Chiang Mai J. Sci. 2013; 40(5) 923

Chiang Mai J. Sci. 2013; 40(5) : 923-937http://it.science.cmu.ac.th/ejournal/Contributed Paper

Microstructural Characterization of Lean DuplexStainless Steel UNS S32101 Welded Joints UsingElectron Backscatter DiffractionZbigniew Brytan*[a] and Jerzy Niagaj [b][a] Division of Materials Processing Technology, Management and Computer Techniques in Materials

Science, Institute of Engineering Materials and Biomaterials, Silesian University of Technology,ul. Konarskiego 18a, 44-100 Gliwice, Poland.

[b] Institute of Welding, ul. B . Czes awa 16/18, 44-100 Gliwice, Poland.*Author for correspondence; e-mail: zbigniew.brytan@polsl.pl

Received: 03 July 2012Accepted: 05 April 2013

ABSTRACTMicrostructural characterization of lean duplex stainless steel UNS S32101

welded joints using scanning electron microscopy (SEM) and electron backscatterdiffraction (EBSD) is discussed in the present paper. The influence of isothermal agingheat treatment at 650oC on the microstructures of weld, heat affected zone (HAZ) andparent metal was investigated. The welded joint in the as-welded condition shows Cr2Nnitride precipitation in the HAZ, while isothermal aging at 650oC for 15 min causesfurther precipitation of nitrides, both in the parent metal, as well as in the HAZ and theweld area. Increasing the aging time at this temperature to 90 min causes the formationof carbides at the grain boundaries and nitride precipitation inside ferritic grains in eachzone of the welded joint.

The EBSD analysis of the welded joint after aging at 650oC for 90 min revealedthat ferrite content decreased from 45% in parent metal to 40% in HAZ and 39% in theweld. The higher austenite content in the weld, closer to equilibrium, optimized themechanical properties of welded joints. Grain size of austenite increased gradually fromthe parent metal over the HAZ to the weld where it was the highest. The ferrite showssmallest grain size in the weld, in the parent metal and the weld it was similar. Theorientation relationship between adjacent grains of austenite and ferrite observed fromEBSD in the weld metal and the HAZ follows both (K-S) and (N-W) orientationrelationship but favors the (N-W) relationship. The presence of such relationship betweenaustenite and ferrite in the weld metal together with the decrease of high-angle grainboundaries in both phases in the welded joint can possibly positively influence thereduction of residual stresses and decrease the susceptibility for crack initiation in theweld metal.

Keywords: EBSD, lean duplex stainless steel, MAG welded joints

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1. INTRODUCTIONStainless steel is the most popular

material applied where high corrosionresistance, high surface finish quality,elevated hygienic standards and moderateprice are required. The group consists ofsingle phase alloys: austenitic, ferritic,martensitic and precipitation hardeningalloys, as well as double phase alloys –duplex stainless steels. The duplex stainlesssteels combine the properties and advantagesof two single phase alloys (ferritic andaustenitic) and show higher mechanicalproperties than the single phase alloys andequivalent or superior corrosion resistance.Duplex stainless steels are less sensitive tostress corrosion cracking than austeniticalloys and show magnetic properties. Theduplex stainless steels, based on chemicalcomposition, can be sub-divided intostandard duplex, super duplex, hyper duplexand lean duplex stainless steel grades. Thestandard duplex grades contain 22%Cr,5%Ni and 3%Mo and the addition of0.2%N, where the most popular grade isUNS S31803 – designation according toUNS (Unified Numbering System). Thenext group named super duplex consistsof grades with higher alloy content i.e.25%Cr, 7%Ni and 3-4%Mo, for examplegrades S32550, S32750, S32760. The latestdevelopment includes the new highlyalloyed grades called hyper duplex, such asS32707 SAF 2707 HD that contains evenhigher amounts of alloying elements andthe nitrogen content is up to about 0.5%.The high contents of Cr, Mo and N togethercan give the alloy very high strength andsimultaneously good workability forextrusion into seamless tubes used for subseaapplications, such as subsea umbilicals toconnect platform control stations andwellheads on the seabed [1,2].

In lean duplex stainless steel nickel is

partially replaced by manganese. They alsocontain a high content of nitrogen. Induplex stainless steels Ni, Mn and Nstabilize austenite and maintain a balancebetween austenite and ferrite at about 50%.The use of manganese can ensure properferrite-austenite phase balance, whileallowing a reduction in nickel content andhence cost. As a result lean duplex stainlesssteels such as S30403 and S31603 are pricedcompetitively with standard types ofaustenitic stainless steels - S32101 is the mostpopular lean duplex stainless steel designedfor general-purpose use and like otherduplex stainless steels provides both superiormechanical strength and greater chloridestress corrosion cracking resistancecompared to austenitic stainless steels - 300series. A new grade of lean duplex stainlesssteel was launched in 2010, called LDX2404® (UNS S82441). This grade contains3.0% of manganese and 3.6% nickel and hashigher chromium and nitrogen contentsthan standard austenitic grade S31603.Such composition offers higher levels ofcorrosion resistance making it suitable towithstand the salty atmospheric conditionswhich prevail in buildings located near thesea. The combination of improved corrosionresistance and high mechanical strengthmakes UNS S82441 very cost-effective [3].

During the welding of duplex stainlesssteels the thermal cycle of welding can causechanges in the microstructure not only inthe weld material, but also in the heataffected zone. These changes affect both thecorrosion resistance of welded jointsand their mechanical properties. Sometimesthe production cycle involves the heattreatment of welded components orstructures made of lean duplex stainlesssteel. In such situations the additionalheating of the welds and heat affected zone

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can produce carbides, nitrides or sigmaphase precipitation – the extent of whichdepends on the temperature and time ofheat treatment. These issues are widelyreported in relation to the base materialbut not when considering welded joints,which may behave differently because theyposses initial microstructures that aredifferent from that of the base materialdepending on the applied welding method,type of filler metal, the amount of heatinput [4,5].

Electron Back Scattering Diffraction(EBSD) is a technique based on the analysisof the Kikuchi pattern formed by electrondiffraction from individual crystals inspecimens in a scanning electron microscope(SEM). The EBSD technique providesquantitative microstructural informationabout the crystallographic nature of thecrystalline materials, where the grain size,grain boundary character, grain orientation,misorientation, texture, phase identity ofthe sample can be revealed together withstatistical measurement and quantitativeanalysis. Consequently, it has become avery important experimental technique inmaterials science and engineering [6,7].

The EBSD technique has found a widefield of applications in multiphase materialslike duplex stainless steels, where by usingthis method the nature of ferrite, austeniteand any intermetallic phases can be identified.The area fraction and grain size of eachphase can also be calculated and the texturein each phase can be identified. This cangive detailed information about the thermaland processing history and the mechanicalproperties of the steel [8-12].

The austenite (γ) can have a specificcrystallographic orientation relationshipwith the ferrite (α) due to phase transforma-tion occurring during the heating andcooling in a duplex stainless steel weld

joint. The relationships present in duplexstainless steel between neighboring austeniteand ferrite grains are often close toKurdjumov – Sachs (K-S) {111}γ // {011}α,<011>γ // <111>α and Nishiyama –Wasserman (N-W) relationships {111}γ //{011}α, <112>γ // <011>α [13]. Thesetwo relationships differ by only with fewdegrees in rotation in the interface planebetween austenite and ferrite, where K-Srelationship corresponds to a 42.8o

misorientation angle and N-W relationshipto 45.9o [14]. The presence of suchrelationships between austenite and ferritein the weld metal may have influence onthe easier propagation of dislocation slipfrom one phase to the other and thusreduce the distribution of residual stresses[15] Presence of the K-S relationshipbetween austenite and ferrite was recentlyconnected with lower tendency tomicrocracks nucleation on the phaseboundaries [16].

This paper presents a few examples ofSEM and the EBSD analysis applied in themicrostructure characterization of differentzones of the welded joints of lean duplexstainless steel S32101 after isothermal agingat 650oC.

2. MATERIALS AND METHODSLean duplex stainless steel as cold

rolled and annealed sheet according toASTM A240 S32101 (EN 1.4162) with athickness of 6 mm was used for the study(Table 1). The welding joints were producedusing the metal active gas (MAG) methodwhere the filler metal was in wire formgrade Avesta LDX 2101 with a diameterof 1.2 mm, as in Table 2. During theprocess a shielding gas mixture of Ar +2.5% CO2 was applied and as a forminggas pure technical argon according tostandard EN ISO 14175-I1-Ar was used.

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Table 2. Chemical composition of wire electrode, grade AVESTA LDX 2101.

Wire Wire

grade diameter,mm C Si Mn P S Cr Mo Ni N2 Cu

AVESTALDX 2101 1.2 0.016 0.53 0.75 0.029 0.001 23.12 0.25 7.27 0.117 0.17

Chemical composition, wt %

The welding joints were producedusing a mechanized welding stand consistingof a semi-automatic welding machine withsynergistic control (Kemppi Pro 5,200), awelding trolley (DC20 PROMOTECH)with precisely adjustable travel speed oftorch and an attachment device to positionwelded pieces. The shielding gas flow duringMAG welding was adjusted between 14 and

16 l/min, while the forming gas was set to5-6 l/min. The welding was carried outusing direct current with the positive endof the consumable electrode (DC +). Theparameters of MAG welding for butt jointsof 6 mm thickness made of lean duplexstainless steel S32101 are summarized inTable 3.

Table 3. The MAG welding parameters of testing butt joints.Joint Number Welding Arc voltage, Welding speed, Heat input,

symbol of beads current, A V cm/min kJ/mm6-20 1 116 22.6 9.2 1.368

Table 1. Chemical composition of lean duplex stainless steel S32101.

Grade Sheet thickness, mm Chemical composition, wt %C Si Mn Cr Ni Mo Cu N

S32101 6 0.028 0.70 4.90 21.34 1.50 0.19 0.25 0.21

After welding the test joints weresubjected to heat treatment by aging attemperatures of 650oC for 15 and 90 min.in a neutral atmosphere.

Microstructure observations of weldedjoints were carried out using light andscanning electron microscopy. Lightmicroscope observations involved etchingusing Aqua Regia reagent and wereperformed at 400x and 1,000x magnifica-tions using a LEICA MEF4A microscope.The scanning electron microscope (SEM)involved in the studies was a SUPRA 25 ofZEISS Company equipped with an EDSprobe. The SEM Inspect F with EBSDequipped with the automatic OIM

(orientation imaging microscopy) softwarefrom TSL has been used to obtain moreinformation about grain structure thanfrom optical microscopy. The samples forEBSD analysis were ground, polished withsilicon carbide paper from grade 220 tograde 4,000, and then with diamond paste(1 and 0.25 μm) on polishing clothes andfinally argon-ion beam polished. TheEBSD measurements were performed at20 keV with a step size of 20 μm. Themicrostructural analysis was performed inthe middle of the welded zone, around theroot of weld, heat affected zone (HAZ) andin parent metal (Figure 1).

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Figure 1. The areas of microstructural studies in the welded joint, a) the arrangement ofstudied zones, b) cross-sectional photo of studied joint (1 – the weld, middle; 2 – theweld, root; 3 – the heat affected zone HAZ; 4 – the parent metal PM).

During SEM observations the EBSDanalysis was applied in three different zonesof each welded joint i.e. in the weld, HAZand the parent metal. Crystal orientationmaps (COM) were generated includingthe distribution of low- and high-angleboundaries, distribution of misorientationangles between all adjacent points, distribu-tion of phases, grain size, grain size aspectratio (defined as the minor axis of an ellipsedivided by its major axis of a fitted ellipse,so a grain with a high aspect ratio has aneedle-like shape), inverse pole figures forindividual phases and the phase content inindividual zones of the welded joint. TheEBSD analyses were performed on the planeperpendicular to the welding direction onthe transverse section of the weld, wherethe axis designation for EBSD, the (TD)axis correspond to horizontal directionand (RD) axis correspond to verticaldirection (Figure 1b).

3. EXPERIMENTAL RESULTS AND DISCUSSIONMicrostructural analysis of the

individual zones of the welded joints bylight microscopy revealed that the weld areacontained more austenite than the HAZand parent material (Figure 2).

The microstructural analysis by SEMconfirmed that in the as-welded conditionsweld material is characterized by mixeddistribution of both phases (ferrite andaustenite), where in ferritic matrix theaustenite of different shape and orientationprecipitated. There are also small areas ofintracellular austenite with characteristictrapezoidal and orthorhombic shape(Figure 3a). Secondary phase precipitationhas not been observed at grain boundariesof either phase. However in the HAZ, verynumerous small precipitates probably ofchromium nitride Cr2N [4, 8-10] wereobserved in the ferrite grains and in ferrite/ferrite grain boundaries (Figure 3b). Thisphenomenon occurred only in the HAZand its range does not exceed beyond thiszone. The parent metal microstructure wasfree of secondary phase precipitation. Themicrostructure of material in the weld areais characteristic for the welded duplexstainless steels with uneven distribution ofboth phases. The chemical compositionmeasured with EDS analysis in the weldzone show very uniform distribution ofalloying elements, expressed by partialcoefficient (α/γ wt%), which for Cr = 1.1and Mn = 0.81 and Ni= 0.97 indicating

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Figure 3. The microstructure of lean duplex stainless steel S32101 of the welded joint inas-welded condition, evaluated by SEM, where a) the weld area and b) the HAZ.

c) d)

Figure 2. The microstructure in as-welded conditions of lean duplex stainless steelS32101 in various zones, where a) the weld centre (point 1 acc. to Figure 1), b) the weldroot (point 2 acc. to Figure 1), c) HAZ (point 3 acc. to Figure 1), d) parent material(point 4 acc. to Figure 1).

a) b)

a) b)

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that ferrite (α) is dominated by chromiumand austenite (γ) is enriched in nickel andmanganese and has lower concentrations ofchromium.

The isothermal heat treatment of thewelded joint at 650oC for only 15 minresults in precipitation of chromiumnitrides Cr2N in the material, both in theweld metal (Figure 4a) and heat affectedzone (HAZ) and in the parent metal (Figure4b). After this heat treatment the presenceof carbides at austenite/ferrite grainboundaries was not observed. Extendingthe holding time in this temperature to 90min results in the precipitation of theincreased amount of nitrides Cr2N andaccording to some reports [4, 11] ofcarbides, probably M23C6 type at the grain

boundaries of austenite and ferrite, whichoccurs in a balanced way in each zone ofwelded joint - in the weld, HAZ andparent metal. The precipitation of nitridesand carbides after isothermal aging are lessharmful to mechanical properties, thanprecipitation of sigma phase (σ) [12], sinceits formation can cause rapid deteriorationin toughness and corrosion resistance. Theformation of sigma phase (σ) occurs whenthe cooling rate during welding is not fastenough and the more highly alloyed theduplex stainless steel, the higher is theprobability of its formation. Therefore,hyper and superduplex steels are moreprone to this problem than lean duplexstainless steel.

a) b)

c) d)

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e) f)

Figure 4. The microstructure of the welded joint after aging at 650oC, a) and b) afteraging for 15 min, a) the weld (area 1) and b) the parent metal (area 4), and c) and d) afteraging for 90 min, c) the weld and d) the parent metal, e) and f) the HAZ, e) after agingfor 15min and f) after aging for 90min.

Detailed EBSD analysis of the sampleafter aging at 650oC for 90 min was performed(Figure 5-7) to evaluate any differences inthe nature of austenite and ferrite in particularzones of welded joints. This condition wasselected due to marginal changes in mainphase balance, the austenite and ferrite,compared to the as-weld condition and afteraging for 15 min. Small precipitates suchcarbides or nitrides will be analyzed infuture work. The phase map in the parentmetal (Figure 5) shows the ferrite contentin this zone to be 45% with predominatedlow-angle boundaries, about 84% (theboundaries with misorientation anglebetween 2o and 15o) of all boundariespresent in the ferrite. The inverse polefigure of ferrite shows strong alignment tothe <110> axes (Figure 5b). Average grainsize diameter of ferrite was calculated as9.04 μm (7.88 - excluding edge grains),where the equivalent area of fitted ellipsewas 64.18 (48.81) μm2 (Table 4). Averagegrain shape (excluding edge grains) wascalculated by computing the average areaand aspect ratio and then using a fittingapproach to determine the angle between

the major axis to the horizontal. The grainshape was described by the major andminor axis of an equivalent ellipse. Themajor axis was measured as 10.18 (8.70) μm,and minor axis 3.77 (3.40) μm, where theangle of major axis to the horizontal was66.18 (70.25) degrees. The austenite phasein the parent metal shows predominantlyhigh-angle boundaries, about 70% (theboundaries with misorientation angleabove 15o ). The inverse pole figure ofaustenite shows strong orientation of thisphase between <111> and <001> axes. Theaverage grain size of austenite was smallerthan that of ferrite. The average grain shapeexpressed by major axis of equivalent ellipsewas 6.16 μm and the minor axis was 2.71μm,smaller than for ferrite. Detailed comparisonof the grain size and grain shape of austeniteand ferrite in particular zones of the weldedjoint are summarized in the Table 4. Thegrain shape aspect ratio of austenite in eachzone of the weld was in the range between0.42 to 0.44, while the grain shape aspectratio of ferrite was increased from 0.37,characteristic for parent metal to 0.50 inHAZ and 0.46 in the weld area.

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Zone ofwelded Phase The average Equivalent Major Minor Grain Angle tojoint grain size, μμμμμm area, μμμμμm2 axis, μμμμμm axis, μμμμμm shape horizontal,

aspect ratio degrees**

Parent 9.04 64.18 10.18 3.77 0.37 66.18metal (7.88) (48.81) (8.70) (3.40) (0.39) (70.25)

HAZ α 6.91 37.51 9.22 4.64 0.50 356.37(5.81) (26.50) (7.11) (3.67) (0.51) (168.42)

Weld 9.45 70.08 12.67 5.90 0.46 352.20(8.19) (52.64) (9.47) (4.54) (0.47) (163.80)

Parent 7.32 42.07 6.16 2.71 0.43 18.04metal (7.31) (41.91) (6.09) (2.74) (0.45) (15.97)

HAZ γ 11.99 112.85 14.28 6.40 0.44 358.91(11.40) (102.09) (13.64) (6.33) (0.46) (179.48)

Weld 13.44 141.86 15.99 6.80 0.42 340.11(12.36) (119.97) (13.83) (6.09) (0.44) (158.60)

Table 4. The grain size and grain shape of austenite and ferrite in particular zones of thewelded joint.

Grain size * Average grain shape*

* Grain size estimated based on the fitted ellipse and its diameter and equivalent area,where the ellipse with minor and major axis represent shape aspect ratio** Grain shape orientation refers to the angle of the major axis from the horizontalThe values in brackets ( ) refers to values excluding edge grains – excluding pointsconsidered as edge grains

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Figure 5. The EBSD maps of the parent metal microstructure, a) phase map and colorcoded map - inverse pool figure [001] of b) ferrite and c) austenite, the color code forpresented microstructures is as follows: the red means a [001] crystal direction, blue a[111] direction, and green a [101] direction, respectively.

(a) (b) (c)

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The EBSD mapping in the weld zone(Figure 6a) confirmed lower ferrite contentin this zone, about 39%, about 93% of theboundaries in the ferrite phase were ratedlow-angle and 7% high-angle boundaries.Austenite contained both low- and high-angle boundaries, where the number ofhigh-angle boundaries was decreasedcompared to parent metal and contained

Figure 6. The EBSD maps of the weld microstructure, a) phase map and color codedmap - inverse pool figure [001] of b) ferrite and c) austenite, the color code for presentedmicrostructures is as follows: the red means a [001] crystal direction, blue a [111] direction,and green a [101] direction, respectively.

56% of high-angle boundaries. The ferriticphase shows preferential grain orientationaccording to <001> direction (Figure 6b),while austenite grains shows mixedorientation (Figure 6c). The austenite showsan increase in average grain size and 3xincrease of equivalent area of grain size(Table 4).

(a) (b) (c)

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The HAZ shows a ferrite content of40% (figure 7a). The grain shapes in thiszone of the welded joint are more elongatedwith higher grain shape factor comparedto the weld and parent metal (Table 4) andboth phases show either low- and high-angle

grain boundaries. The ferritic phase (Figure7b) as well as austenitic (Figure 7c) showsmixed orientation. The respective amountsof high-angle boundaries in ferrite andaustenite are 35% and 52%.

Figure 7. The EBSD maps of the HAZ microstructure, a) phase map and color codedmap - inverse pool figure [001] of b) ferrite and c) austenite, the color code for presentedmicrostructures is as follows: the red means a [001] crystal direction, blue a [111] direction,and green a [101] direction, respectively.

Studied lean duplex stainless steel weldmetal shows regions of Widmanstatten-type austenite (Figure 6 and 7) typical of afully ferritic solidification mode thatprovide a positive influence on the weldimpact toughness. The duplex weld metal

solidifying completely as ferrite showsgenerally higher toughness level thanstructures solidifying in mixed austenitic-ferritic mode [17].

For each EBSD map, the proportionsof small (2o - 15o) and large (45o - 180o)

(a) (b) (c)

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misorientations between all adjacent pointswere calculated, thus the boundaries witha misorientation angle less than 15o weredefined as low angle boundaries and thesewith an angle greater than 45o were termedas high angle boundaries. Figure 8 showsthe distribution of the misorientation anglebetween austenite and ferrite boundariesfor each studied zone. The misorientationangle between austenite and ferriteboundaries in the parent material liespreferentially near low angles below 5o andgradually increases near 40-45o and showsrandom misorientation angle distribution(Figure 8a). The average grain misorienta-tion angle in the weld and also in the HAZbetween phases of lean duplex stainless steelwelded joint lies near 40-45o , whichcontains both Nishiyama-Wassermann

(N-W) and Kurdjumov-Sachs (K-S)crystallographic orientation relationships(Figure 8b and 8c). The austenite phase canprecipitate with near (N-W) or (K-S)orientation relationship. This result agreeswith earlier results from the literature [18].The (K-S) orientation relationshipscorrespond to a 42.8o misorientation angleand (N-W) relationship to 45.9o so detailedanalysis in this range was performed. Thisindicates that the orientation relationshipbetween austenite and ferrite in the studiedlean duplex stainless steel weld metal andHAZ is closer to the (N-W) orientationrelationship than to the (K-S) one. TheHAZ shows even slightly higher numberof analyzed fractions that follows (N-W)orientation relationship than that in theweld zone.

The high-angle boundaries are knownto be cracking-susceptible boundaries.High-angle boundaries content for bothphases was reduced from the parent metal,over the HAZ into the weld, thus if a crackis initiated in the HAZ it cannot thenreadily propagate into the weld metal viasuch continuous, cracking-susceptibleboundaries. The-high boundaries inaustenite were decreased from 70% inparent metal to 52% in the HAZ and 56%in the weld. The low-angle boundaries for

which content increased in both phases inthe weld metal show the ability to induceslip in neighboring grains thus reducinggrain boundary stress and susceptibility forcrack initiation.

4. CONCLUSIONSThe effects of aging at 650oC on the

microstructure of welded joints of leanduplex stainless steel S32101 can besummarized as follows:1. The welded joint in as-welded condition

Figure 8. The grain misorientation angle between austenite and ferrite in a) parent metal,b) the HAZ, c) the weld.

(a) (b) (c)

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shows few Cr2N nitride precipitateslocated only in the HAZ. The isothermalheat treatment at 650oC for 15 mincauses formation of numerous nitrideprecipitates, both in the parent metal, aswell as in HAZ and the weld area.Increasing aging time at this temperaturefrom 15 to 90 min causes the formationof carbides on the grain boundaries andnitride precipitation inside the ferriticgrains in each zone of the welded joint.

2. The EBSD analysis of the welded jointafter aging at 650oC for 90 min revealedthat ferrite content decreased from 45%in parent metal to 40% in HAZ and 39%in the weld. The higher austenite thecontent in the weld, closer to equilibrium,the optimal are the mechanical propertiesof welded joints.

3. Grain size of austenite increasedgradually from 7.32 μm in parent metalto 11.99 μm in the HAZ and 13.44 μmin the weld. The ferrite shows thesmallest grain size in the HAZ, about6.91μm, while in parent metal it wasclose to 9μm and slightly increased inthe weld to 9.45 μm. The grain shapeaspect ratio of austenite in each zone ofstudied weld was included in the rangebetween 0.42 to 0.44, while grain shapeaspect ratio of ferrite was increased from0.37, characteristic for parent metal to0.50 in HAZ and 0.46 in the weld area.The increase of grain shape aspect ratioindicates that grains of both phasesbecome more elongated and they showsimilar shape in the weld and the HAZ.

4. The orientation relationship betweenneighboring grains of austenite andferrite observed from EBSD in the weldmetal and the HAZ follows both (K-S)and (N-W) relationship favoring (N-W)orientation relationship. The presence ofsuch relationship between austenite and

ferrite in the weld metal together withthe decrease of high-angle grainboundaries in both phases in the weldedjoint can make a contribution toreduction of grain boundary stresses anddecrease the susceptibility for crackinitiation in the weld metal, thusincreasing overall weld metal mechanicalproperties.

ACKNOWLEDEMENTSThe authors gratefully acknowledge

the financial support from the NationalScience Centre granted based on thedecision number DEC-2011/01/B/ST8/06648.

The authors are grateful to DrKlaudiusz Go ombek form SilesianUniversity of Technology in Gliwice forhis valuable support in SEM micro-structural testing.

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