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Surfaces and Interfaces

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Microstructural and electrochemical behavior of 2205 duplex stainless steelweldments

M. Atif Makhdooma, A. Ahmadb, M. Kamrana, K. Abidc, W. Haiderd,*a Department of Metallurgy and Materials Engineering, University of the Punjab, Quaid e Azam Campus, 54590, Lahore, Pakistanb Department of Metallurgical and Materials Engineering, University of Engineering and Technology, 54000, Lahore, Pakistanc Department of Electrical Engineering, University of the Punjab, Quaid e Azam Campus, 54590, Lahore, Pakistand School of Engineering and Technology, Central Michigan University, Mount Pleasant, MI 48859 USA

A R T I C L E I N F O

Keywords:Duplex stainless steel 2205Gas tungsten arc weldingShielded metal arc weldingPotentiodynamic Tafel scanCorrosion

A B S T R A C T

The paper describes the joining of rolled plates of 2205 Duplex stainless steel (DSS) by Gas tungsten arc welding(GTAW) and shielded metal arc welding (SMAW). The development of different phases upon welding withoutany post heat-treatment – especially in the heat affected zone (HAZ) – and their consequent different corrosionresistance were investigated through optical microscopy, scanning electron microscopy (SEM), X-ray diffraction(XRD) method and potentiodynamic Tafel scan in aqueous NaCl solution. Comparison of mechanical propertiesand fracture surface morphology of the welded joints were also made. GTAW weldment was found more ef-fective towards corrosion resistance due to the presence of relatively larger amount of secondary austenite (γ2)i.e. acicular austenite in HAZ – when compared with the HAZ of SMAW.

1. Introduction

Duplex stainless steel (DSS) is an important class of stainless steelbearing a dual microstructure of primary ferrite (α1) and primaryaustenite (γ1) in approximate equal volume fractions. These steelspossess resistance to corrosion, especially in chloride containing en-vironments due to the phase balance between γ1 and α1 [1]. Thewelding is the most widely accepted fabrication technique and it is alsoutilized for DSS. Rapid thermal cycles during welding unbalances the γ/α phase ratio from 1:1 in either fusion zone (FZ) or HAZ thereby af-fecting negatively chemical and mechanical properties of DSS [2,3].High heat input together with slow cooling rate in DSS preserves thedesired γ/α phase 1:1 balance in welding zone (WZ) and HAZ but it alsoproduces coarse grains and undesirable deleterious phases like chi (χ)and sigma (σ) as well [3–5]. On the other hand, low heat input togetherwith fast cooling rate in DSS makes α phase supersaturated with ni-trogen, which is already present in base metal (BM). The resultantmicrostructure is metastable and consists of coarse grains of α, sec-ondary austenite as allotriomorphic austenite, called grain boundaryaustenite (GBA) [6], Widmanstäten austenite (WA) and large quantityof detrimental intragranular chromium nitrides (Cr2N) [7]. This Cr2Nprecipitates form α by nucleation and growth mechanism and forms atdislocations, grain-boundaries and inclusions [6]. It is therefore notonly mandatory to maintain γ/α phase 1:1 balance in weldment but

also welding thermal cycles must be chosen in such a way that coolingshould not be too fast to promote undesirable coarse grains of α, γ2 asallotriomorphic γ as GBA, WA and Cr2N precipitates formation but alsonot too slow that coarse grains, χ and σ phases can form.

Corrosion kinetics [8,9] and morphological changes [10,11] inweldments have been recently investigated. Correlations of DSS weldedmicrostructures of HAZ and WZ with mechanical properties and re-sistance towards corrosion have also been extensively documented[12,13] but - to the best of our knowledge - a comparison of “as-welded” DSS microstructures i.e. without any post-heat treatment andtheir corresponding corrosion response is still not reported. This con-tribution reports the investigation on “as-welded” microstructures ofDSS using standard industrial GTAW and SMAW and their responsetowards corrosion under conditions similar to immersion in sea water.The microstructural changes upon GTAW and SMAW welding techni-ques and the emergent microstructural phases that are responsible ofdifferent mechanical properties and resistance towards corrosion aredescribed.

2. Experimental

2.1. Welding

Welding of 8mm thickness hot rolled plates of 2205 DSS was

http://dx.doi.org/10.1016/j.surfin.2017.09.007Received 10 December 2016; Received in revised form 16 July 2017; Accepted 14 September 2017

* Corresponding author.E-mail addresses: haide1w@cmich.edu, whaid001@fiu.edu (W. Haider).

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Available online 15 September 20172468-0230/ © 2017 Elsevier B.V. All rights reserved.

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carried out by using GTAW & SMAW welding processes using Argon(99.99% pure) as shielding gas at a flow rate of 12 l/min. Sound buttjoints were made after visual examination as per BS EN ISO 17637. Inboth techniques, filler metal ER2209 was used and weld was depositedwhile insuring full weld penetration, proper shape and contour.

2.2. Metallography

Optical and scanning electron microscopy of weld zone, heat af-fected zone and base metal (BM) of each welded plate was carried out.For this purpose, each sample was first macro etched to identify WZ,HAZ and BM according to ASTM E340-00. The specimens from the topsurface of each section were taken for the evaluation of microstructuralchanges. HCl and HNO3 solutions were used as etchant for optical mi-croscopy [2]. For SEM analysis specific electrolytic etching techniqueusing 40 vol.% HNO3 was used [14]. This technique is reported todevelop a step between γ1 and γ2 and hence aids in further character-ization. Qualitative XRD phase analyses were carried out using XPertPro PANalytical® XRD having Cu Kα radiations (Ni filtered) to in-vestigate the HAZ regions of specimens.

2.3. Electrochemical measurements

To investigate the corrosion behavior three different zones i.e. WZ,HAZ and BM were sectioned and samples were cold mounted by usingcopper wire soldered at the back and exposing area of interest on theother side. SiC paper (600 grit) was used to ground up the exposedsurface area of 1.6 cm2. Samples were rinsed with ethanol beforeelectrochemical measurement. Potentiodynamic Tafel scan was carriedout to investigate corrosion kinetic parameters of samples in simulatedsea water application i.e. 3.5% (w/v) NaCl solution at room tempera-ture. Electrochemical measurements were carried out using GAMRYۛPotentiostat version PC/750. The potentiodynamic current-potentialcurves were swept from −250 to 250mV vs open circuit potential(OCP) at a scan rate of 1mV/s. OCP was monitored within initial10 min. period before instigating the said activity.

2.4. Mechanical properties

Samples for mechanical tests were sectioned from the weldedsamples using wire cut electric discharge machine (EDM). Tensile andBend tests were performed according to ASME IX 451.1. Tensile testswere performed at a strain rate of 0.1s−1 using Shimadzu universaltesting machine, model: UMH-200A T.V having 200T capacity.Hardness across the weldments was measured using Shimadzu HMV-2micro-Vickers hardness tester. All the tests were performed at roomtemperature.

3. Results and discussion

3.1. Composition and welding parameters

Chemical compositions of base and filler metal as well as differentparameters for both GTAW and SMAW processes are reported inTables 1 and 2, respectively.

3.2. Metallography

Optical micrographs of the BM – see Fig. 1 – shows banded structurein the rolling direction is evident in Fig. 1 of base metal (BM). Thefigure shows island like austenite phase (γ) as white, surrounded bycontinuous matrix of ferrite (α) phase (dark region). Results using“Phase Analysis®” showed 46.2% and 53.8% of γ-α volume fraction,indicating a good balance between these two.

The rapid heating and cooling cycles during fusion welding processmake the microstructures of HAZ and WZ difficult to assess due to theirnon-equilibrium nature and hence equilibrium phase diagram does notrigorously apply. It is interesting to estimate the initiation of weldmicrostructure especially in case of duplex steel where γ and α havetransient phase stability in the temperature range from room to themelting point. Contrary to equilibrium phase diagram, slow diffusion of

Table 1Nominal chemical composition (wt.%, balance Fe) of base metal (AISI 2205) and filler metal (ER2209).

C Si Mn Cr Ni Mo N Cu S P

Base Metal 0.023 0.48 0.8 22.13 5.87 3.427 0.148 0.08 0.004 0.02Filler Metal 0.02 0.5 1.6 23.0 8.5 3.1 0.11 – <0.005 <0.010

Table 2Welding parameters as recorded for GTAW and SMAW processes, performed with 2.4mmdiameter electrode.

No. ofpass

Inter-passTemperature (°C)

Voltage (V) Current (A) Heat input(KJ/mm)

GTAW 1 32 9 81 1.4582 52 10 110 1.0813 78 10 122 1.4354 78 11 134 2.1895 69 12 148 1.7336 71 12 147 1.7787 65 13 149 1.9838 39 10 121 1.5259 67 11 129 1.139

SMAW 1 32 9 93 1.4772 63 10 101 1.3313 74 29 128 1.5154 81 30 134 1.1415 79 30 143 1.0556 72 31 141 1.2737 68 29 103 0.8158 73 30 111 0.8199 78 30 107 0.729

Fig. 1. Optical micrograph of the BM. α is shown as a matrix and γ as islands.

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austenite above solution annealing temperatures also confirms the non-applicability of phase diagram during welding processes [15,16].

At high thermal cycles, most of γ-phase gets dissolved whileα–phase remains stable. HAZ microstructures show comparatively largeamount of ferrite in SMAW than that of GTAW. These microstructuresshow different morphologies of γ-phase. In case of SMAW, thin allo-trimorphic austenite as grain boundary austenite (GBA) formed be-tween the interfaces of ferrite grains. Widmanstatten austenite (WA)also nucleated at α grain boundaries and extends into the un-transformed α grains interiors as shown in Fig. 2. The heterogeneousnucleation of intra-granular austenite (IGA) as acicular austenite is alsoobserved. Higher magnification image using SEM – see Fig. 3 – showsprecipitates of chromium nitrides at α/γ grain boundaries. HAZ mi-crostructures of GTAW – see Fig. 4 – exhibits relatively large amount ofWA and IGA as compared to SMAW. The GBA also became coarsenedwith some evidence of partially transformed austenite (PTA). Highermagnification SEM image – see Fig. 5 – shows presence of secondaryaustenite (γ2) at the grain boundaries of acicular austenite. The for-mation of γ2 is closely associated with Cr2N dissolution [17].

On the basis of recorded welding parameters – see Table 2 – it isconfirmed that the observed microstructural changes in both weld-ments can be attributed to the calculated heat input using formula

given elsewhere [18] and associated cooling rate, which is different inboth welding techniques. In case of SMAW process, low heat input wascalculated and thereby relatively high cooling rate was experienced bythe weldment when compared with GTAW. The dissolution of Cr2N andsubsequent formation of γ2 required time, which was provided by slowcooling rate in case of GTAW. However, comparatively fast cooling rate,as in case of SMAW, suppresses the transformation of γ-phase therebyformation of higher contents of ferrite and chromium nitride results[19].

Microstructure of the WZ of both SMAW and GTAW showed co-lumnar grains of ferrite with significant amount of austenite as WA,GBA and IGA as evident in Fig. 6.

3.3. Electrochemical corrosion studies and microstructural effect oncorrosion behavior

Potentiodynamic scan results of GTAW and SMAW for both HAZ

Fig. 2. Optical micrograph of HAZ of SMAW sample (9 welding passes).

Fig. 3. SEM image of HAZ of SMAW sample.

Fig. 4. Optical micrographs of HAZ of GTAW sample (after 9 welding passes).

Fig. 5. SEM image of HAZ of GTAW Sample.

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Fig. 6. Optical micrographs of weld zones of (a)GTAW and (b) SMAW samples.

1E-8 1E-7 1E-6 1E-5 1E-4-0,6

-0,5

-0,4

-0,3

-0,2

-0,1

0,0

0,1

0,2

Current Density (A/cm2)

V v

s E oc

(V)

SMAW-HAZ SMAW-WZ GTAW-HAZ GTAW-WZ

Fig. 7. Comparison of potentiodynamic polarizationTafel scans of SMAW and GTAW samples (HAZ andWZ).

Table 3Potentiodynamic polarization Tafel scans results of SMAW and GTAW.

Tafel parameters

Sample ID βa (mV/decade) βc (mV/decade) Icorr (µA/cm2) Ecorr (mV) Corrosion rate (mpy) Chi squared (error)

GTAW Heat Affected Zone (HAZ) 347.3 237.7 4.00 −348.0 1.829 55.60 e−15Weld Zone (WZ) 211.8 109.3 0.719 −223.0 0.328 20.75 e−15

SMAW Heat Affected Zone (HAZ) 426.6 624.6 33.90 −192.0 15.48 86.80 e−15Weld Zone (WZ) 347.9 157.3 0.877 −276.0 0.400 8.672 e−15

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and WZ are presented in Fig. 7. The corrosion kinetics parameters wereevaluated with the help of Tafel Extrapolation method on potentiody-namic polarization curves in 3.5% NaCl solution and are presented inTable 3. The results show that corrosion current densities of HAZ in-creases in both types of welding techniques when compared with theircorresponding WZ. Higher value of corrosion current (Icorr) in case ofHAZ upon SMAW indicates higher metal dissolution rate when com-pared with GTAW. HAZ microstructure in case of SMAW is enrichedwith ferrite – see Fig. 2 – therefore, higher value of βa in this caseconfirms that ferrite is electrochemically more active than austenite.However, this effect is less in case of GTAW. Additionally, the corrosioncurrent of WZ is very small as compare to HAZ in both type of weldingtechniques and therefore it has small corrosion rate.

Variations in corrosion resistance among HAZ and WZ of bothGTAW and SMAW are significant. The reason for such behavior istwofold: chemical composition and γ/α ratio. High thermal cycleduring fusion welding makes the γ/α ratio lopsided. It has been ob-served that increasing the ferrite contents, enhances the corrosion rate[10]. Hence, presence of relatively large volume fraction of ferrite incase of HAZ upon SMAW registered higher corrosion currents. Thequalitative XRD results of HAZ of both weldments also confirm thelarge volume fraction of ferrite in case of SMAW as shown in Fig. 8.Nitrogen is one of the key elements for corrosion resistance but theprecipitation of chromium nitride is at the expense of nitrogen diffusionfrom the ferrite matrix. Therefore, it is proposed that the low corrosionresistance of HAZ upon SMAW is also associated with the chromiumdepletion around the Cr2N precipitates in ferrite [14,20]. Micro-structure of GTAW-HAZ shows higher volume fraction of austenite. Thisis the austenite with increased nitrogen contents that improves corro-sion resistance [21–24], therefore low corrosion current was recorded.Among other forms of austenite, the morphology of acicular austeniteoffers less exposure area toward corrosive media. Also, the precipitationof acicular austenite is at the expense of intra-granular nitrides andtherefore it is concluded that this is the phase which is rich in nitrogenand hence more resistance to corrosion [6]. Microstructure of WZ – seeFig. 6 – shows higher volume fraction of austenite that resulted in lowcorrosion currents.

3.4. Mechanical behavior

Variations in microstructure as illustrated above have direct impacton mechanical properties. Formation of different phases during welding

has great impact on toughness and tensile properties. SMAW weldmentshowed comparatively low value of toughness when compared withGTAW as shown in Fig. 9(a) and (b). The low value of toughness in caseof SMAW can be attributed to the presence of large amount of ferrite – asoft phase [22]. The Fig. 9(b) indicates an increase in elongation withcorresponding decrease in the tensile strength in case of SMAW. Ingeneral, formation of precipitates hinders the motion of dislocationsand makes the material tough and less ductile but in case of SMAW theprecipitates have been precipitated out, no or very less interaction withdislocation results in lower strength than expected. Moreover, thepresence of large volume fraction of ferrite which is BCC (body centeredcubic) crystallographic in structure makes the material less tough. TheBCC structure of iron there is no well-defined slip system and slip mayoccur in {110}, {112} and {123} directions on ⟨110⟩ planes. However,the interaction of dislocations mostly result in movement of pure edgedislocation laying on ⟨001⟩ planes which is not a closed pack slip planein BCC lattice, resulting in brittle fracture with low impact energy. Onthe other hand, FCC structure of austenite gives higher toughness due toits well defined inherent slip system composing of closely packed ⟨111⟩plane with [110] direction which helps the material to absorb energybefore fracture and hence making it tough [23]. The fracture mode –see Figs. 10 and 11 – also shows that failure behavior change fromquasi-cleavage to dimple rupture as we moved from SMAW to GTAW.

4. Conclusion

In summary, the growth of different phases in as-welded samples asa result of two selected techniques and their response towards corrosionwere studied. It is concluded that the existence of sigma and chi phasesare not evident in as-welded SMAW and GTAW weldments. The highcorrosion rate of SMAW weldment is attributed to the formation ofrelatively large amount of ferrite and chromium nitride contents whencompared with GTAW, which was also confirmed by more negativevalues of Ecorr. Among different phases, acicular austenite is found to bemore responsible in impeding corrosion. Inferior mechanical propertiesof SMAW weldment is also attributed to the presence of large amount offerrite. Hence, corrosion behaviour under seawater solution and me-chanical properties of DSS greatly depends upon the morphology ofphases and their corresponding volume fractions.

30 40 50 60 70 800

100

200

300

400

500

600

700

γ (200)

α (200)

γ (220)α (110)

Inte

nsity

/ (a

.u)

Angle (2-theta)

GTAWSMAW

γ (111)

Fig. 8. XRD of HAZ of GTAW and SMAW samples.

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268.03

151.7

GTAW SMAW

(a)GTAW SMAW

24 29

790.88 765.05

GTAW SMAW

(b) Elonga�on (%) Strength (MPa)

Fig. 9. Average mechanical properties of GTAW and SMAWsamples (a) Toughness behavior (b) Tensile properties.

Fig. 10. SEM fractograph of SMAW sample obtained after fracture. Fig. 11. SEM fractograph of GTAW sample obtained after fracture.

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