Effects of Solution-Annealing Temperature on the Precipitationof Secondary Phases and the Associated Pitting Corrosion Resistancein Hyper Duplex Stainless Steel

Soon-Hyeok Jeon, Soon-Tae Kim, Se-Young Kim, Min-Seok Choi and Yong-Soo Park+

Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea

The effects of the solution-annealing temperature on the precipitation of secondary phases and the associated pitting corrosion resistance inhyper duplex stainless steel after aging at 1123K were investigated in a highly concentrated chloride solution. Increasing the solution annealingtemperature from 1333 to 1413K retarded the formation of the secondary phases owing to a decrease in both the number of preferentialprecipitation sites such as the ferrite/austenite interface and the activities of Cr, Mo and W. The slow rate of degradation of the pitting corrosionresistance of the alloy with the increase in the solution annealing temperature appears to be caused by the retardation of the precipitation of thesigma phase. Based on the results of the precipitation of secondary phases and electrochemical tests, the optimal solution annealing temperaturewas determined to be 1373K. [doi:10.2320/matertrans.M2012402]

(Received December 7, 2012; Accepted May 20, 2013; Published June 28, 2013)

Keywords: duplex stainless steel, pitting corrosion, solution heat treatment, secondary phase

1. Introduction

Duplex stainless steels (DSSs) have been increasingly usedin the oil, chemical, petrochemical, nuclear and marineindustries owing to their excellent combination of mechanicaland corrosion properties and relatively low cost, resultingfrom the addition of low amounts of Ni as compared toaustenite stainless steels (ASSs).1,2)

Standard stainless steel such as UNS S32205 are definedas DSSs with a pitting resistance equivalent number(PREN = [mass% Cr] + 3.3 ([mass% Mo] + 0.5 [mass%W]) + 16 [mass% N]3,4)) of 35 and super duplex stainlesssteels (SDSSs) such as UNS S32750 are defined as DSSswith a PREN of 40­45. However, in a heat exchangerapplication, the pitting corrosion resistance of SDSSs with aPREN value above 40 is insufficient for higher-temperatureservice or for a long service life, and thus materials with evenhigher corrosion resistance are required. Hyper duplexstainless steels (HDSSs) such as UNS S32707 are definedas highly alloyed duplex stainless steels with a PREN inexcess of 45. Hence, HDSSs with PREN values above 45have been developed to provide high resistance to pittingcorrosion, combined with improvements in mechanicalproperties.

DSSs are stainless steels having a microstructure in whichboth ferrite (¡) and austenite (£) phases are present inapproximately equal volume fraction. For DSSs, the tendencyfor secondary phase precipitation is crucial, because thepresence of an ¡ phase in them will enhance the kinetics forprecipitation of secondary phases such as sigma (·) and chi(») phases. HDSSs contain a significant amount of Cr, Moand W which improves corrosion resistance. However, thesealloying elements facilitate precipitation of secondary phasessuch as the · and » phases. These phases are intermetallicphases that form at high temperature, 873­1223K.5,6) Theylower the fracture toughness and the pitting corrosion

resistance of the steel significantly.7­9) With the increaseduse of Cr, Mo and Was alloying elements in HDSSs, limitingthe precipitation of secondary phases has become moreimportant.

In HDSSs, the · phase preferentially precipitates into the¡ phase owing to the higher Cr and Mo concentration inthe ¡ phase which is thermodynamically metastable atthe temperature at which the · phase precipitates.10,11) Inaddition, the diffusion rates of the alloying elements in the¡ phase are 100 times faster than the corresponding valuesin the £ phase.12,13)

Because the precipitation kinetics of the · phase in HDSSsis very sensitive to the amounts of Cr, Mo and W, appropriatemethods are needed in order to prevent both the formation ofa · phase and changes to the chemical composition of thesteel. Changes in chemical composition affect other impor-tant properties such as pitting corrosion resistance, which isbased on phase balance, and mechanical properties. There-fore, at a given chemical composition, the prevention ofsecondary phases is generally achieved by applying appro-priate solution-annealing temperatures and cooling rates.

The aim of this study is to elucidate the effects of solution-annealing temperature on the precipitation of secondaryphases and the associated pitting corrosion resistance, thusestablishing the optimal solution-annealing temperature atwhich HDSSs exhibit the best pitting corrosion resistance andretard the precipitation of secondary phases. This informationwould facilitate further improvement of HDSSs.

2. Experimental Procedures

2.1 Material and heat treatmentIngots weighing 50 kg with dimensions 150 by 150 by

300mm (width by length by height) were manufacturedusing a high frequency vacuum induction furnace. After theseingots were hot rolled at 1523K, plates of 6mm thicknesswere manufactured. The specimens were cut into dimensionsof 15 by 15 by 6mm (width by length by thickness) and+Corresponding author, E-mail: yongsoop@yonsei.ac.kr

Materials Transactions, Vol. 54, No. 8 (2013) pp. 1473 to 1479©2013 The Japan Institute of Metals and Materials

solution heat-treated for 30min at following temperatures:1333K, 1353K, 1373K, 1393K, 1413K and then quenchedin water. The specimens were then isothermally aged at1123K for 10min. Chemical analysis was performed byARL 3460 optical emission spectrometer (OES). N concen-tration was analyzed using a LECO N/O (TC-300) analyzer.Chemical composition of the HDSS is presented in Table 1.

2.2 Corrosion testPotentiodynamic anodic polarization test was carried out

using an EG&G PAR 263A potentiostat model. The pittingpotential (Ep) was obtained from the potentiodynamicpolarization curves. The pitting potential marked the end ofthe passive potential region and the transition from passiveto transpassive behavior. The potentiodynamic anodic polar-ization test was conducted in a deaerated 4M NaCl at 343Kaccording to the ASTM G 5.14) Test specimens were joinedwith copper wire through soldering (95mass% Sn­5mass%Sb), and then mounted with an epoxy resin. One side of thesample was ground to 600 grit using SiC abrasion paper.After defining the exposed area of the test specimen as0.5 © 10¹4m2, the remainder was painted with a transparentlacquer. The test was conducted at a potential range of ¹0.65to +1.1V vs. SCE (saturated calomel electrode) and at ascanning rate of 1 © 10¹3 V s¹1, using a SCE.

2.3 Micro-structural characterizationTo observe the microstructures of the alloy, they were

ground to 2000 grit using SiC abrasive papers, polished withdiamond paste. The samples were then etched with a 10%potassium hydroxide and 10% oxalic acid solutions. Thesecondary phases were observed using a JEOL JSM 700scanning electron microscope (SEM) in a JEOL JSM 840Aback scattered electrons (BSE). In addition, the chemicalcompositions of the secondary phases were analyzed by anOXFORD instruments INCA X-art (51-ADD0069) energydispersive X-ray spectroscope (EDS) attached to a SEM. Thecontents of Cr, Mo and W in ¡ and £ phases were analyzedusing a SEM-EDS. The nitrogen content was analyzed usinga scanning Auger multi-probe (SAM).

2.4 Thermodynamic equilibrium calculationTo predict the effects of solution annealing temperature

on the chemical composition of ¡ and £ phases and activitiesof Cr, Mo and W of ¡ phase in the HDSS, thermodynamiccalculations using Thermo-Calc software were conducted.It should be stressed that such calculations give theequilibrium state of system with the TCFE5-TCS Steels/Fe-alloys database.15) The POLY and POST modules inThermo-Calc software were used to perform the calculation.The POLY module can calculate various complex heteroge-neous equilibrium states. And the POST module makes itpossible to plot many kinds of phase diagrams and propertydiagrams.16)

3. Results and Discussion

3.1 Microstructural analysisFigure 1 shows optical micrographs of the HDSS samples

after solution heat treatment for 30min at 1333, 1373 and1413K. The £ phase can be seen as an isolated phase on thebackground of the ¡ phase, which looks relatively dark. Nosecondary phases were observed in either alloy. As a resultof hot rolling, the specimen has an elongated texture parallelto the rolling direction. It is observed that the grain size of the¡ phase increased significantly with increasing solution-annealing temperature as compared to the grain size of the£ phase. In other words, increasing the solution-annealingtemperature increased the grain size of the ¡ phase anddecreased the number of ¡ phase grains.

The area fraction of the ¡ and £ phases as measuredby image analyzer was plotted against solution-annealingtemperature and the results are shown in Fig. 2. Increasingthe solution-annealing temperature from 1333 to 1413Kresulted in a gradual increase of the area fraction of the ¡

phase from 46 to 57%.Figure 3 shows BSE images of the microstructure of the

HDSS samples after solution heat treatment at differenttemperatures for 30min and aging at 1123K for 10min.The precipitates formed continuous networks along the grainboundaries; they also randomly appeared within the grains.

Table 1 Chemical composition of the HDSS (mass%).

Cr Ni Mo W Si Mn S N Fe PREN*

27.29 7.06 2.58 3.39 0.22 1.46 0.0037 0.33 Bal. 51.3

*PREN (Pitting Resistance Equivalent Number) = mass% Cr + 3.3 © (mass% Mo + 0.5 © mass% W) + 30 © mass% N

αα

γ

200 μm

α

γ

200 μm

α

γ

200 μm

(a) (b) (c)

Fig. 1 Optical micrographs of the HDSS samples after solution heat treatment at (a) 1333K, (b) 1373K and (c) 1413K for 30min.

S.-H. Jeon, S.-T. Kim, S.-Y. Kim, M.-S. Choi and Y.-S. Park1474

These precipitates were identified as · and » phases by SEM-EDS analyses. The chemical compositions of the · and »

phases were also analyzed using SEM-EDS and the resultsare presented in Table 2. In all the specimens, the · phasesgrew extensively along the ¡/£ phase boundaries and withinthe grains of the ¡ phase whereas the » phase precipitatedmainly along the ¡/£ phase boundaries. Increasing thesolution-annealing temperature from 1333 to 1413K resultedin a gradual decrease of · phase precipitation.

Figure 4 shows the effects of solution-annealing temper-ature on the area fraction of the » and · phases in the HDSSsamples as determined by image analyzer. An increase in thesolution-annealing temperature did not affect the area fractionof the » phase. Dobranszky et al.17) reported that a smallamount of » phase formed earlier than the · phase on the ¡

grain boundaries, but later the » phase transformed into ·

phase. However, as seen in Fig. 4, the higher the solution-annealing temperature with the same aging process, the lowerthe area fraction of the · phase. That means that theprecipitation of the secondary phases is retarded if thesolution-annealing temperature is increased.

3.2 Effect of solution annealing temperature on thepitting corrosion resistance

The · phase is a secondary phase enriched in Cr andMo.18,19) Due to its high Cr and Mo contents, theprecipitation of · phase depletes the surrounding regions ofCr and Mo, which deteriorates the corrosion resistance.20)

Figure 5 shows the effect of solution-annealing temper-ature on the pitting corrosion resistance in a deaerated 4MNaCl solution at 343K after solution heat treatment ofthe HDSS samples at different temperatures for 30min(Fig. 5(a)) and after aging at 1123K for 10min (Fig. 5(b))according to ASTM G 5. In general, the pitting potential(Ep) is defined as the breakdown potential destroying apassive film. As the Ep of an alloy increases, the resistanceto pitting corrosion of the alloy increases. The resistanceto pitting corrosion of the specimens is ranked in thefollowing order: 1333K ; 1353K ; 1373K > 1393K >1413K. When solution heat-treated in the range of 1333­1413K, the specimens that were solution annealed at1333, 1353 and 1373K showed the best pitting corrosionresistance. In Fig. 5(b), the resistance to pitting corrosion is

1333 1353 1373 1393 141335

40

45

50

55

60

65A

rea

frac

tio

n (

%)

Temperature, T / K

Ferrite Austenite

Fig. 2 Effect of solution-annealing temperature on the area fraction of the¡ and £ phases.

(a) (b) (c)

Fig. 3 BSE images after solution heat treatment at (a) 1333K, (b) 1373K and (c) 1413K for 30min and aging at 1123K for 10min.

Table 2 Chemical compositions of the secondary phases formed in HDSS samples after aging at 1123K (mass%).

Secondary phase1333K 1373K 1413K

Fe Cr Mo W Fe Cr Mo W Fe Cr Mo W

Chi (») 45.6 26.9 10.9 13.4 47.4 27.8 9.8 11.8 46.4 27.2 10.4 12.6

Sigma (·) 55.1 32.2 4.3 4.9 54.7 33.1 4.2 5.1 54.1 31.7 4.6 5.0

1333 1353 1373 1393 14130

2

4

6

8

10

12

14

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18

20

Temperature, T / K

Are

a fr

acti

on

(%

)

Chi phase Sigma phase

Fig. 4 Effect of solution annealing-temperature on the area fraction of» and · phases precipitated in the HDSS samples after solution heattreatment at different temperatures for 30min and aging at 1123K for10min.

Effects of Annealing Temperature on the Precipitation of Secondary Phases and Pitting Corrosion in Hyper Duplex Stainless Steel 1475

ranked in the following order: 1373K > 1353K > 1393K >1333K > 1413K.

With increasing aging time at 1123K, the value of Ep

decreased and thus the resistance to pitting corrosion of theHDSS samples decreased. The resistance to pitting corrosionof the specimen solution heat treated at 1373K was superiorto that of the specimen solution heat treated at 1353K due tothe retardation of secondary phase precipitation.

Figure 6 shows the SEM surface morphology of the HDSSsamples after the potentiodynamic polarization test indeaerated 4M NaCl solution at 343K, after solution heattreatment at 1333K for 30min and aging at 1123K for10min. The pits initiated around the · phase formed in the¡ phases because of the formation of the Cr-, Mo- and W-depleted zones. The · phase is a secondary phase enriched inCr, Mo and W; thus, the precipitation of the · phase depletesthe surrounding regions of Cr, Mo and W, which deterioratesthe pitting corrosion resistance. Park et al.20) reported thatpits were found near the ¡/· phase boundaries, suggestingthat the region around the · phase where Cr and Mo weredepleted was sensitive to the initiation of pitting.

To clarify the reason why pitting corrosion resistance isreduced with increasing the temperature of solution heattreatment, the contents of Cr, Mo and W in the £ and the ¡

phases in the HDSS were quantitatively measured usingSEM-EDS analysis and the N content was measured usinga SAM analysis. Then, thermodynamic calculations of thechemical compositions of ¡ and £ phases using Thermo-Calcsoftware were conducted. Figure 7 shows the chemicalcompositions of the ¡ and £ phases after solution heattreatment for 30min at different temperatures. The thermo-dynamic calculations of the chemical compositions of the ¡

and £ phases were in good agreement with the experimentalresults. As the amount of the ¡ phase increases and that of the£ phase decreases, Cr, Mo and W, which act as ¡ stabilizers,are diluted in the ¡ phase and enriched in the £ phase. Incontrast, N, a £ stabilizer, is restricted to a maximum of0.05mass% in the ¡ phase and is enriched in the £ phase.21) Nis nearly completely solutionized in the £ phase in DSSs, butit is rarely solutionized in the ¡ phase. Because the octahedralinterstice of the face-centered cubic (FCC) lattice are largerthan the octahedral interstices and tetragonal interstices ofthe body-centered cubic (BCC) lattice, the N atoms in the £

phase with FCC lattices occupy the octahedral interstitialsites, leading to an N-saturated £ phase as compared to the ¡phase with BCC lattices.

To determine the influence Cr, Mo, W and N have on thepitting corrosion, the PREN value was calculated by usingthe results of the SEM-EDS analysis for Cr, Mo and W andthe results of the SAM analysis for N in eq. (1):22­24)

PREN ¼ ½mass% Cr� þ 3:3 ð½mass% Mo�þ 0:5 ½mass% W�Þ þ 30 ½mass% N� ð1Þ

Some authors25,26) reported that an expression of the PRENvalue would allow establishing a correlation among thecorrosion resistance and the chemical composition of thealloy. Merello et al.27) suggested that the higher PREN valueleads to a higher pitting potential and the PREN value andpitting potential have a relation of an exponential function.Figure 8 shows the PREN values of the ¡ and £ phases as afunction of solution-annealing temperature. To predict theeffects of solution-annealing temperature on the PREN valuesof the ¡ and £ phases in the HDSS samples, thermodynamic

10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2-600

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1353 K

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1373 K1333 K

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Current Density, I / A · cm-2 Current Density, I / A · cm-2

Po

ten

tial

, E /

mV

vs.

SC

E

Po

ten

tial

, E /

mV

vs.

SC

E

(a) (b)

Fig. 5 Potentiodynamic curves in deaerated 4M NaCl solution at 343K after (a) solution heat treatment at different temperatures for 30min and (b) aging at1123K for 10min.

15 μm

γ phase

σ phase formed in the α phase

5 μm

Pit initiation around the σ phase due to the Cr, Mo and W depleted zone

Not pitted in the γ phase

Fig. 6 SEM surface morphology after potentiodynamic polarization test indeaerated 4M NaCl solution at 343K after solution heat treatment at1333K for 30min and aging at 1123K for 10min.

S.-H. Jeon, S.-T. Kim, S.-Y. Kim, M.-S. Choi and Y.-S. Park1476

calculations using Thermo-Calc software were conducted.As shown in Fig. 8, as the solution-annealing temperatureincreased, the PREN value of the ¡ phase graduallydecreased, whereas that of the £ phase gradually increased.The thermodynamic calculation of the PREN values of the ¡and £ phases was in good agreement with the experimentalresults. In DSSs, it is important to balance the corrosionresistance of the ¡ and £ phases to reduce the galvaniccorrosion effect caused by the difference in the corrosionresistance of the ¡ and £ phases.28) As the solution-annealingtemperature increased, the difference in the PREN values ofthe ¡ and £ phases increased within the solution-annealingtemperature range of 1333­1413K, and the higher the

solution-annealing temperature, the lower the pitting corro-sion resistance of the HDSS samples.

In order to investigate the effects of solution-annealingtemperature on the precipitation of secondary phases and theassociated pitting corrosion resistance of HDSSs, the pittingpotential of the HDSS after aging at 1123K for 10min wasdetermined from potentiodynamic anodic polarization curvesmeasured in 4M NaCl solution at 343K. The results arepresented in Fig. 9.

With increasing aging time at 1123K, the pitting potentialof the HDSS samples decreased significantly, graduallyderiving a · phase. Although the pitting potential of the

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1333 1353 1373 1393 14131333 1353 1373 1393 1413

1333 1353 1373 1393 14131333 1353 1373 1393 14131

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erce

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%)

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Ferrite Austenite Ferrite by Thermo-Calc Austenite by Thermo-Calc

(a) (b)

(c) (d)

Fig. 7 Effect of solution-annealing temperature on the chemical compositions of the ¡ and £ phases: (a) Cr, (b) Mo, (c) W and (d) N.

1333 1353 1373 1393 141340

44

48

52

56

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Temperature, T / K

Ferrite Austenite Ferrite by Thermo-Calc Austenite by Thermo-Calc

PR

EN

Fig. 8 Effect of solution-annealing temperature on the PREN values of the¡ and £ phases.

0 5 10-100

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Pit

tin

g P

ote

nti

al, E

/ m

V v

s. S

CE

Fig. 9 Effect of solution-annealing temperature and aging time on thepitting potentials as determined from potentiodynamic polarization test indeaerated 4M NaCl solution at 343K.

Effects of Annealing Temperature on the Precipitation of Secondary Phases and Pitting Corrosion in Hyper Duplex Stainless Steel 1477

specimen solution heat-treated at 1353K decreased afteraging at 1123K for 10min, the pitting potential of thespecimen solution heat-treatment at 1373K remained rela-tively high in spite of aging. The decrease in the pittingpotential of the alloys with aging is due to the precipitation ofthe · phase and the resultant depletion of Cr around the ·

phase. On the other hand, the slow rate of degradation of theresistance to pitting corrosion of the alloys with an increaseof solution-annealing temperature appears to be due to theretardation in the precipitation rate of the · phase. Basedon the results of the precipitation of secondary phases andelectrochemical test, the optimal solution-annealing temper-ature was determined to be 1373K.

3.3 Mechanism of the effects of solution annealingtemperature on the precipitation of the secondaryphases

Thermodynamic studies of the precipitation of intermetal-lic compounds such as · and » phases and Cr2N throughthe addition of various elements have been investigated.Pettersson29) demonstrated that the effects of alloyingelements on the precipitation of secondary phases couldbe explained by the elemental activities. Retardation ofsecondary phase precipitation is explicable by the reductionof activity of Cr and Mo with increasing N content. Theeffect of N on the activity of Cr and Mo was suggested to bethe determining factor for the precipitation of the · and »

phases.30) Cu addition retards the precipitation of the · phasedue to the decrease of activity of Mo in HDSS.31) However,Cu addition to the alloy facilitates the precipitation ofdeleterious Cr2N, and increases the high-temperature limitof precipitation of Cr2N by increasing the activity of Cr.It also reduces the pitting corrosion resistance owing to theprecipitation of Cr2N in HDSS.32)

Since Cr, Mo and W in the ¡ phase are the key elements inthe formation of secondary phases such as · and » phases, thedecrease of the activity of Cr, Mo and W in the ¡ phase hasan effect on the retardation of the precipitation of secondaryphases. The effect of the solution-annealing temperature onthe activity of Cr, Mo and W of the ¡ phase in the HDSSsamples is shown in Fig. 10. As the solution-annealing

temperature increased, the area fraction of the ¡ phaseincreased, which decreased the concentration of Cr, Mo andW in the ¡ phase somewhat. This decrease in concentrationin turn reduced the activity of Cr, Mo and W, thus retardingthe precipitation of secondary phases.

The effect of the solution-annealing temperature on theprecipitation of secondary phases of the HDSS is schemati-cally presented in Fig. 11. Increasing the solution-annealingtemperature from 1333 to 1413K retarded the formation ofsecondary phases because it promoted the growth of the ¡

phase, in which the grain size of the ¡ phase increased andthe number of ¡ phase grains decreased. In general, the ·

phase forms more easily in high-energy regions, such as atthe ¡/£ interfaces, through a heterogeneous nucleationprocess.33) Thus, increasing the solution-annealing temper-ature increased the ¡ grain size and thus reduced the numberof preferential precipitation sites for the · phase such as the¡/£ interfaces.

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

Cr

W

Mo

Act

ivit

y

Temperature, T / K1333 1353 1373 1393 1413

Fig. 10 Effect of solution-annealing temperature on the activity of thealloying elements of the ¡ phase as calculated using Thermo-Calcsoftware.

α

Solution annealed at 1333 K

σ

α

Solution annealed at 1413 K

σσχγ

χγ

A few of α / γ phase boundaries:a few of nucleation sitesLarge volume fraction of α phase (57 %):Small activity of Cr, Mo and W in α phase

Small amount of secondary phases

A lot of α / γ phase boundaries: a lot of nucleation sitesSmall volume fraction of α phase (46 %):Large activity of Cr, Mo and W in α phase

Large amount of secondary phases

Fig. 11 Schematics of effect of solution-annealing temperature on the precipitation of secondary phases.

S.-H. Jeon, S.-T. Kim, S.-Y. Kim, M.-S. Choi and Y.-S. Park1478

4. Conclusions

(1) Increasing the solution annealing temperature from1333 to 1413K retarded the formation of secondary phasesdue to the decrease of preferential precipitation sites such asthe ¡/£ phase boundaries and the activity of Cr, Mo and Win the ¡ phase. The slow rate of degradation of the pittingcorrosion resistance of the alloy with an increase in thesolution-annealing temperature appears to be due to theretardation in precipitation of the · phase.

(2) Based on the results of the precipitation of secondaryphases and electrochemical tests, the optimal solutionannealing temperature was determined to be 1373K. Theoptimal solution annealing temperature was determinedbased on factors such as pitting corrosion resistance andretardation of secondary phase precipitation.

Acknowledgement

This work has been supported by Ministry of KnowledgeEconomy of the Republic of Korea.

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Effects of Annealing Temperature on the Precipitation of Secondary Phases and Pitting Corrosion in Hyper Duplex Stainless Steel 1479