Baosteel Technical ResearchVolume 4, Number 3, September 2010. Page 32Mechanical behavior of electric resistance welded pipes in pipe-making processTIAN Qingchao' , DONG Xiaoming"，SHI Hongde2) and DING Weijun'1) Tube & Pipe Technology Center ,Research Institute. ,Baoshan Iron & Steel Co,Ltd, Shanghai 201900 ,China2) HFW Plant ,Tube & Pipe Business Unit , Baoshan lron & Steel Co, Ltd, Shanghai 201900 ,ChinaAbstract : The changes in the mechanical behavior of electric resistance welded( ERW) pipes before and after the cage rollforming process were investigated through tensile experiments. It is found that the Bauschinger efect does not exist in thepipe product, while the work hardening effect introduced by pipe-making is the direct cause of the mechanical changes.The prestrain introduced during different pipe making processes are accumulative to the work hardening efect. And theincrement of the yield strength for making φ 244. 48 x8. 94 pipe is approximately 45 MPa , higher than that of hot rolledlplates. It is verified that the stain ε = is an efficient index representing the work hardening effect from theD-Iengineering viewpoint.Key words: electric resistance welding; mechanical properties; work hardening; Bauschinger efct; prestraindoi: 10. 3969/j. ssn. 1674 - 3458.2010. 03.005serious work hardening effect, and even lead to afracture failure according to the mechanical constitutive1 Introductionrelation. In this study , only several important strainswere considered according to the final deformationWhen compared with seamless pipes ,high frequencyresults , and the process variants were ignored in orderelectric resistance welded ( ERW ) pipes have manyto simplify the calculation process.advantages , such as its low energy-consumption, thehigh precision of its dimensions and its high-produc-tivity. This is mostly due to moderm developments in2 Experiment and resultswelding processes, prototyping technology, qualitydetection as well as developments in control techno-The tensile specimens were cut from at the 1/4 widthlogy. Against this background,a φ 610 ERW mill wasposition of a hot-rolled plate and were prepared accord-introduced at Baosteel in 2005 in expectation that seaming to the requirements of the American Society forpipes would eventually replace seamless pipes becauseTesting and Materials ( ASTM) standard. A φ 244. 48 Xof their low costs and excellent performance. Th8.94 55 ksi (1 ksi= 6.84 N/mm2 ) steel coil forφ 610 ERW mill adopts cage roll forming of hot-rolledmaking ERW pipe was uncoiled and sampled forplate. In comparison with other advanced cold-rolltesting the mechanical properties in the longitudinalforming techniques, flexible roll forming, cage rol( rlling) and the transverse directions , respectively.forming is especially beneficial to obtaining a highTensile deformations were applied to the specimensquality seam line because it allows for complete plasticof the as-rolled plate at a strain rate of 0.01 m/s todeformation.induce different prestrains , and an extensometer wasDuring the cage roll forming. process, the hot-olledused to record the strain change in the loading->late experiences the consecutive processes of edgeunloading process. Consequently , the work hardeningformation，pre-formation , linear formation and precisioneffect could be determined by observing the elevatedformation. This is a continuous and dynamic defor-yield strength of the subsequent tensile test.mation process ，inwhich tensile , compressive，bendingFig.1 shows the stress-strain curves of the hot-as well as torsion deformations are accompanied t600 rrebounding in both the transverse and the longitudinalCurve I .directions. There have been disputes on whether theCurve 2Bauschinger effect or/ and the work hardening effectexist during the pipe-making process. Therefore ,it wasnecessary to clarify this confusion first ，and make surewhich effect most affects the mechanical properties ,200before further developing high performance pipeproducts.中國煤化工It was found that the equivalent strain in the ERWcage roll forming of a thin-wall 406 pipe might reachMHCNMHG20as high as 28% - 78% using finite element simula-tion. Obviously, such large strain might cause aFig. 1 Stress-strain curves of the hot-rolled plateCorresponding author: TIAN Qingchao; E-mail: tianqc@ bastel. com.TIAN Qingchao, et al. Mechanical behavior of eletric resistance welded pipes in pipe-_making process33rolled plate in the tensile test. Curve 1 monotonicallyplastic region , the loading path drops approximatelyrises to 18%，and Curve 2 shows a set of stress-strain5 MPa. As shown in Table 1 ,the yield strength of thecurves in a consecutive loading-unloading process atsecond loading curve is small, approximately 5 MPa,some intervals of strain. V arious prestrains werewhich is higher than the tensile strength of the lasttherefore introduced in the tensile specimens. In theloading curve. Under the same strain condition, theelastic region，every loading path but the first one liesyield strength in Curve 1 is almost the same as theon the prior unloading path to form a straight line , andyield strength in Curve 2( Table 1).eventually all the straight lines become parallel. In theTable 1 Mean value of yield stress before and after pipe productionTensile to variedActual value in Curve 2/MPaStress ( yield strength) in Curve 1/MPastrains/%Yield strengthTensile strengthYield drop0-3.04634794643.0-4.548451254804.5-6.0519531176.0-7.5536540347. 5 - failure54345The experiment indicates two important facts: oneφ244.48 mmx 8. 94 mm 55 ksi grade pipe. Afteris that the elevated yield strength caused by a certainpipe-making, the yield strength of the pipe bodyprestrain can be determined conveniently according toincreased to 473 MPa from 428 MPa of the hot-rolledthe obtained stress- strain curve of the specific material.plate, while the seam yield strength decreased toThe other is that the prestrains introduced during402 MPa,much lower than that of the pipe body. Thisdifferent pipe-making processes accumulatively contri-phenomenon was caused by the welding and thermalbute to the work hardening effect.mechanical processes of pipe-making. It is found thatthe tensile strength is maintained at approximately3 Pipe-making and discussion550 MPa.Table 2 Mean value of the hot-rolled plate3.1 Mechanical properties of the hot-rolled plateFig.2 shows the yield and tensile strength variationsYield strength MPa Tensile strength/ MPafrom the inside to the outside of the hot rolled coil inTransverse446550the longitudinal ( rolling) and the transverse direc-tions, respectively. It can be seen that the yieldLongitudinal428547strength in the transverse direction is approximately19 MPa higher than that in the longitudinal direction,500 ras shown in Table 2 ,while the tensile strength in boththe directions are at the same level. This is consistent400with the observations of other researchers300 t- -Longitudinal600-Tensile stuength十+ Transverse號200| -ylongitudina)+ Seantransvere)edinal)500100σ 51152025Testing number二穴工工Fig. 3 Yield strength of the hot-olled plate and the ERWpipeTo further clarify the variation , tensile specimens inInnerthe circumferential direction at an equal angle of 45°3008(were cut from the φ 244. 48 mmx 8. 94 mm ERWPosition/ mpipes. Figs. 4 -5 show the tensile stress fluctuation ofFig.2 Variation of the yield and tensile strength from thehe p中國煤化工he transverseinside to the outside of the hot-rolled steel coildirecticThe .iYHC N M H Gal direction fluc-3.2 Strength variation during pipe-makingtuates,as shown in Hig. 4, approximately 473 MPa,Fig. 3 shows the tested yield strength of hot-olledwhich is 45 MPa higher than that of the hot-olledplate and the finished pipe during the industrialplate and thus clearly demonstrating the work harden-production of American Petroleum Institute ( API )ing effect. The mean value of the tensile strength34Baosteel Technical Research, VolL4, No.3, Sep. 2010546 MPa, which is the same level as the plate.L-L=△L≈D/2L(1The maximum tensile strain produced by cage roll600 r Longitudinal+ - Yield strength.forming is as follows:-Tensile strengthε≈D*/2L2(2)S50-r.一r00 t50-400 L一。9080 270 360Hot-olled plate_LSkelpAngle to sean/(° )Note:L=11 m.e≈0.02% for making φ 248. 73 mm pipeFig.4 Circumferential distribution of tensile stress in theFig.6 A schematic drawing of the hot-rolled plate and thelongitudinal direction (X =0 represents the position of theskelp during cage roll forming processweld seam)The wall thickness after cage roll forming tw can beFig. 5 shows that the average yield strength in themeasured at the marked place on the hot-olled platetransverse direction is 438 MPa, lower than that of thehot-rolled plate , while the tensile strength still remainsbefore and after the welding process , and the radialat the same level. The Bauschinger effect existedstrain can be obtained by the equation 82 = ln一,when the tensile stress in the transverse direction wastested because bending took place during pipe-makingwhere to is the wall thickness of the plate at theand straightening occurred during the tensile specimenmarked place. Fig. 7 shows the result of this calcu-preparation. As a result, the yield strength of the pipelation, where the mean value of the radial strain isbody in the transverse direction decreased from446 MPa to 438 MPa. Obviously , the BauschingerThe plate, which is 771 mm in width , is plasticallyeffect does not exist in the finished pipe product,deformed to the pipe shape, and the introducedmainly because the work hardening effect was intro-circumferential strain is 83 = , where↑is theduced during pipe-making.D-!thickness. As for φ 248.73 ,83 =3. 75%.60Transverse- -Yield strengthr -Tensile strengthAfter sizing 1.75550‘2 500品450Afler welding 1.040090 135180Angle to seam/(° )90 180 270 360Angle lo seam/(° )Fig. 5 Circumferential distribution of tensile stress in thetransverse direction(X = 0 represents the position of theFig.7 Variation of radial strain in the wall thickness dire-ctionThe equivalent strain is estimated using the equation3.3 Pipe-making strainsThe key process parameters for making φ 244.48 mmEa=: l(e:-82)*+(82-83) +(ei-es). Forx8. 94 mm 55 ksi ERW pipe are listed below. Theφ 248.73 pipe ,if 8r=1.0% ,8a =2.9%.width of the hot-rolled plate is 771 mm, the outerdiameter (OD) of the welded pipe is中248. 73 mm,3.3.2 Strains introduced by sizingand the OD after sizing is φ 245.86 mm.During the longitudinal deformation process , longi-3.3. 1 Strains introduced during cage rollingtudin中國煤化工overall equivalentstrainguations'3):The hot-olled plate and the skelp pipe during the;YHC NMH G.cage roll forming process are schematically shown in5↑=In(3)(D,一4,)1 .Fig. 6, where the plate and the skelp constitute a right-angled triangle. Due to elongation, the hypotenuse LB2 =In(4)is the greatest in the longitudinal direction:1TIAN Qingchao, et al. Mechanical behavior of electric resistance welded pipes in pipe- making process35of a hot-rolled plate is approximately 19MPa higherD。-t。(5)than that in the longitudinal directions, while thewhere the subscript W and s represents before(w) andtensile strength in both directions are at the samelevel , which is also observed after pipe-making.after(s) sizing deformation, respectively.(3) The work hardening effect exists in the longi-The measured results of radial strains are alsotudinal direction after pipe-making , and the increment ofshown in Fig. 7. The mean value of the equivalentyield strength for making φ 244 48 mm x8.94 mm pipestrain introduced by sizing is approximately 1. 8%is approximately 45 MPa higher than that of the hot-according to the above data.rolled plate . which is 428 MPa.3.3.3 Pipe-making strains and the work hardening(4) The Bauschinger effect does not exist in theeffecpipe product. It exists in the transverse direction whenThe equivalent strain during the pipe-making oftesting the tensile stress. The yield strength of the pipeφ 244.48 mmx8. 94 mm is added up to approxima-body in the transverse direction decreases totely 4.7%. A 4.7% prestrain should make the yield438 MPa, while that of the corresponding plate isstrength increase to approximately 500 MPa according446 MPa for making φ 244. 48 mm x8. 94 mm pipe.(5) The equivalent strain is a good index , indicat-to the plate stress-strain curve. In fact, the industrialproduction of φ 244. 48 mm x8. 94 mm sees an aver-ing the maximum deformation strain whileg=n-,isage increment of 45 MPa in yield strength, whichshould correspond to an introduced prestrain ofan efficient one representing the work hardening effect3.8%.from an engineering viewpoint.The above-mentioned prestrain introduced by cagerolling is the maximum values in the longitudinal , thecircumferential and the radial directions. A neutralReferencesdeformation band also exists , where zero prestrain is[ 1] Gu Zhongying. Research on FEM based roll pass designintroduced. Therefore，the inferred equivalent strain isof cage rll-orming of ERW pipe [ D ]. Shanghai:a good index indicating the maximum deformationstrain but not an efficient one representing the work2] Huang Wei, Zhang Xu and Wang Zexiao. Analysis offluctuation in mechanical properties of hot rlled coilshardening effect.[J]. Iron and Steel ,2010,36( 10) :36-38.In fact . from an engineering perspective , the ratio of[3] Zhu Jingqing . Fu Chenguang and Liu Yukun. Calculationwall thickness to diameter , which in the present case isnd application of deformation amount during pipe3. 66%，is usually used to characterize the formabilityroling operation[ J]. Steel Pipe ,2002 ,29(5):17-22Tian Qingchao, Dong Xiaoming and Ding Weijun. Deve-of pipe-making , while the equation ε=一一, which inlopment of K55 eletric resisance welding casing [J].D-tSieel Pipe ,2008 ,37(4) :28-32.the present case is 3. 8%，is employed to predict thestrain produced by bending deformationl. It isproved that ε =- is an efficient index representingthe work hardening effect in this study. Superficially,the strain contributes most to the equivalent strain,nevertheless, the good applicability of the equationshould be atributed to the microstructural changes,such as the motion of dislocation and multiplicationTIAN QingchaoDONG XiaomingSHI Hongdeduring the bending process. Further investigation willbe conducted to gain a better understanding.4 Conclusions(1) The elevated yield strength corresponding to acertain prestrain can be determined according to theobtained stress-strain curve of the specific material.And the prestrain introduced during different pipe-DING Weijunmaking processes are accumulative to the work harden-ing effct.中國煤化工(2) The yield strength in the transverse directionsMHCNMHG