The aim of this paper is to define optimization of welding conditions for Submerged Arc Welding (SAW) of steel pipes for gas transportation. Fine grain steel X-52 with thickness of 8 mm were used as a base material. Welding was performed from inner and outer side. Two wires, inclined under different angles, were feed separately. Eleven samples divided in three series were experimentally welded. Performed investigations indicated that the best properties showed weldments from series III, welded with the highest heat input. On the contrary of our expectations, welds from series II, using self made equipment, showed pretty bead properties and improper geometry. So, improving of this this equipment and obtaining welds with better properties is the target in future investigations.Cilj ovog rada je definirati optimizaciju parametara zavarivanja pod praškom čeličnih cijevi za plinovode. Finozrnati čelik X-52 debljine 8 mm je korišten kao osnovni materijal. Zavarivanje je izvedeno s vanjske i unutarnje strane. Dvije žice, pod različitim kutom su dodavane odvojeno. Jedanaest uzoraka, podeljenih u tri serije ekserimentalno je zavareno. Ispitivanja su pokazala da najbolja svojstva imaju zavari iz serije III, zavareni s najvećom količinom unešene toplote. Suprotno očekivanjima, zavari iz serije II, kod kojih je korištena oprema koju su izradili autori rada, pokazali su vrlo loše karakteristike i neadekvatnu geometriju spoja. Stoga je osnovni cilj u sljedećim istrazivanjima poboljšanje ove opreme u cilju dobijanja kvalitetnijih zavara

D. Slavkov

S. Cvetkovski

V. Grabulov

Z. Odanovic

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S. CVETKOVSKI, V. GRABULOV, Z. ODANOVIC, D. SLAVKOVOPTIMIZATION OF WELDING PARAMETERSFOR GAS TRANSPORTATION STEEL PIPESReceived – Prispjelo: 2009-09-22Accepted – Prihva}eno: 2009-11-05Preliminary Note – Prethodno priop}enjeINTRODUCTIONSteel pipes still have the main roll in petrochemicalindustry. Submerged Arc Welding (SAW) processcould be treated as one of the most important processesfor obtaining longitudinally welded steel pipes, mainlydue to the very good quality of welded joints and highproductivity. However the defects in welded joints oftenlead to damage of installations and even more dangerous- human victims 1-3.Therefore the optimization of welding parameters isthe most important task in order to obtain welded pipeswith high exploitation safety, which is also the main aimof the investigations presented in this paper.SETUP OF EXPERIMENTSteel plates of 8 mm thickness with designationX-52, were used as a base material for the production ofwelded pipes for gas transportation. Specification ofsteels for pipeline production, for petrochemical indus-try is in accordance with American standard API 5L 4.According to this standard, “X” represents longitudi-nally welded pipes, and “52” shows minimal tensilestrength of 520 N/mm2. Chemical composition and me-chanical properties of the base material are given in Ta-bles 1 and 2.Prior to welding, plates of a base material wereformed as pipe segments and tack welded using ShieldMetal Arc Welding (SMAW) process with basic elec-trode, classified as E 42 4 B 32 H 5 according EN 499,( 3,25 mm). SAW welding process was performed us-METALURGIJA 49 (2010) 4, 331-334 331The aim of this paper is to define optimization of welding conditions for Submerged Arc Welding (SAW) of steelpipes for gas transportation. Fine grain steel X-52 with thickness of 8 mm were used as a base material. Wel-ding was performed from inner and outer side. Two wires, inclined under different angles, were feed separa-tely. Eleven samples divided in three series were experimentally welded. Performed investigations indicatedthat the best properties showed weldments from series III, welded with the highest heat input. On the contraryof our expectations, welds from series II, using self made equipment, showed pretty bead properties and im-proper geometry. So, improving of this this equipment and obtaining welds with better properties is the targetin future investigations.Key words: welding parameters, microalloyed steel, tandem processOptimizacija parametra zavarivanja ~eli~nih cijevi za plinovode. Cilj ovog rada je definirati optimizaci-ju parametara zavarivanja pod pra{kom ~eli~nih cijevi za plinovode. Finozrnati ~elik X-52 debljine 8 mm jekori{ten kao osnovni materijal. Zavarivanje je izvedeno s vanjske i unutarnje strane. Dvije `ice, pod razli~itim ku-tom su dodavane odvojeno. Jedanaest uzoraka, podeljenih u tri serije ekserimentalno je zavareno. Ispitivanja supokazala da najbolja svojstva imaju zavari iz serije III, zavareni s najve}om koli~inom une{ene toplote. Suprotnoo~ekivanjima, zavari iz serije II, kod kojih je kori{tena oprema koju su izradili autori rada, pokazali su vrlo losekarakteristike i neadekvatnu geometriju spoja. Stoga je osnovni cilj u sljede}im istrazivanjima pobolj{anje oveopreme u cilju dobijanja kvalitetnijih zavara.Klju~ne rije~i: parametri zavarivanja, mikrolegirani ~elik, zavarivanje s dvije `iceISSN 0543-5846METABK 49(4) 331-334 (2010)UDC – UDK 621.791.75:643.2-034.14 =111S. Cvetkovski, D. Slavkov, Faculty of Technology and Metallurgy,Skopje, Republic of Macedonia; V. Grabulov, Z. Odanovic, Institute formaterials testing, Republic of SerbiaTable 1 Chemical composition of base material for pi-peline production, steel X52 4Chem.elem.C Si Mn P S Nb TiMas.% 0,059 0,27 0,9 0,007 0,013 0,023 0,01Table 2 Mechanical properties of base material in rol-ling (R) and transverse (T) directions, steelX52 4Re / N/mm2 Rm / N/mm2 A5 / % KV / J -40 °CR dir. 439 510 27,0 275, 301, 341, 305T dir. 422 492 28,0 174, 170, 171, 171ing welding wire L-70 (4 mm) with chemical composi-tion given in Table 3 4, 5. The welding wire has thefollowing mechanical properties: Re = 400 N/mm2, Rm =520 N/mm2, A5 = 25 % and KV = 133 J at -40 °C.The welding wire L-70 is generally used in order tosatisfy the high toughness requirement of weldingjoints. Neutral welding flux Lincoln 995 (granulation:0,2-2,5 mm) was used for single layer butt welding. Theflux can be used five times in welding process, and it isrecommended to perform draying at temperature of 300°C/2h before welding 5. Welding was conducted at theboth sides of the pipe, with two wires for each run,which is known as tandem process. The first wire wasconnected to direct current (DC, + pole) and the secondwire to the alternating current (AC). There was no gapbetween the edges of the pipes. Experimental welding ofthe steel pipes was performed for 11 samples, welded onautomatic SAW machines ELLIRA 6. One machinewas used for inner and other for outer welding, while themaximum current for the both machines was 1200 A.The first weld was performed from inner side and wasmade under the same conditions for all segments.Inclination of torches, distance between torches andtip of the wire (stick out) is shown in Figure 1.The following parameters were used for this weldingprocess: Wire I (welding current 460 A, welding voltage26 V, welding speed 1,16 m/min) and Wire II (weldingcurrent 480 A, arc voltage 32 V and welding speed 1,16m/min). Since the basic task in this investigation is toobtain optimal geometry of outer welds, three types ofexperiments (series) were performed.Series I. Welding was performed under the condi-tions shown in Figure 2, with manual regulation ofwelding geometry along the weld length. Two segments1 and 2 were welded in this way. Welding parameterswhich were used are given in Table 4. As can be seenfrom the table 4, higher amperage and arc voltage wereused for segment 2.Series II. Welding was performed using self-madeequipment which aimed to maintain the uniform geome-try. Four segments with different geometry were weldedusing this approach. Segments 3 and 4 were welded asshown in Figure 2, while segments 5 and 6 were weldedas shown in Figure 3.Series III. Five segments (7-11) were welded underthe conditions shown in Figure 3, with manual maintainof welding geometry. In order to obtain dipper penetra-tion for this series, higher welding parameters wereused. The torch angle for the first and second wire was900 and 600, respectively. Welding parameters for seg-ment 7-11 are given in Table 6.Macro, micro and fractographic analysis were per-formed to the welded joints produced in these investiga-tions. Testing of the mechanical properties and hardnesswere also performed at the sample of the Serie III.332 METALURGIJA 49 (2010) 4, 331-334S. CVETKOVSKI et al: OPTIMIZATION OF WELDING PARAMETERS FOR GAS TRANSPORTATION STEEL PIPESTable 3 Chemical composition of weld metal (L-70wire) 4, 5Ch. elem. C Mn SimaxMass. % 0,109 0,91 0,14Momax Cumax Smax Pmax0,5 0,77 0,009 0,007Figure 1 Welding geometry for inner tandem weldingTable 4 Welding parameter for segments 1 and 2from Series ISerNoWeld. Param.Segment 1 Segment 2Wire I Wire II Wire I Wire IIICurrent / A 460 480-500 510 520Arc voltage / V 26 28 30-32 36-38Weld speed / (m/min) 1,12 1,12 1,12 1,12Figure 2 Welding geometry for series II, (left)Figure 3 Welding geometry for series III, (right)Table 5 Welding parameter for segments 3, 4, 5 and 6from Series IISerNoWeld. Param.Segment 3 and 4 Segment 5 and 6Wire I Wire II Wire I Wire IIIICurent / A 510 500-520 680 580Arc voltage. / V 30 36 30-32 40-42Weld speed / (m/min) 1,12 1,06 1,12 1,16Table 6 Welding parameters for segment 7-11, SeriesIIISer.NoWeld. Param.Segment 7Wire I Wire IIIII Current / A 680 580Arc voltage. / V 32 40Weld speed / (m/min) 1,4 1,4ANALYSIS OF RESULTSMacrostructure investigations of all welded jointswere performed and the results for Series I to III are pre-sented in Figures 4 to 6. Metallographic analyses of theSeries I segments has shown low penetration betweenthe welds, especially for the first segment (Figure 4a),because of the low current 460 A. In addition, misalign-ment between the welds was observed (the second seg-ment, Figure 4b). Reinforcement of the faces were 1,8 -2 mm for the first and 1,8 - 2,5 mm for the second seg-ment. It is observed that weld shapes in both segmentsare not regular. The row of pores in both segments wasobserved by radiographic control. Pores can be seen onmetallographic specimen in Figure 4 (arows).Macroscopic results of the Series II are presented inFigure 5. Rows of pores, insufficient root penetrationand misalignment of welds are the defects which werefound in all welded segments. In addition, it was shownthat with this equipment and welding procedure thisequipment was not able to completely maintain the uni-form geometry. Higher penetration which can be seen inFigure 5 is a result of the fact that probes are taken at thebeginning of the weld, when the geometry of welding isstill irregular. Proper penetration between the welds isobtained in Serie (Figure 6), as a result of the higherwelding parameter use. Dimensional, radiographic andmetallographic control was conducted for each segment.Radiographic control showed small number of isolatedpores in the welds. Weld penetration was recorded to bebetween 4,2 and 5 mm and all segments showed goodwelded joints characteristics. It was observed that thesegment 7 in Figure 6a had the best characteristics.Welding speed during this process was 1,4 m/min. Ascan be seen from Figure 6b, segment 10 showed just lit-tle lower penetration.Metallographic observations were conducted on seg-ment 7, (Series III), in order to analyze microstructuralcharacteristics of different areas in SAW joints. Figure 7shows the locations of the microstructural observations.Microstructure of those areas of welded joint can beseen in Figure 8 (a-f).Figure 8a show dendritic microstructure of weldmetal. It was observed that the elongated dendrites prop-agate parallel with cooling direction. Traces ofproeutectoide ferrite (PF) on the boundaries of primaryaustenitic grains, and acicular ferrite (AF) inside thegrains can be seen too 8. Micro photo, Figure 8b pres-ents the point of penetration between inner and outerwelds. The influence of the second (outer) weld to thefirst one is clearly seen on the image. In the upper part ofthe picture, microstructure is dendritic, but in the lowerpart it can be seen that dendritic microstructure is par-tially destroyed as a result of a heat input of the secondweld. Reaustenitisation contribute to the formation ofequiaxed grains as seen in coarse grained HAZ.Microstructure in Figure 8c shows the coarse grainedHAZ. It consists of coarse, equiaxed grains of primaryaustenite formed as result of retransformation. Proeu-tectoide ferrite and Widmanstaten ferrite are formed onthe boundary of primary austenitc grains (black arrows)9, 10. Micro constituent found inside the grains isidentified to be acicular ferrite. Point 8d represent finegrained normalized HAZ which generally poses verygood mechanical properties, strength and impact tough-ness 9. Figure 8e presents intercritically HAZ i.e. zoneof partially transformation of perlite where max. tem-perature is between A1 and A3 points. Figure 8f showsmicrostructure of base material X52 with very finegrains (9-10 according to ASTM), which was not ex-posed to the influence of the welding thermal cycle.Furthermore Charpy impact testing was performedwith the notch located in different areas (weld metal,heat affected zone and base metal) followed byfractographic analyses. Fractographic analyses of thefractured surfaces (segment 7 serie III are presented inFigure 9. Figure 9a shows that base material has a duc-tile (dimple) type of fracture.METALURGIJA 49 (2010) 4, 331-334 333S. CVETKOVSKI et al: OPTIMIZATION OF WELDING PARAMETERS FOR GAS TRANSPORTATION STEEL PIPESFigure 4 Macroscopic observation of segments 1 and 2from series I weldments (a, b respectively), et-ched in NitalFigure 5 (a and b) Macro photos of series II weldments,segments 3 and 5, etched in NitalFigure 6 (a and b) Macro photos of series IIIweldments, segments 7 and 10, etched in NitalFigure 7 Macro photo ofwelded joint (segment 7, se-rie III) and locations of micro-structural observations,etched in NitalThe weld metal shows mixed type of fracture, con-taining both areas of brittle and ductile fracture (see Fig-ure 9b). It was observed that the micro crack found inthis region is parallel to the proeutectoid ferrite, markedwith black arrow.Testing of mechanical properties was performed ac-cording to GOST 20295 7. These investigations con-cerns segments 7-11 from Series III. Generally the fol-lowing values are obtained Re = 389-403 N/mm2, Rm=519-531 N/mm2, A5 = 27-31% and KU = 106-125 J, at-40 oC. Hardness measurements (Vickers method) wereperformed on the cross section containing base metaland weld joint. The measurements were defined in away to investigate all characteristic zones of weldedjoint. In summary, it can be said that the lowest hardnessvalues were recorded in the base metal (145-160 HV)because of the low carbon content, while the highest val-ues were found in the weld metal (200 HV). It shouldbe noted that increase in hardness in the HAZ was notrecorded. Generally, the highest measured value is 202HV, which is much lower than 300 HV that is treated asa critical value for this type of steels 10.CONCLUSIONSExperiments showed that increase of the weldingcurrent improved penetration. Increasing of arc voltageresulted in a broadening of the weld face without a sig-nificant influence to the penetration, while the increas-ing of the welding speed lowers the welds. The bestproperties were found in the sample 7 (Series III)welded with the following parameters: wire I weldingcurrent 680 A, arc voltage 32 V. Wire 2 welding current580 A, arc voltage 40 V. Welding speed is 1,4 m/min.Torches inclination: 90° for the first and 60° for the sec-ond wire.Performed welding with self made equipment (serieII) didn’t satisfy requirements. Pure welds quality andbed geometry was obtained. So improving weld proper-ties and geometry, using this equipment will be the maintarget in our next investigation.REFERENCES1 American Petroleum Institute: Recommended pipeline ma-intenance welding practices. API RP 1107, USA.2 G. Tither and W. E. Lauprecht, Pearlite-reduced HSLA ste-els for line pipe, Metal Science and Heat Treatment, De-cember 07, 2004.3 V. N. Marchenko and B. F. Zin’ko, Current trends in the de-velopment and production of steels and pipes for gas and oilpipelines; Translated from Metallurg, (2008) 3, 49-55.4 American Petroleum Institute: API 5L standard5 Lincoln Electric, Welding Handbook6 Linde ELLIRA handbook, 1988.7 Standard GOST 20295/858 G: M: Evans, N Bailey, Metallurgy of Basic Weld Metal;Abington Publishing, TWI, Cambridge England 1997.9 Norman Bailey, Weldability of Feritic Steels, Abington Pu-blishing, TWI, Cambridge, England 1994.10 S. Shanmugam, R.D.K. Misra, J. Hartmann and S.G. Jansto,Microstructure of high strength niobium-containing pipeli-ne steel, Materials Science and Engineering, 441(2006) 1-2,215-229.ACKNOWLWDGMENT - The part of this study wasconducted under support of Ministry of Science andTechnological Development, Republic of SerbiaNote: English language lecturer: Biljana Mostrova, Faculty of Technol-ogy and Metallurgy, Skopje.334 METALURGIJA 49 (2010) 4, 331-334S. CVETKOVSKI et al: OPTIMIZATION OF WELDING PARAMETERS FOR GAS TRANSPORTATION STEEL PIPESFigure 8 Microstructure of the locations presented on Fi-gure 7, a. weld metal, b. penetratration betwe-en the welds, c. CGHAZ, d. FGHAZ, e. ICHAZ, f.base metal fine grained microstructure, magnifi-cation x200, etched in NitalFigure 9 Fractured surface (segment 7 Serie III), a. basemetal, b. weld metal

Optimizacija parametra zavarivanja čeličnih cijevi za plinovode

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