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04. Effects of microstructure alteration on corrosion behavior of welded joint in API X70 pipeline steel - Bordbar - 2013.pdf

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Effects of microstructure alteration on corrosion behavior of welded joint in API X70 pipeline steel Sajjad Bordbar b , Mostafa Alizadeh a,b,⇑ , Sayyed Hojjat Hashemi c a Department of Metals, International Centre for Science, High Technology & Environmental Sciences, PO Box 76315-117, Kerman, Iran b Department of Materials Science and Engineering, Kerman Graduate University of Technology, PO Box 76315-115, Kerman, Iran c Department of Mechanical Engineering, The University of Birjand, PO Box 9
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  Effects of microstructure alteration on corrosion behavior of welded jointin API X70 pipeline steel Sajjad Bordbar b , Mostafa Alizadeh a,b, ⇑ , Sayyed Hojjat Hashemi c a Department of Metals, International Centre for Science, High Technology & Environmental Sciences, PO Box 76315-117, Kerman, Iran b Department of Materials Science and Engineering, Kerman Graduate University of Technology, PO Box 76315-115, Kerman, Iran c Department of Mechanical Engineering, The University of Birjand, PO Box 97175-376, Birjand, Iran a r t i c l e i n f o  Article history: Received 24 July 2012Accepted 18 September 2012Available online 6 October 2012 Keywords: SteelGas pipelineCorrosion resistanceHeat treatment a b s t r a c t In the present work, a heat treatment process was used to modify corrosion behavior of heat affectedzone (HAZ) and weld metal (WM) in welded pipe steel of grade API X70. A one-step austenitizing withtwo-step quenching and subsequent tempering treatment was performed to alter the microstructureof HAZ and WM. The hardness and strength values were controlled to be in the standard range aftertheheattreatment process. Inorder toinvestigatethe effect of the heattreatment onthe corrosionprop-erties of welded joint, the samples were immersed in a mixture of naturally aerated 0.5M sodium car-bonate (Na 2 CO 3 ) and 1M sodium bicarbonate (NaHCO 3 ) solution with pH of 9.7 for 45days. Theelectrochemical impedance spectroscopy (EIS) measurements were carried out then to study the protec-tivepropertiesofthecorrosionproductslayer.TheX-raydiffraction(XRD)investigationdepictedthatthecorrosion products layer composition includes FeCO 3 , FeO(OH), Fe 3 O 4  and Fe 2 O 3 . The EIS results showedthat, the corrosion resistance of HAZ and WM increased after heat treatment. This can be attributed toformation of uniformly distributed polygonal ferrite (PF) and to the decrease in the volume fraction of bainite (B) after heat treatment.   2012 Elsevier Ltd. All rights reserved. 1. Introduction Generally in pipeline industry, coating and cathodic protectionare used together to maintain the integrity of buried pipelines.An incompatible cathodic protection and also a disbanded coatingcan lead to formation of a local corrosive environment under thedisbanded coating. In other words, the disbanded coating can bean appropriate place for corrosion, especially localized corrosion[1,2]. It has been reported that stress corrosion cracking (SCC) of buried pipelines (i.e., high-pH SCC and near-neutral pH SCC) ishighly dependent on the local environment developed under thedisbandedcoating[3–5].Thehigh-pHSCCofburiedpipelinestakesplace commonly in a concentrated carbonate/bicarbonate solutionin the pH range of 9–11, under a disbanded coating [6]. In particu-lar, most of SCC damages in the pipelines are observed under highpHconditions[7]. Anodicdissolutionisthecommonmechanismof high-pH SCC in the pipelines [8,9] where formation and rupture of apassivefilmisfrequentlyoccurred[10].Thecharge-transferreac-tions and mass-transfer process in a thin solution layer results in acomplicated condition for investigating the corrosion of steel un-deradisbandedcoating[11,12].Inthecarbonate/bicarbonatesolu-tion, the bicarbonate species plays a critical role in the dissolutionreactions at internal and external sides of pipeline structures.Welding is the most commonly technique which is used forconstruction of long-distance pipeline projects. Due to weldingprocess, the microstructure and the mechanical properties of weldedzonedifferssignificantlyfromthoseofthebasemetal.Con-sequently,thecorrosionbehavioroftheweldedzoneisexpectedtobe different from the other zones in corrosive media [13].Due to different corrosion activities in the various zones of thewelded steel, the corrosion product layers with different thick-nesses and protective properties are formed in the various weldsub-zones [14]. Electrochemical characterizations have been re-vealed that, the base metal (BM) has higher charge-transfer resis-tance with respect to the HAZ and WM [14]. This makes theanodic dissolution activity of HAZ and WM to be higher than thatof the BM. This behavior can be related to the metallurgical trans-formations across the WMand HAZ [14]. Also it has been reportedthat, the corrosion product layer protects the steel surface fromcorrosive species through a physical blocking effect. In this rela-tion, the structure of the corrosion product layer plays an essentialrole in the corrosion mode of the steel [13]. 0261-3069/$ - see front matter    2012 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.matdes.2012.09.051 ⇑ Corresponding author at: Department of Metals, International Centre forScience, High Technology & Environmental Sciences, PO Box 76315-117, Kerman,Iran. Tel.: +98 3426226611, mobile: +98 9133541004; fax: +98 3426226617. E-mail addresses:  mostafa_alizadeh56@yahoo.com, alizadeh@icst.ac.ir (M. Ali- zadeh).Materials and Design 45 (2013) 597–604 Contents lists available at SciVerse ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes  The main goal of the present study is to modify the corrosionbehaviorofweldedpipesteelproducedinusingathermo-mechan-ical control-rolled API X70 steel. In fact, the objective of this workis to design an appropriate heat treatment cycle to get a suitablemicrostructure and uniform hardness, thus enhancing the corro-sionresistanceof the weldedjoint of X70 steel. To dothis, the pro-tective properties of corrosion products layer generated on thewelded joint before and after heat treatment are investigateseparately. 2. Experimental details  2.1. Test material The material under investigation was API grade X70 gas pipe-line with 1422mm outside diameter and 19.8mm wall thicknessformed by spiral welding. The srcinal coil used for pipe manufac-ture produced by thermo-mechanical control-rolled process(TMCR). The chemical analysis of BM was determined by opticalemission spectroscopy. The measured chemical composition is gi-ven in Table 1 together with target values for the test materialspecified by API 5L  [15]. Note that all elements had measured val-ues below (or close to) the maximum values set by standard code.ThepipelinewasweldedwithadoubleV-shapeof weldpool bythe submerged arc welding (SAW) technique. In the SAW process,bothweldelectrode andthe BMare meltedbeneatha layer of flux.This layer protects the weld metal from contamination and con-centrates the heat into the joint. The molten flux rises throughthe weld pool, deoxidising and cleaning the molten metal. Twoweld passes were applied to complete the joint. Four-wire sub-merged arc welding with low carbon content wires were used forwelding. The measured chemical composition of the fusion zone(using optical emission spectroscopy) together with target valuesspecifiedbyAPI 5L  [15] aregiven inTable 1. Note that all elements had measured values below the maximum values of the standardcode.  2.2. Heat treatment procedure A100  20  19.8mmspecimenwas obtainedfromtheweldedpipe so that the weld metal was placed in the middle of the spec-imen. Beforeheat treatment, thissampleis calledas-receivedweld joint and after heat treatment this sample is called heat treatedweld joint. Fig. 1a depicts the as-received welded joint aftermacro-graphy in 2% nital solution as suggested by ASM MetalsHandbook [16]. As can be seen in this figure, the as receivedwelded joint includes three recognizable zones of BM, HAZ andWM. A one-step austenitizing with two-step quenching and tem-pering treatment was performed on the as-received specimen asshown schematically in Fig. 2.  2.3. Mechanical properties The Vickers hardness test and standard tensile experimentswere performed on test material to measure its mechanical prop-erties for both as-received and heat treated weld joints. Everyhardness data was an average of three measurements with 100Nindentation load (HV10). The tensile samples (with 50mm gaugelength and 10mm gauge diameter) were machined in the loopdirection before and after heat treatment from the srcinal pipeas suggested by API 5L standards [15]. To conduct the tensileexperiments, an INSTRON 5586 testing machine under low dis-placement rate of 0.05mm/s at roomtemperature was used. Inor-dertoensurethattheweldedjointwaslocatedinthemiddleofthespecimens, the tensile specimens were etched in 2% nital solution.This revealed the desired zones as shown in Fig. 3.  2.4. EIS measurements of corrosion product  The test samples (of 7  7  3mm dimensions) were cut fromBM, HAZ and WM of both as-received and heat treated welded joint. The samples were soldered to copper wires and thenmounted in cold-cured epoxy resins. They were sequentially wet-grounded with 120, 320, 500 and 1000 grit silicon carbide emerypapers and then decreased ultrasonically with ethyl alcohol for10min. Afterwards, they were rinsed with distilled water and fi-nally dried with cool air. The behavior of corrosion products layer  Table 1 Chemical composition of the base metal, welding wire and target values specified by API 5L. Cu V Cr Ni Ti Mo Nb Al S P Si Mn C Element0.01 0.04 0.01 0.18 0.018 0.24 0.05 0.03 0.015 0.008 0.2 1.5 0.05 wt.% (BM)0.036 0.03 0.015 0.13 0.009 0.31 0.03 0.02 0.003 0.008 0.25 1.4 0.06 wt.% (WM)– – – – 0.06 – – – 0.015 0.025 – 1.4 0.24 Maximum Fig. 1.  The macro-etched welded joint and the procedure of sample preparation forcorrosion test. Fig. 2.  The schematic illustration of heat treatment cycle.598  S. Bordbar et al./Materials and Design 45 (2013) 597–604  formation was studied in a mixture of naturally aerated 0.5M so-dium carbonate (Na 2 CO 3 ) and 1M sodium bicarbonate (NaHCO 3 )solution with pH of 9.7 after 45days immersion.Electrochemical impedance spectroscopy (EIS) measurementswere conducted using a typical three-electrode electrochemicalcellsystemwiththesteelspecimenastheworkingelectrode,asat-urated calomel electrode as the reference electrode and a coiledplatinum wire as the counter electrode. EIS measurement fre-quency was selected to be in the range of 100kHz to 10MHz withan applied AC perturbation of 10mV. The ZSimpWinV3.21 imped-ance analysis software was used to fit the achieved data. 3. Results and discussion  3.1. Microstructural observation The microstructures of the as received and the heat treated BM,HAZ and WM were observed by using scanning electron micros-copy (SEM). Fig. 4 shows three main recognizable zones of theas-received welded joint. The as-received BM zone exhibits amicrostructure including very fine grains of bainite and acicularferrite (AF) as shown in Fig. 4a. This microstructure caused byTMCR process under which the base metal was produced. TheHAZ microstructure contained a mixture of acicular ferrite andbainitic ferrite (BF) as shown in Fig. 4b. The grain size of HAZmicrostructure was considerably more than that of the BM area.As it can be seen in Fig. 4c, the melted and the resolidified WMzone microstructure included mainly acicular ferrite and grainboundary ferrites (GBFs), such as Widmanstatten and polygonalferrites.Fig. 5 shows the SEM microstructure of heat treated BM, HAZand WM. comparing the Fig 4a with Fig 5a revealed that, the microstructure of heat treated BM differ with that of as-receivedBM in the grain size. But they are similar to each other in the typeof phases. As Fig. 5b depicts, the microstructure of heat treatedHAZincludedaconsiderableamountofuniformlydispersedpolyg-onal ferrite, acicular ferrite and fine bainite. In other words, it dif-fered with the microstructure of as-received HAZ. Also, themicrostructure of heat treated WM had mainly differences withthat of as-received WM. In spite of as-received WM, the ferritesin the heat treated WM did not locate in the grain boundary.  3.2. Hardness profile and mechanical properties Fig.6comparesthehardnessprofilemeasuredinthemid-thick-ness of the as-received and the heat treated welded joint. Afterheat treatment, the BM hardness was decreased slightly due tograingrowthof steel matrix(seeFig. 5a). Consideringthehardnessprofileofas-receivedweldedjoint,theminimumvalueofhardnesswas related to HAZ. As it has been reported elsewhere [17,18], thepresence of fine precipitates such as NbC, VN and TiN in the pri-mary sheet led to grain boundary pining. The welding thermal cy-cle provided an adequate driving force for grain growth bycoarseningandpartially/completelydissolutionoftheprecipitates,this caused reduction of hardness in the HAZ in comparison withthe BM [17,18]. After heat treatment, the HAZ hardness was in-creased due to formation of bainite and the refined grain sizemicrostructure, asshowninFig. 5b. The WMhadthe highesthard-ness in both as-received and heat treated specimens. The as-re-ceived WM hardness of 228 HV in its centre line can beattributed to the presence of lower temperature transformationproducts such as Widmanstatten ferrite and bainite [19]. In addi-tion to microstructural transformation, plastic deformations duetoresidualstresses increasedtheWMhardness. Asaresult ofplas-ticdeformations,thedislocationdensityincreasedthroughoutWM Fig. 3.  The tensile sample representing various zones of welded joint. Fig. 4.  The SEM micrographs of as-received (a) BM, (b) HAZ and (c) WM. S. Bordbar et al./Materials and Design 45 (2013) 597–604  599  [17,19]. The hardness of WMdecreased during heat treatment dueto removing the residual stress, reduction of lattice defects gener-ated during welding, grain growth and formation of considerableferrite in the microstructure, as shown in Fig. 5c.Thetensilestress–strainbehaviorsof as-receivedandheat trea-ted welded joint are presented in Fig. 7. The yield and tensilestrength of as-received sample were higher than that of heat trea-ted sample while the elongation of as-received sample was lessthan the heat treated sample. In fact, the acicular ferrite causedhigher yield and tensile strength in the as-received welded joint.Increasing the volume fraction of polygonal ferrite led to decreas-ing the strength and increasing the elongation of heat treatedweldedjoint.Inspiteoftheas-receivedweldedjoint,theheattrea-ted welded joint exhibited yield point phenomenon. This may beattributed to elimination of secondary phases and formation of polygonal ferrite [20].Investigation of the mechanical properties of heat treatedwelded joint revealed that, the heat treatment cycle designed inthe present work (see Fig. 2) was a proper cycle. In other words,the hardness data of heat treated welded joint satisfied the maxi-mum hardness limitation of 350 HV given by API standard code[15]. Also, the hardness profile of the heat treated welded jointwas more uniform with respect to the as-received welded joint.Moreover, the tensile properties of heat treated samples were con-sistent with the API specifications (yield strength>483MPa, ten-sile strength>565MPa) for X70 steel pipeline [15].  3.3. EIS measurements The EIS investigations were done to study the protective prop-erties of the corrosion products layer. In the first step the as-re-ceived and heat treated BM, HAZ and WM were immersed in amixture of 0.5M Na 2 CO 3  and 1M NaHCO 3  solutions for 45days.A corrosion products layer was generated uniformly in macro-scopic scale on the surface of the specimens in this period of time. Fig. 5.  The SEM micrographs of heat treated (a) BM, (b) HAZ and (c) WM. Fig. 6.  The hardness profile of as-received and heat treated welded joints. Fig. 7.  The nominal stress–strain behavior of as-received and heat treated welded joint.600  S. Bordbar et al./Materials and Design 45 (2013) 597–604
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