Influence of Welding Speeds on the Morphology, Mechanical Properties

materials

Article Inﬂuence of Welding Speeds on the Morphology, Mechanical Properties, and Microstructure of 2205 DSS Welded Joint by K-TIG Welding

Shuwan Cui 1,*, Shuwen Pang 1, Dangqing Pang 2 and Zhiqing Zhang 1

1 School of Mechanical and Transportation Engineering, Guangxi University of Science and Technology, Liuzhou 545006, China; [email protected] (S.P.); [email protected] (Z.Z.) 2 Guangxi Zhuang Autonomous Region Tobacco Company Liuzhou Tobacco Company, Liuzhou 545006, China; [email protected] * Correspondence: [email protected]; Tel.: +86-13597-0666-15

Abstract: In this paper, 8.0 mm thickness 2205 duplex stainless steel (DSS) workpieces were welded with a keyhole tungsten inert gas (K-TIG) welding system under different welding speeds. After welding, the morphologies of the welds under different welding speed conditions were compared and analyzed. The microstructure, two-phase ratio of austenite/ferrite, and grain boundary characteristics of the welded joints were studied, and the microhardness and tensile properties of the welded joints were tested. The results show that the welding speed has a significant effect on the weld morphology, the two-phase ratio, grain boundary misorientation angle (GBMA), and mechanical properties of the welded joint. When the welding speed increased from 280 mm/min to 340 mm/min, the austenite content and the two-phase ratio in the weld metal zone (WMZ) decreased. However, the ferrite Citation: Cui, S.; Pang, S.; Pang, D.; content in the WMZ increased. The proportion of the Σ3 coincident site lattice grain boundary Zhang, Z. Influence of Welding (CSLGB) decreased as the welding speed increased, which has no significant effect on the tensile Speeds on the Morphology, strength of welded joints. The microhardness of the WMZ and the tensile strength of the welded joint Mechanical Properties, and gradually increased when the welding speed was 280–340 mm/min. The 2205 DSS K-TIG welded Microstructure of 2205 DSS Welded joints have good plasticity. Joint by K-TIG Welding. Materials 2021, 14, 3426. https://doi.org/ 10.3390/ma14123426 Keywords: K-TIG welding; weld morphology; grain boundary characteristics; microhardness; tensile strength Academic Editors: Frank Czerwinski and Massimiliano Avalle

Received: 10 May 2021 1. Introduction Accepted: 14 June 2021 2205 Duplex Stainless Steel (DSS) is a functional integrated material with high strength Published: 21 June 2021 and excellent corrosion resistance. It is composed of two phases of ferrite and austenite, and the content of both is close to 1:1 [1,2]. 2205 DSS takes into account the advantages Publisher’s Note: MDPI stays neutral of both ferrite and austenite. In recent years, it has been widely used in papermaking, with regard to jurisdictional claims in construction, structural materials, nuclear reactors, the petrochemical industry, and un- published maps and institutional affil- derwater engineering. As an important thermal processing technology in manufacturing, iations. welding can change the two-phase structure balance in the 2205 DSS welded joint, which affects the performance of the welded joint [3–6]. Therefore, the organization and control of the welding process of 2205 DSS has received extensive attention from many disciplines. At present, in the medium-thick 2205 DSS plate welding process, the multi-layer and Copyright: © 2021 by the authors. multi-pass welding process is often used. It is a traditional welding method, which has Licensee MDPI, Basel, Switzerland. low work efficiency and easily destroys the two-phase balance of the heat-affected zone This article is an open access article (HAZ). High-energy beam welding is an ideal method for welding medium-thick metal distributed under the terms and plates. It has high welding efficiency and excellent welding seam shape. The welding conditions of the Creative Commons heat input required for high-energy beam welding is relatively low, so the cooling rate is Attribution (CC BY) license (https:// faster. The austenite content in the weld metal zone (WMZ) was too low, and coarse ferrite creativecommons.org/licenses/by/ grains were formed, and the grain boundaries are easily corroded. Under low temperature 4.0/).

Materials 2021, 14, 3426. https://doi.org/10.3390/ma14123426 https://www.mdpi.com/journal/materials Materials 2021, 14, x FOR PEER REVIEW 2 of 10

The austenite content in the weld metal zone (WMZ) was too low, and coarse ferrite grains were formed, and the grain boundaries are easily corroded. Under low temperature con- Materials 2021, 14, 3426 ditions, the WMZ is prone to embrittlement, and the toughness and corrosion resistance2 of 10 of the WMZ decrease. Westin [7] et al. applied CO2 laser welding to weld 2205 duplex stainless steel. Coarse ferrite grains formed in the 2205 duplex stainless steel welded joint. Excessiveconditions, ferrite the content WMZ is and prone coarse to embrittlement, grain structure and seriously the toughness affect the and toughness corrosion and resistance cor- rosionof the resistance WMZ decrease. of the welded Westin joint. [7] etZhang al. applied [8] et al. CO pointed2 laser out welding that the to two-phase weld 2205 struc- duplex turestainless was unbalanced steel. Coarse in the ferrite WMZ grains of the formed S31803 in DSS the electron 2205 duplex beam-welded stainless steeljoint, welded and there joint. is aExcessive Cr2N precipitated ferrite content phase. and Singh coarse [9] et grain al. used structure electron seriously beam welding affect the to toughnessweld 18 mm and andcorrosion 12 mm thick resistance 2205 ofDSS the steel welded plates. joint. They Zhang reported [8] et that al. pointedthe fine-grained out that thefusion two-phase zone withstructure a higher was γ/α unbalanced ratio showed in thebetter WMZ impact of the toughness, S31803 DSS while electron the coarser-grained beam-welded joint, fusion and zonethere with is a Crhigher2N precipitated α/γ ratio showed phase. better Singh fatigue [9] et al.properties. used electron The above beam research welding shows to weld that18 the mm microstructure and 12 mm thick and 2205 mechanical DSS steel proper plates.ties They of the reported welded that joints the are fine-grained different fusiondue to zonethe difference with a higher in theγ cooling/α ratio rate showed corresponding better impact to different toughness, high-energy while the beam coarser-grained welding methods.fusion zoneThe current with a higher researchα/ γmainlyratio showedfocuses betteron the fatigue effect of properties. cooling rate The on above the micro- research structureshows thatof the the 2205 microstructure DSS welded and joint mechanical and lacks properties the mechanism of the welded analysis joints of the are micro- different structuredue to evolution the difference during in the the welding cooling process. rate corresponding to different high-energy beam weldingKeyhole methods. tungsten The inert current gas (K-TIG) research welding mainly is focusesa kind of on high-energy the effect of beam cooling welding rate on method.the microstructure It can be welded of the in 2205a single-pass DSS welded welding joint and formed lacks the on mechanism both sides analysiswithout offill- the ingmicrostructure the welding material evolution [10–14]. during In the this welding paper, different process. welding speeds were applied to analyzeKeyhole the influence tungsten of cooling inert gas rate (K-TIG) on morphology, welding is mechanical a kind of high-energy properties, beam and micro- welding structuremethod. of the It can 2205 be DSS welded welds. in The a single-pass evolution law welding of the and two-phase formed ratio, on both grain sides boundary without misorientationfilling the welding angle (GBMA), material and [10– mechanical14]. In this properties paper, different of the welded welding joints speeds under were dif- ap- ferentplied welding to analyze speeds the influencewere studied. of cooling rate on morphology, mechanical properties, and microstructure of the 2205 DSS welds. The evolution law of the two-phase ratio, grain 2. Materialsboundary and misorientation Methods angle (GBMA), and mechanical properties of the welded joints under different welding speeds were studied. 2.1. Material and Welding Procedure 2.Figure Materials 1 shows and Methods the schematic diagram of K-TIG welding. The butt-joint welding method2.1. Material was adopted, and Welding the workpieces Procedure to be welded were fixed before welding, and there was no Figureobvious1 showswrong theedge schematic and gap between diagram the of workpieces. K-TIG welding. The high-purity The butt-joint argon welding was appliedmethod as wasthe protective adopted, the gas workpieces on the front to an bed welded back of were the workpieces, fixed before and welding, the gas and flow there ratewas was no 25 obvious L/min. wrong The specific edge and welding gap between parameters theworkpieces. are shown in The Table high-purity 1. The 2205 argon DSS was platesapplied with as a thickness the protective of 8 mm gas were on the selected front and as the back base of thematerial workpieces, (BM), and and its the chemical gas flow compositionrate was 25 was L/min. shown The in specificTable 2. welding The dime parametersnsions of the are BM shown were in 300 Table mm1. × The 100 2205 mm. DSS plates with a thickness of 8 mm were selected as the base material (BM), and its chemical composition was shown in Table2. The dimensions of the BM were 300 mm × 100 mm.

Cooling water tank

Torch

KUKA K-TIG Welding power and controller robot

Industrial computer Argon Figure 1. Schematic diagram of keyhole tungsten inert gas (K-TIG) welding. Figure 1. Schematic diagram of keyhole tungsten inert gas (K-TIG) welding.

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Table 1. Test parameters. Table 1. Test parameters.

WeldingWelding Parameters Parameters Value Value WeldWeld current current 480 480 (A) (A) ArcArc voltage voltage 16.7 16.7 (V) (V) WeldWeld speed speed 280, 300,300, 320, 320, 340, 340, 360 360 (mm/min) (mm/min) Diameter of of tungsten tungsten electrode electrode 6.4 6.4 (mm) (mm) Electrode gap 2.5 (mm) Electrode gap 2.5 (mm) Shielding gas Pure argon (99.9%) Shielding gas Pure argon (99.9%)

Table 2. Chemical composition of the base material (BM).

ElementElement CrCr Ni Ni MnMn MoMo SiSi C C N N S S P P Fe Fe Wt.Wt. % % 22.4622.46 5.705.70 1.4281.428 3.023.02 0.0570.057 0.0190.019 0.1560.156 0.00050.0005 0.0210.021 Bal.Bal.

2.2. Mechanical Mechanical Properties Properties Test and Microstructure The Vickers Vickers microhardness tests were performed on the cross section of the 2205 DSS welded joints. joints. During During the the test, test, a load a load of 100 of 100g and g anda dwell a dwell time timeof 10 ofs were 10 s used were to used meas- to uremeasure the microhardness. the microhardness. The tensile The test tensile specim testens specimens of the 2205 of DSS the 2205welded DSS joints welded were joints pro- videdwere providedaccording according to the American to the American society of society testing of materials testing materials (ASTM) (ASTM) E8 [15], E8the [15 tensile], the testtensile specimen test specimen dimension dimension was shown was shownin Figure in 2. Figure For each2. For set each of parameters, set of parameters, two tensile two specimenstensile specimens are taken are for taken testing, for testing,and the andstrength the strength and elongation and elongation were averaged. were averaged. A scan- ningA scanning electron electron microscope microscope (SEM) was (SEM) applied was applied to observe to observe the morphologies the morphologies of the tensile of the fracturetensile fracture after tensile after tests. tensile tests.

50 mm

32 mm

6 mm 10 mm

R= 6 mm

140 mm Figure 2. TensileTensile test specimen dimension.

The microstructure microstructure of 2205 2205 DSS DSS welded welded join jointsts were were observed observed by by using using optical optical micro- micro- scopescope (OM). 180#, 240#, 400#, 600#, 600#, 800#, 800#, 1000#, 1000#, 1200#, 1200#, 1500#, 1500#, and and 2000# 2000# water water sandpapers were used used to to grind grind the the cross cross section section of ofthe the welded welded joint joint step step by bystep. step. Finally, Finally, a 2 μ am 2 dia-µm monddiamond polishing polishing agent agent was was used used for mechanical for mechanical polishing polishing on the on polishing the polishing machine. machine. Be- Beraha etchant (30 mL H O + 60 mL HCL + 1 g K S O ) was applied to etch the polished raha etchant (30 mL H2O2 + 60 mL HCL + 1 g K22S2O2 5)5 was applied to etch the polished sample,sample, and and the the etch time was ~11 s s [16]. [16]. We We rinsed the welded joints with water after corrosioncorrosion and and then then dried dried them them with with a a hair hair dryer for for observation of of their their microstructure. microstructure. According to ASTM E1245-03 (2016) standard, the austenite content, ferrite content, and two-phasetwo-phase ratio ratio were were analyzed using using Image Pro software [[17].17]. The characteristics of the grain boundary was characterized by electron backscatter diffraction (EBSD) technique. 3. Results and Discussion 3. Results and Discussion 3.1. Inﬂuence of Welding Speed on the Weld Morphology 3.1. Influence of Welding Speed on the Weld Morphology There were obvious differences in the morphology of the weld under different welding speedThere conditions, were obvious as shown differences in Figure3 in. When the morphology the welding of speed the weld was 280–340under different mm/min, weld- the ingfront speed and back conditions, sides of theas shown welds havein Figure reinforcements, 3. When thethe weldwelding seams speed were was well 280–340 formed, mm/min,and the cross the front section and of back the weldssides of were the nail-shaped.welds have reinforcements, As the welding the speed weld increased, seams were the wellhead formed, of the nail-shaped and the cross gradually section narrowof the welds and short, were thatnail-shaped. was, the As weld the width welding gradually speed increased,decreased, the as shownhead of in the Figure nail-shaped4. When grad the weldingually narrow speed and was short, 360 mm/min, that was, thethe crossweld

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width widthgradually gradually decreased, decreased, as shown as shown in Figure in Figure 4. When 4. Whenthe welding the welding speed speedwas 360was 360 mm/min,mm/min,section the cross of the the sectioncross weld section wasof the triangular, ofweld the wasweld and tr wasiangular, the tr workpiecesiangular, and the and wereworkpieces the notworkpieces completely were notwere penetrated.com- not com- pletelypletelyThis penetrated. was penetrated. because This was theThis higherbecause was because welding the higher the speed higher welding results welding speed in a speedresults lack of results in welding a lack in aof heatlack welding of input, welding and the arc penetration ability was weakened. It can be seen that the welding speed can heat input,heat input, and the and arc the penetration arc penetration ability ability was weakened. was weakened. It can Itbe can seen be that seen the that welding the welding signiﬁcantly affect the morphology of the weld. speed speedcan significantly can significantly affect affectthe morphology the morphology of the ofweld. the weld.

FigureFigure Figure3. Cross 3. 3. sectionCrossCross section sectionof 2205 of ofDSS 2205 2205 K-TIG DSS DSS K-TIGwelded K-TIG welded welded joints at joints joints different at at different different weld speeds: weld weld speeds: speeds: (a) 280 ( (mm/min,aa)) 280 280 mm/min, mm/min, (b) 300( (bmm/min,b)) 300 300 mm/min, mm/min, (c) 320 (mm/min,c (c) )320 320 mm/min, mm/min, (d) 340 ( dmm/min, ()d 340) 340 mm/min, mm/min, and (e )and 360 and (mm/min.e) ( e360) 360 mm/min. mm/min.

Figure 4. Weld width at different welding speeds. FigureFigure 4. 4. WeldWeld width at different welding speeds.

3.2. Microstructure3.2.3.2. Microstructure Microstructure and Two-Phase and and Two-Phase Two-Phase Ratio inRatio the WMZ in the WMZ When WhenWhenthe welding the the welding welding speed speedwas speed 360 was wasmm/min, 360 360 mm/min, mm/min, the workpieces the theworkpieces workpieces were not were completely were not completely not completelypen- pen- etrated,etrated,penetrated, which which did whichnot did meet not did the meet not welding meet the welding the requir welding requirements. requirements.ements. Therefore, Therefore, this Therefore, was this conducted was this conducted was only conducted only to studytoonly studythe to microstructure study the microstructure the microstructure and property and property and of property the ofwelded the of welded the joint welded when joint jointwhenthe welding when the welding the speed welding speedwas speed was from 280fromwas to from280 340 to 280mm/min. 340 to mm/min. 340 Figure mm/min. Figure 5 shows Figure 5 shows the5 showsmicrostructure the microstructure the microstructure of the ofBM the ofand BM the WMZ BMand and WMZat dif- WMZ at dif- at ferent ferentdifferentwelding welding weldingspeeds. speeds. The speeds. BM The Thewas BM BMcomposed was was composed composed of austenite of of austenite austenite and ferrite. and and ferrite. ferrite.The light The The phaselight light phase was austenitewas austenite and the and dark the phase dark phase was ferrite; was ferrite; the austenite the austenite was distributed was distributed on the on ferrite the ferrite matrix in the form of long strips, as shown in Figure5a. When the welding speeds were

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matrix in the form of long strips, as shown in Figure 5a. When the welding speeds were 280–340280–340 mm/min, mm/min, the theaustenite austenite in the in the WMZ WMZ exis existsts in three in three forms: forms: intergranular intergranular austenite austenite (IGA),(IGA), grain grain boundary boundary austenite austenite (GBA), (GBA), and and Widmanstätten Widmanstätten austenite austenite (WA). (WA). The The ferrite ferrite in inthe the WMZ WMZ mainly mainly came came from from the the solidifica solidificationtion process process of ofthe the liquid liquid phase, phase, while while the the austeniteaustenite was was mainly mainly produced produced from from the the solid solid phase phase transformation transformation process process of offerrite. ferrite. The The arrangementarrangement of ofatoms atoms at the at the ferrite ferrite grain grain boundary boundary was was disordered, disordered, and and the the free free energy energy waswas relatively relatively high. high. Austenite Austenite precipitates precipitates at the at the grain grain boundary boundary to toform form GBA. GBA. As As the the coolingcooling progresses, progresses, the the austenite austenite content content at th ate the grain grain boundary boundary increased, increased, and and the the location location at atthe the boundary boundary that that was was beneficial beneficial for for nuclea nucleationtion decreased. decreased. Therefore, Therefore, austenite austenite grew grew rapidlyrapidly into into the the ferrite ferrite grains grains in inparallel parallel laths laths at the at the boundary, boundary, so soit formed it formed WA. WA. As As the the temperaturetemperature decreases, decreases, IGA IGA gradually gradually forms. forms. Figure Figure 6 displayed6 displayed the the austenite austenite content, content, ferriteferrite content content and and the the two-phase two-phase ratios ratios of ofthe the BM BM and and the the WMZ WMZ under under different different welding welding speeds.speeds. When When the the welding welding speed speed was was 280–340 280–340 mm/min, mm/min, the the ferrite ferrite content content of ofthe the WMZ WMZ underunder different different welding welding speeds speeds were were significantly significantly higher higher than than that that of of the the BM. BM. The The ferrite ferrite is is aa body-centered body-centered cubic structure andand austeniteaustenite isis aa face-centeredface-centered cubic cubic structure. structure. Therefore, There- fore,the the welds welds without without filling filling metal metal have have reinforcementsreinforcements on the the front front and and back back sides. sides. With With thethe increased increased of ofwelding welding speed, speed, the the content content of ofaustenite austenite and and the the two-phase two-phase ratio ratio in inthe the WMZWMZ gradually gradually decreased, decreased, and thethe contentcontent of of ferrite ferrite gradually gradually increased. increased. This This was mainlywas mainlybecause because as the as welding the welding speed speed increased, increas theed, welding the welding cooling cooling rate increased, rate increased, and austenite and austenitedid not did have not enough have enough time to time precipitated to precipitated from the from ferrite. the ferrite.

Figure 5. Microstructure of 2205 DSS K-TIG welded joint. (a) BM. WMZ under different welding speeds (mm/min): (b) 280, (c) 300, (d) 320, and (e) 340.

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Materials 2021, 14, 3426 6 of 10 Figure 5. Microstructure of 2205 DSS K-TIG welded joint. (a) BM. WMZ under different welding speeds (mm/min): (b) 280, (c) 300, (d) 320, and (e) 340.

Figure 6. Phase content and austenite/ferrite ratio in the BM and WMZ under different welding Figure 6. Phase content and austenite/ferrite ratio in the BM and WMZ under different speeds. welding speeds.

3.3. Grain Boundary Characteristics Park et al. proved that the grain boundaries have a very important influenceinfluence on thethe mechanical and physical propertiesproperties ofof polycrystallinepolycrystalline materialsmaterials [[18].18]. PreviousPrevious studiesstudies showed that the difference in the GBMAGBMA cancan causecause thethe materialmaterial toto produceproduce completelycompletely different deformation behaviors behaviors when when the the mate materialrial was was deformed deformed [19]. [19 ].In Inthe the process process of ofdeformation, deformation, high-angle high-angle grain grain boundaries boundaries (HAG (HAGB;B; misorientation misorientation angle angle greater greater than than 10°) 10can◦) completely can completely hinder hinder the mo thevement movement of dislocations of dislocations [20–2 [2].20 –However,22]. However, the low-angle the low- anglegrain boundaries grain boundaries (LAGB; (LAGB; misorientation misorientation angle angleless than less 5°) than can 5 ◦react) can with react moving with moving dislo- dislocationscations to form to formnew dislocations. new dislocations. Compared Compared with the with LAGB, the LAGB, HAGB HAGB mainly mainly made madecrack crackpropagation propagation more difficult. more difficult. Therefore, Therefore, whether whether the material the material has good has toughness good toughness depends dependson the proportion on the proportion of HAGB. of Under HAGB. the Under same the conditions same conditions of other of pa otherrameters, parameters, the higher the higherthe proportion the proportion of HAGB, of HAGB, the bette ther the better toughness the toughness of the material. of the material. Figure 77 shows shows thethe GBMAGBMA distributiondistribution mapsmaps ofof thethe BMBM andand WMWM underunder differentdifferent welding speeds. In the BM, the GBMA was mainly distributed in HAGB. The proportionproportion ◦ of the Σ3 coincident site lattice grain boundary (CSLGB, misorientation angle equal to 6060°)) was 49.8%, as shownshown in FigureFigure7 7a.a. FigureFigure7 b–e7b–e displays displays the the GBMA GBMA distribution distribution maps maps of of the WM. It can be seen that in all the WMZ, WMZ, the the GBMA GBMA were were mainly mainly distributed distributed in in LAGB. LAGB. The proportion of the Σ3 CSLGB in the WMZ under different welding speeds were 14.2%, 10.1%, 8.8%, and 7.1%, respectively. ItIt provedproved thatthat thethe HAGBHAGB inin thethe WMZWMZ havehave migratedmigrated to LAGB after K-TIG welding, and part of thethe Σ3 CSLGBCSLGB hashas beenbeen consumedconsumed duringduring thethe migration process. 3.4. Mechanical Properties 3.4.1. Microhardness Figure8 shows the microhardness distribution trend map of WMZ under different welding speeds. Take the center line of the weld as the axis of symmetry, and take three points on each side, with an interval of 0.5 mm between each point. When the welding speeds were 280–340 mm/min, the average value of the microhardness of the WMZ were 264.6 ± 3.7 HV, 274.1 ± 3.4 HV, 290.5 ± 3.8 HV, and 300.3 ± 3.8 HV, respectively. However, the average value of the microhardness of the BM was 229.4 ± 2.6 HV. The microhardness of all WMZ were higher than that of the BM. This was because the two-phase ratio of austenite/ferrite in the WMZ decreased significantly after welding. In the microstructure of 2205 DSS, the microhardness of ferrite is greater than that of austenite. Therefore, the microhardness of the 2205 DSS K-TIG WMZ was inversely proportional to the two-phase ratio of austenite/ferrite. The lower the two-phase ratio of austenite/ferrite, the higher the microhardness corresponding to the area. Figure6 shows that as the welding speed increases, the two-phase ratio of austenite/ferrite gradually decreases. Therefore, the microhardness of the WMZ increased with the increase of welding speed.

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Figure 5. Microstructure of 2205 DSS K-TIG welded joint. (a) BM. WMZ under different welding speeds (mm/min): (b) 280, (c) 300, (d) 320, and (e) 340.

Figure 6. Phase content and austenite/ferrite ratio in the BM and WMZ under different welding speeds.

3.3. Grain Boundary Characteristics Park et al. proved that the grain boundaries have a very important influence on the mechanical and physical properties of polycrystalline materials [18]. Previous studies showed that the difference in the GBMA can cause the material to produce completely different deformation behaviors when the material was deformed [19]. In the process of deformation, high-angle grain boundaries (HAGB; misorientation angle greater than 10°) can completely hinder the movement of dislocations [20–22]. However, the low-angle grain boundaries (LAGB; misorientation angle less than 5°) can react with moving dislocations to form new dislocations. Compared with the LAGB, HAGB mainly made crack propagation more difficult. Therefore, whether the material has good toughness depends

Materials 2021, 14on, x FOR the PEERproportion REVIEW of HAGB. Under the same conditions of other parameters, the higher 7 of 10 the proportion of HAGB, the better the toughness of the material. Figure 7 shows the GBMA distribution maps of the BM and WM under different welding speeds. In the BM, the GBMA was mainly distributed in HAGB. The proportion of the Σ3 coincident site lattice grain boundary (CSLGB, misorientation angle equal to 60°) was 49.8%, as shown in Figure 7a. Figure 7b–e displays the GBMA distribution maps of the WM. It can be seen that in all the WMZ, the GBMA were mainly distributed in LAGB. The proportion of the Σ3 CSLGB in the WMZ under different welding speeds were 14.2%, 10.1%, 8.8%, and 7.1%, respectively. It proved that the HAGB in the WMZ have migrated Materials 2021, 14, 3426 7 of 10 to LAGB after K-TIG welding, and part of the Σ3 CSLGB has been consumed during the migration process.

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Figure 7. GBMA distribution maps of the welded joint. (a) BM. WMZ under different welding speeds (mm/min): (b) 280, (c) 300, (d) 320, and (e) 340.

points on each side, with an interval of 0.5 mm between each point. When the welding speeds were 280–340 mm/min, the average value of the microhardness of the WMZ were 264.6 ± 3.7 HV, 274.1 ± 3.4 HV, 290.5 ± 3.8 HV, and 300.3 ± 3.8 HV, respectively. However, the average value of the microhardness of the BM was 229.4 ± 2.6 HV. The microhardness of all WMZ were higher than that of the BM. This was because the two-phase ratio of austenite/ferrite in the WMZ decreased significantly after welding. In the microstructure of 2205 DSS, the microhardness of ferrite is greater than that of austenite. Therefore, the microhardness of the 2205 DSS K-TIG WMZ was inversely proportional to the two-phase ratio of austenite/ferrite. The lower the two-phase ratio of austenite/ferrite, the higher the microhardness corresponding to the area. Figure 6 shows that as the welding speed increases, the two-phase ratio of austenite/ferrite gradually decreases. Therefore, the micro- Figure 7. GBMA distribution maps of the welded joint. (a) BM. WMZ under different welding speeds (mm/min): (b) 280, Figure 7. GBMAhardness distribution of the maps WMZ of the increased welded joint. with (a the) BM. increase WMZ under of welding different speed. welding (c) 300, (d) 320,speeds and ((mm/min):e) 340. (b) 280, (c) 300, (d) 320, and (e) 340.

3.4. Mechanical Properties 3.4.1. Microhardness Figure 8 shows the microhardness distribution trend map of WMZ under different welding speeds. Take the center line of the weld as the axis of symmetry, and take three points on each side, with an interval of 0.5 mm between each point. When the welding speeds were 280–340 mm/min, the average value of the microhardness of the WMZ were 264.6 ± 3.7 HV, 274.1 ± 3.4 HV, 290.5 ± 3.8 HV, and 300.3 ± 3.8 HV, respectively. However, the average value of the microhardness of the BM was 229.4 ± 2.6 HV. The microhardness of all WMZ were higher than that of the BM. This was because the two-phase ratio of austenite/ferrite in the WMZ decreased significantly after welding. In the microstructure of 2205 DSS, the microhardness of ferrite is greater than that of austenite. Therefore, the microhardnessFigureFigure of the 8.8. 2205 MicrohardnessMicrohardness DSS K-TIG distributiondistribution WMZ wa trendtrends inversely mapmap ofof proportionalWMZ WMZunder underdifferent different to the two-phase welding welding speeds. speeds. ratio of austenite/ferrite.3.4.2. Tensile The Property lower the two-phase ratio of austenite/ferrite, the higher the microhardness corresponding to the area. Figure 6 shows that as the welding speed in- Table3 displays the tensile test results of 2205 DSS BM and K-TIG welded joints under creases, the two-phase ratio of austenite/ferrite gradually decreases. Therefore, the micro- different welding speeds. The tensile strength of the BM was 804.0 ± 2.12 MPa. Under hardness of the WMZ increased with the increase of welding speed. different welding speed conditions, the tensile strength of 2205 DSS K-TIG welded joints were signiﬁcantly different. When the welding speeds were 280–320 mm/min, the tensile strength of the welded joints gradually decreased, and the tensile fracture positions were all in the WMZ. It can be seen that when the welding speeds were 280–320 mm/min, the tensile strength of WMZ in the welded joints were lower than that of the BM. This

Figure 8. Microhardness distribution trend map of WMZ under different welding speeds.

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was mainly because the grains of the WMZ grew up under the action of the welding thermal cycle. The coarse grain size can also reduce the tensile strength of the welded joint. When the welding speed was increased to 340 mm/min, the tensile strength of the welded joint was 800.3 ± 1.76 MPa, and the tensile fracture position was the BM, which was far away from WMZ. At this time, the tensile strength of the WMZ in the welded joint was signiﬁcantly greater than that of the BM. Therefore, the tensile fracture position occurs in the BM. This was because as the welding speed increased, the content of ferrite in the WMZ increased, and the content of austenite decreased. As ferrite has a body-centered cubic structure, the ferrite in the DSS determined the tensile strength of the material. In addition, as the welding speed increased, the cooling rate increased, and the grains in the WMZ did not have enough time to grow. Therefore, the tensile strength of the WMZ gradually increased as the welding speed increased. Combining the characteristics of the grain boundary in the WMZ under different welding speeds, it can be seen that the increased of the LAGB proportion and decreased of the Σ3 CSLGB proportion no signiﬁcant effect on the tensile strength of the welded joint.

Table 3. Tensile test results of the 2205 DSS BM and welded joint.

Welding Speed Tensile Strength Tensile Fracture Test Area Elongation (%) (mm/min) (MPa) Position 280 752.3 ± 1.98 27.9 ± 0.99 WMZ 300 789.5 ± 1.34 24.7 ± 0.99 WMZ Weld joint 320 794.5 ± 1.69 23.1 ± 0.42 WMZ 340 800.3 ± 1.76 32.3 ± 0.57 BM BM / 804.0 ± 2.12 46.6 ± 0.71 /

The elongation of the BM was 46.6 ± 0.71%. when the welding speeds were 280–340 mm/min, the elongation of the welded joint were 27.9 ± 0.99%, 24.7 ± 0.99%, 23.1 ± 0.42%, and 32.3 ± 0.57%, respectively. When the welding speeds were 280–320 mm/min, the elongation of welded joints were gradually reduced. This showed that when the welding speeds were 280–320 mm/min, the plasticity of the welded joint decreased as the welding speed increased. However, when the welding speed was 340 mm/min, the elongation of the welded joint was larger. It was mainly because when the welding speed was 340 mm/min, the tensile strength of WMZ was better than that of the BM, so the tensile fracture position occurred in the BM. Therefore, the amount of deformation of the welded joint during the tensile process was different. At this time, the elongation cannot be used to compare and analyze the plasticity of welded joint. Figure9a shows the morphologies of tensile fracture of BM. The tensile fracture surface of the BM has a typical ductile fracture morphology. Figure9b,c shows the morphologies of tensile fracture of WMZ in the welded joints under welding speeds were 280–320 mm/min. Figure9e shows the morphology of tensile fracture of BM in the weld joint when the welding speed was 340 mm/min. When the welding speeds were different, the tensile fracture positions of the welded joints were different. However, all the tensile fracture surfaces of the welded joints showed dimples and tearing edges, which indicated that plastic deformation occurred at the fracture positions of the welded joints before the tensile fracture. Therefore, it can be explained that the 2205 DSS K-TIG welded joints have good plasticity. Materials 2021, 14, 3426 9 of 10 Materials 2021, 14, x FOR PEER REVIEW 9 of 10

Figure 9. SEM image of tensile fracture. (a) BM. Welded joints under different welding speeds Figure(mm/min): 9. SEM (b image) 280, (ofc) tensile 300, (d )fracture. 320, and (a ()e )BM. 340. Welded joints under different welding speeds (mm/min): (b) 280, (c) 300, (d) 320, and (e) 340. 4. Conclusions 4.1. ConclusionsWhen the welding speeds were 280–340 mm/min, the front and back sides of the 2205 DSS welds were well formed. 1. When the welding speeds were 280–340 mm/min, the front and back sides of the 2205 2. Under different welding speeds, the austenite morphologies of the WMZ mainly exist DSS welds were well formed. in three forms: WA, GBA, and IGA. The content of austenite and the ratio of the 2. Under different welding speeds, the austenite morphologies of the WMZ mainly ex- two-phase in the WMZ decreased, the content of ferrite in the WMZ increased as the ist in three forms: WA, GBA, and IGA. The content of austenite and the ratio of the welding speeds increased. two-phase in the WMZ decreased, the content of ferrite in the WMZ increased as the 3. When the welding speeds were 280–340 mm/min, the proportion of Σ3 CSLGB de- weldingcreased speeds as the weldingincreased. speed increased. However, the change of Σ3 CSLGB proportion

3. Whenhas no the signiﬁcant welding speeds effect on were the tensile280–340 strength mm/min, of weldedthe proportion joints. of Σ3 CSLGB de- 4. creasedWhen as the the welding welding speed speed increased increased. from However, 280 mm/min the change to 340 of mm/min,Σ3 CSLGB the propor- micro- tionhardness has no ofsignificant the WMZ effect and the on tensilethe tensile strength strength of the of welded welded joint joints. gradually increased. 4. WhenThe 2205the welding DSS K-TIG speed welded increased joints from have 280 good mm/min plasticity. to 340 mm/min, the microhardness of the WMZ and the tensile strength of the welded joint gradually increased. The 2205 DSS K-TIG welded joints have good plasticity.

Materials 2021, 14, 3426 10 of 10

Author Contributions: Methodology, D.P.; data curation, Z.Z.; writing—original draft preparation, S.P.; writing—review and editing, S.C.; funding acquisition, S.C. All authors have read and agreed to the published version of the manuscript. Funding: This work was financially supported from the [Science and Technology Base and Talent Special Project of Guangxi Province] under Grant [number AD19245150] and [Guangxi University of Science and Technology Doctoral Fund] under Grant [number 19Z27]. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data sharing is not applicable to this article. Conflicts of Interest: The authors declare no conflict of interest.

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