have prevented the Cr enriched phases to be etched away before SEM analysis.

It is important to point out that the absence of Cr₂N peaks in the diffraction pattern of the sample aged 4 h at 650ºC (Figure 2b) may be related to the small amounts of Cr₂N observed in the BSC images (Figure 3c). So, considering the small amounts of Cr₂N, their volume fraction was neglected in phase quantification, and σ and χ phases will be designated as intermetallic phases, since they are both Cr- and Mo-rich phases, probably associated to areas depleted in those elements.

Figure 4 shows the volume fractions of phases obtained by quantitative metallography of BSC images. For samples aged for times up to 4 h at 650ºC an increase in γ volume fraction as well as a decrease in α volume fraction is observed (Figure 4), but only small amounts of Cr₂N are observed mainly at α/α phase boundaries (Figure 3). Intermetallic phase formation between 4 h and 360 h of aging at 650ºC leads to a great drop in α phase volume fraction combined with an increase in γ volume fraction (Figure 4). The increase in γ volume fraction observed for samples aged up to 4 h at 650ºC may be related to γ₂ formation in cooperation with Cr₂N, while between 4 h and 360 h of aging the increase observed in γ fraction is probably related to γ₂ formation as a result of Cr₂N, σ and χ phases formation. Moreover, the reduction observed in α phase fractions (Figure 4) indicates that the intermetallic phases and Cr₂N are probably formed by the consumption of α phase.

A comparison between the experimental volume fractions obtained after 360 h of aging at 650ºC (Figure 4) and the equilibrium volume fractions (Figure 1) shows that the remaining volume fraction of α phase (17.4 ± 1.0%) is higher than expected (12%) while the volume fraction of formed intermetallic phases (24.3 ± 1.1%) is lower than the value predicted for σ phase at the equilibrium (32%). Moreover, after 360 h of aging at 650ºC a slightly higher volume fraction value of γ phase (58.3 ± 1.5) is observed than the expected at equilibrium (54%). So, it is important to highlight that as the equilibrium phases expected at 650ºC are α, γ, σ and Cr₂N (Figure 1), the differences found in the comparison between experimental and calculated volume fractions of phases, as well as the presence of χ phase (Figure 3), indicate that aging at 650ºC for time periods up to 360 h is not enough to establish the thermodynamic equilibrium in the UNS S31803 DSS studied.

DL-EPR tests in 1 M H₂SO₄ + 0.25 M NaCl + 0.01 M KSCN, for the solution treated sample and for samples aged at 650ºC, produced the typical curves presented in Figure 5.

It can be observed in Figure 5 that there is an evident increase in i_a and i_r for the sample aged 360 h at 650ºC. This increase may be related to the development of chromium and molybdenum depleted areas as a consequence of intermetallic phase formation.

Another important observation is the existence of a second current density peak during reactivation for the sample aged 360 h at 650ºC. Magnabosco, Santos and Mélo²⁵ show that the potentiodynamic polarization curves, in acid solution, for a solution treated sample of an UNS S31803 duplex stainless steel present two distinct anodic current density peaks, indicating that the one with a lower potential corresponds to α phase and the other with a higher potential represents γ phase. Moreover, it is expected that a multiphase material presents an anodic polarization curve which represents a sum of the individual curves of existing phases²⁶. So, the appearance of a second peak may be related to the existence of α and γ phases, both with distinct electrochemical behaviours. Also, a high Cr and Mo depletion in the original α matrix in the sample aged 360 h at 650ºC could explain why the α phase peak (lower potential) is highly evidenced in this sample, indicating the possibility of α₂ formation.

However, it is important to point out that, as there is only one well defined peak in the activation portion of the curve, even though a second peak appears during reactivation, only the reactivation peak with higher potential was used to calculate the degree of sensitization for the sample aged for 360 h at 650ºC in order to properly compare the values of i_r and i_a.

Optical micrographs of samples surfaces after DL-EPR tests, for the solution treated sample and for the samples aged at 650ºC, are presented in Figure 6. A more intense etching at α/α grain boundaries, α γ interfaces and inside α grains is observed when the solution treated sample and the samples aged at 650ºC are compared (Figure 6).

While the etch at α/α grain boundaries is enhanced from 1 h of aging at 650ºC on (Figure 6c), at α/γ interfaces it is better observed from 4 h of aging at 650ºC on (Figure 6d). Also, some areas inside α grains are slightly etched after 1 h of aging at 650ºC and this etching becomes more intense with aging time, leading to a great material removal inside α grains in the sample aged 360 h at 650ºC.

A comparison between SEM-BSC images (Figure 3) and samples surfaces after DL-EPR tests (Figure 6) shows that etching provided by DL-EPR tests promotes material removal at intermetallic phase adjacent areas. Therefore, samples surfaces behaviour after DL-EPR tests are absolutely consistent with the microstructural observations as the material removal is probably taking place at chromium and/or molybdenum depleted areas adjacent to intermetallic phase formation. It is important to point out that γ phase remains with no signs of etching after DL-EPR tests, reinforcing that the secondary phases formation observed are mainly related to phase transformations taking place at α phase.

As the analysis of the volumetric fraction of phases (Figure 4) indicates not only an increase in γ phase fractions but also a remaining α phase fraction (even after aging 360 h at 650ºC), the chromium and/or molybdenum depleted areas may be related to α₂ and/or γ₂ phases formation as a result of intermetallic phase formation. This statement is valid as it is not possible to separate the contributions of the original α matrix and a possible chromium and/or molybdenum α phase coexistence in the analysed samples.

Moreover, as the main precipitation observed for samples aged for times up to 4 h at 650ºC is related to Cr₂N formation, probably, for such samples, the etched areas are depleted in chromium, although the slightly etched areas observed inside ferrite grains may indicate an early stage of some secondary phase formation, presenting particles yet too small to be distinguished by SEM-BSC images. However, when the sample aged 360 h at 650ºC is analysed, the etched areas may be depleted in chromium and/or molybdenum as the intermetallic phases formed are chromium and molybdenum enriched.

DOS evolution as a function of aging time at 650ºC is compared to the solution treated sample behaviour in Figure 7. It can be observed in Figure 7a that aged samples until 4 h at 650ºC present DOS values a little higher than that of the solution treated sample, but values are still very low. However, for the sample aged for 360 h at 650ºC it is observed a great increase in DOS values when compared to the solution treated sample or even other samples aged at 650ºC (Figure 7a).

In addition, a better observation of DOS values of samples aged up to 4 h at 650ºC is presented in Figure 7b. Besides the small increase in DOS values until 4 h of aging at 650ºC, when compared to the solution treated sample, apparently between 1 h and 4 h of aging, there is a slight reduction tendency in DOS values, already observed by other authors^20,23,27. This reduction tendency in DOS values may indicate healing up of chromium/ molybdenum depleted areas due to Cr and Mo redistribution between the surrounding matrix and depleted areas, as stated elsewhere²⁷. This Cr and Mo redistribution is more likely to happen between the surrounding matrix and depleted areas than between intermetallic phases and depleted areas as intermetallic phases do not behaviour as solid solutions when it comes to alloying elements redistribution.

In addition, the great increase observed for DOS values for the sample aged 360 h at 650ºC is consistent with the observations of higher intermetallic phase fractions values for this sample, showed in Figure 4, as higher intermetallic phase fractions may indicate the presence of more areas subjected to Cr and Mo depletion. Moreover, taking into consideration that the γ phase is not etched during DL-EPR tests and the fact that the highly evidenced α phase peak during reactivation (in the sample aged 360 h at 650ºC) is probably due to α₂ phase formation, the main Cr-/Mo-depleted phase responsible for the increase in DOS values seems to be α₂.

Thus, this small increase in DOS values until 4 h of aging, besides the slight decrease observed between 1 h and 4 h of aging, as well as the great increase for sample aged for 360 h at 650ºC point out that intermetallic phase formation at 650ºC leads to the formation of chromium and/or molybdenum depleted regions, which are considerably deleterious to corrosion resistance of UNS S31803 duplex stainless steel studied in this work.

4. Conclusions

Acknowledgements

The authors would like to thank the Brazilian National Council of Scientific and Technological Development - CNPq (Process nº 141097/2011-6) for the given financial support.

Received: February 22, 2013

Revised: June 5, 2013

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