The Experimental Study and Finite Element Simulation of Residual Stress in Welded Sections of Steel P91 Pipes with Multi-pass Welding

Transcription

1 Int J Advancd Dsign and Manufacturing Tchnology, Vol. 9/ No. 3/ Sptmbr Th Exprimntal Study and Finit Elmnt Simulation of Rsidual Strss in Wldd Sctions of Stl P91 Pips with Multi-pass Wlding Davood Azadi, Nosratollah Solhjoi Dpartmnt of Mchanical Enginring, Najafabad Branch, Islamic Azad Univrsity, Najafabad, Iran Davidfrdom65@yahoo.com, Solhjoi@yahoo.com SaydAli Mousavi* Dpartmnt of Mchanical Enginring, Najafabad Branch, Islamic Azad Univrsity, Najafabad, Iran saydali_mousavi@yahoo.com *Corrsponding author Rcivd: 0 Dcmbr 015, Rvisd: 1 January 016, Accptd: 8 Fbruary 016 Abstract: All th producing and assmbling procsss xrt rsidual strss on th pics that may lad to structural failur. Thrfor, calculating th rsidual strss in such structurs has bn common in rcnt yars. In this articl, distribution of tmpratur and rsidual strss rsulting from arc wlding in thr-pass butt joint in P91 austnitic stainlss stl pips is calculatd and stimatd using th finit lmnt mthod and xprimntal data. Simulating th wlding procss has bn carrid out thr- dimnsionally using Abaqus softwar. Distribution of th arc thrmal flux has bn idntifid basd on th Goldak two-lliptical modl using DFLUX subprogram in Abaqus softwar. Th numrical mthod has bn mployd by doing thrmolastoplastic analysis and th tchniqu of birth and dath of th lmnts to modl th wlding passs and th mltd lmnts. Thn, using cntral hol drilling mthod, rsidual strss gradint of th thicknss at distanc 3mm from th wlding lin on th pip is masurd. Finally, th maximum prcntag of rror, through th rsults obtaind from xprimntal masurmnts and finit lmnt mthod, was rportd 7% which is scintifically rasonabl.th rsults show that th rsidual nvironmntal strss in th intrnal surfac of th pip from th wlding cntral lin to 8.7mm varis with th gradint of 610MPa from 96MPa to -314MPa. Such a drastic strss distribution lads to gnsising som cracks on th wldd pips. Kywords: Arc wlding, Cntral hol drilling mthod, Finit lmnt mthod, Goldak hat sourc modl, Rsidual strss. Rfrnc: Davood Azadi, Nosratollah Solhjoi and Saydali Mousavi, Th Exprimntal Study and Finit Elmnt Simulation of Rsidual Strss in Wldd Sctions of Stl P91 Pips with Multi-pass Wlding, Int J of Advancd Dsign and Manufacturing Tchnology, Vol. 9/No. 3, 016, pp Biographical nots: D. Azadi rcivd his BSc in Mchanical Enginring from Iran Univrsity of Scinc and Tchnology in 009. H is currntly Mastr Studnt in Mchanical Enginring at th Dpartmnt of Mchanical Enginring, Najafabad Branch, Islamic Azad Univrsity, Najafabad, Iran. N. Solhjoi rcivd his PhD and MSc in Mchanical Enginring from Pirr and Mari Curi Univrsity (Franc) and BSc from Shiraz Univrsity. His currnt rsarch focuss on Finit Elmnt, Computational Mchanics, and manufacturing. S. A. Mousavi rcivd his PhD in Mchatronics from Eastrn Mditirian Univrsity (Cyprus). His currnt rsarch focuss on Industrial Automation and Bio-mchatronics.

2 4 Int J Advancd Dsign and Manufacturing Tchnology, Vol. 9/ No. 3/ Sptmbr INTRODUCTION On of th common typs of joining pips in oil, gas, ptrochmical and powr plant industris is th circumfrntial butt wlding which includs svral passs. Du to concntration of grat hat, gnratd from shildd mtal arc wlding, th aras around th wlding lin ar affctd by th thrmal cyclic loads. During th wlding procss, th hat distribution insid th matrial dirctly influncs th mchanical faturs, distortion and rsidual strss which ar rmaind aftr cooling. Th tnsional rsidual strsss on ach pic ar addd to th imposd loads and caus th failur cracks to appar and dvlop on ach pic. Th rsidual strsss ar intangibl in particular bcaus thy satisfy th balanc quations and on cannot find any tangibl sign of thir xistnc. Thrfor, it is ncssary to find accurat information about th tnsion rsultd from th wlding procss to improv th pip prformanc, dimnsions and incras wlding paramtrs. As th abilitis of th computrs had incrasd, th tchniqus of problm solving rlatd to thrmal transfr updatd, using th finit lmnt mthod. Analyzing th wlds and numrical mthod pionrd in 1960 by Hibbit and Marcal [1], Fridman [], Wstby [3] and Masubushi [4]. In 1978, Andrsson [5] studid th distribution of rsidual strss at high and low lvls of a bas plat nar th wlding lin using th two-dimnsional finit lmnt mthod. H showd that phas transmutations hav an important influnc on th rsidual strsss during th cooling cycl. Marcal [6] providd a summary of xprincs in th wlding simulation. In 197, Chihoski [7] found a thory that xplaind why th wlds crack in crtain conditions. H concludd that thr is a svr biaxial comprssiv strss fild nar th wld pool. Thr may b a gap or crack in front of th comprssiv fild and a tnsional fild or crack bhind it. According to his thory, changing th mthod of wlding, th comprssiv fild can b controlld. In ordr to prov his thory, Chihoski dvlopd th Moir' Fringing Tchniqu to masur th displacmnts during dg and butt wlding. Brickstad t al. [8] could calculat th wlding rsidual strsss in multi-passs joining of thick pips using an axial symmtrical two-dimnsional modl and indpndnt analysis. Yajiang [9] studid th distribution of rsidual strss in wlding joining of high-strngth stl plats and showd that th tnsion along th wlding has a mor influnc on th crack than th tnsion in othr two dirctions. In 010, using th Quick Wldr softwar, Dan Dng t al. [10] also gnralizd th finit lmnt mthod to simulat th thrmal wlding fild and distribution of rsidual strss in th tubular thr-dimnsional modl with multi-pass wlding. In th rsarch, th faturs of rsidual strss distribution in stainlss SUS304 pip that has bn inducd by arc wlding with Tungstn shilding gas, has bn studid numrically. H showd that th rsults of numrical simulation for axial and circumfrntial rsidual strsss nar th start and nd of th wlding aras hav a stp gradint and ar basically diffrnt from othr aras. In 01, thy us numrical simulation and xprimntal mthods for stimating of rsidual strss distribution using th multi-pass wlding in a pip wlding with th avrag diamtr and th dissimilar mtals. Kumar has dlinatd th ffcts of diffrnt thrmomtallurgical procsss lik hating, softning, cooling and solid stat phas transformation (SSPT) on tmporal volution of th strss-fild rsulting from lasr wlding [17]. Columnar and quatd microstructural zons hav bn numrically modlld in th wld rgion of th pip by Yaghi. H shows that FE microstructural rgions hav bn corroboratd by columnar and quatd zons that hav bn mappd out on a cross-sctional macro imag of th wld [13]. Yaghi indicats th importanc of including Solidstat phas transformation in th simulation of rsidual strsss during th wlding of P91 stl as wll as th significanc of PWHT on strss rlaxation [14]. Vnkata using Norton crp law implmntd PWHT and dtrmind th rsidual strsss aftr rlaxation. His modl and th prdictions wr validatd using nutron diffraction masurmnts on wldd and post wld hat tratd plat s rlaxation [0]. Othr rsarchs wr conductd on automatd masurmnts systm for instrumnts that idntify th rsidual strsss caus fatigu in rotary systms [1]. Th goal of this rsarch is th numrical mthodology in multi-passs wlding of P91 stl pips using th Goldak hat modl. Hr, shildd mtal arc wlding is usd to wld. In sction.1, th litratur rviw of wlding hat and invstigating of hat affctd distanc is mntiond. Th rsults of th modl simulating ar compard with th rsults of th practical xprimnt that is conductd using th cntral hol drilling mthod. Rquirmnts of wlding procss modling Th intraction of hat sourc (arc, lctron radiation or lasr) with th wlding pool is a complx physical phnomnon that has not yt bn modlld compltly. Th prssur distribution from th arc sourc, th ffcts of surfac tnsion of th drops of lctrod, th buoyancy and viscosity powrs of moltn mtal mak th pool to distort and lad to significant disturbancs. Du to th influnc of th arc, th hat input is distributd ffctivly throughout a work pic. Thus, th most significant challng, in simulating th wlding procss, is to dfin a hat sourc modl that can prdict th tmpratur fild in th wlding, accuratly. Basd on th obsrvations of th moltn ara, it is suggstd that th thr-dimnsional modl of twolliptical is mor ralistic and flxibl than vry othr modl that has bn introducd for hat sourcs so far. According to this modl, th fractions ff and fr of thrmal dposition in front and back quartrs of th modl ar ncssary for fr + ff =. Distribution of powr intnsity in th front quartr is dtrmind by th quation (1) [11], and similarly,

3 Int J Advancd Dsign and Manufacturing Tchnology, Vol. 9/ No. 3/ Sptmbr distribution of powr intnsity insid th two-lliptical for th back quartr is dtrmind by quation () [11]. For th thr-dimnsional modl, th abov volum flux is dirctly ntrd into th Abaqus softwar and is applid to th wlding lin using th subprogram DFLUX that is writtn in FORTRAN. q( x, y, q( x, y, 6 3 f Q, t) abc f 3x / a 3y / b 3[ z ( t)] / c f (1) 6 3 f Q, t) abc f r 3x / a 3 y / b 3[ z ( t )] / cr () r whr Q = ηvi/, and is th nt nrgy rat, η is th arc fficincy for SMAW wlding qual to 0.6,I symbol in quation is abbrviation (arc voltag),v symbol is abbrviation of amprag, is th wlding spd and is th wlding tim. Th constants a, b, c f and c r ar variabls that ar dtrmind by th siz and form of th wlding pool. Ths constants ar obtaind basd on th wlding pool. Th wlding simulation includs a st of tim stps which ach of thm offr wlding passs. Each pass includs a st of tim dvlopmnts. Th maximum allowabl tmpratur changing in ach dvlopmnt is st to b 40 C. In wlding simulation, th birth and dath tchniqu is usd to modl th changs of fillrs during a spcific amount of tim. In this mthod, all th lmnts including fillrs, that ar aliv in th nxt stags of analyss, must b gnratd. In addition, multiplying dgr of difficulty of filling lmnts and rduction factor (10-6, for xampl) togthr dactivats th lmnts. Similarly, th wight, vibration, spcific hat and othr faturs of dactivatd lmnts bcoms zro. Th wight and nrgy of dactivatd lmnts ar liminatd from th sum of th modls. Whn th lmnts rviv, thy ar not addd to th modl, but thy ar activatd simply through rturning th wight, strngth, loads, tc. back to thir initial valu..1. Numrical Simulation Th SMAW wlding procss is simulatd through th finit lmnt mthod using th Abaqus softwar. Du to th symmtry of th sampl, half of th wldd stl pip (P91) was modlld. Fig. 1 shows th thrmomchanical proprtis rlatd to th tmpratur of th wldd and th bas stl. Th wlding has bn modlld circumfrntially with thr passs. Th boundary conditions wr considrd to transfr th convction and radiation hat. In th simulatd modl, linar cubic ight-nod lmnts wr usd. Th numbr of usd lmnts wr dtrmind basd on th analyzing th snsitivity of th lmnts. Thrfor, th maximum numbr of displacing th pip during a wlding procss was studid. Fig. shows that whn th numbr of lmnts is qual to 17000, th maximum numbr of displacmnts dos not chang. Fig. 1 Th thrmomchanical proprtis of th wldd and bas stl in trms of tmpratur usd in analyzing th finit lmnts of P91 stl pips [1]. Fig. Th lmnts usd in analyzing th finit lmnt, B. Th convrgnc of lmnts in simulating th wlding procss. Tabl 1 Th prcntag of alloying lmnts rsulting from th quantomtr xprimnt for th P91 pip sampls Composition, % N Cb Al Ni V Mo P Cr Si S P Mn C P Fig. 3 Gomtry and dimnsions of th tstd pip Tabl Th wlding faturs for a -inch pip Spd (mm/s) Amprag (A) Voltag (V) Bvl (Dg) Numbr of passs

4 44 Int J Advancd Dsign and Manufacturing Tchnology, Vol. 9/ No. 3/ Sptmbr Exprimnts Th P91 stl pips wr circumfrntially wldd. In this mthod, two pips with an outr diamtr of 50.8, thicknss of 6.35 and lngth of 50mm was usd. In tabl 1, th quantomtr rsults of th sampl ar rprsntd that includ th wight of th composition of chmical lmnts which ar usd. Th pips wr wldd without pr-hating in thr passs. Th ordr of wlding layrs on th pip and th wlding conditions ar shown in Fig. 3 and Tabl. hat affctd zon, (about on cntimtr) th maximum tnsil strss changs to th maximum comprssiv strss, which shows that this ara is th most probabl on for cracking...1 An xprimnt for valuating th rsidual strss First, th imposd strain aftr drilling th cntral hol, is masurd basd on th groov dpth. Thn, using th strains cratd from th xprimnt and rgarding to th quations of cntral hol drilling mthod, th rsidual strss of th pic is valuatd. In ths xprimnts, th strainmtrs of TML Co. (FRS-3-17) wr usd. In ordr to install th strainmtrs, th surfac of th work pic must b highly polishd. Thrfor, th installation location is polishd with soft polishrs (1000). This location must b xactly in th plat in front of th groov. In th sampl, th strainmtr was installd in th part affctd by th hat so that th cntr of strainmtr hol locatd in 3mm of th wlding lin. Fig. 5 Th countrs of tmpratur distribution of th moltn pool 3-D modl during wlding. Fig. 4 Th sampl aftr th xprimntal tst. 3 RESULTS Th thrmal history of th wlding procss on th -inch pip as shown in Fig. 5, in thr points on th innr surfac of th pip at angls of 90, 180 and 70 and in similar distancs from th cntral wlding lin is indicatd in Fig. 6. Th circumfrntial rsidual strsss in th innr and outr surfacs of th -inch pip that ar obtaind from numrical simulation ar rprsntd in Figs.7 and 8. As it can b sn in Figs.7 and 8 strsss at th wlding cntr (zro mm) ar tnsil. Th mor far from th cntr of wlding, th tnsil strss will b rducd and changd to th comprssiv strss. In th short distanc from th wlding lin, in th Fig. 6 Th thrmal history of wlding on th -inch pip. In Fig. 9 th comparison of shar strss is shown. As shown, by comparing rsponss (rsidual strsss and displacmnts) th rsults show that sampls with lss displacmnt hav lss rsidual strsss and vic vrsa. In hoop wlding th larg axial tnsil and comprssiv rsidual strss and radial dformations can b sn whil in th distanc far from th wlding lins, larg axial tnsil

5 Int J Advancd Dsign and Manufacturing Tchnology, Vol. 9/ No. 3/ Sptmbr and comprssiv rsidual strsss ar sn on th innr and outr plats. drilling xprimnts is 35, dpth of mm.as it can b sn in Fig. 10, th axial strains ar tnsil whil hoop strain in th first stps to th dpth of 1.mm is comprssiv and thn in th dpth to mm is tnsil. Th limitation of mm is choosd according to th [19]. In Fig. 11, th strsss obtaind from th finit lmnt mthod and th xprimnts ar compard. Fig. 7 Th circumfrntial rsidual strsss in th outr surfac of th pip at th angls of 30 and 150 from th wlding starting point. Fig. 10 -inch pip, th strains obtaind from th xprimnt. Fig. 8 Th circumfrntial rsidual strsss in th innr surfac of th pip at th angls of 30 and 150 from th wlding starting point. Fig. 9 Th comparison of shar strss In Fig. 10, th rvald strains aftr drilling in xprimntal tst ar indicatd. It is worth mntioning that for valuating th strains, th strainmtr is installd in 3mm from th wlding lin. It should b notd that th numbr of stps in Fig. 11 -inch pip, th strsss obtaind from th xprimnt and simulation Study of strss changs rsulting from th pass wlding Th rsidual strsss causd by wlding in diffrnt passs ar subjctd to chang. In this sction, th changs will b discussd using numrical simulations. For this purpos, on

6 46 Int J Advancd Dsign and Manufacturing Tchnology, Vol. 9/ No. 3/ Sptmbr 016 a pip with th outsid diamtr, thicknss and lngth of, rspctivly,, 1/4, 0 inchs, thr pass wlding is simulatd. Th simulation rsults ar brought in Fig. 1. Ths forms show th nvironmntal and thrust strsss in th innr surfac of th tub. As it is shown, th volum of th passs is incrasd and thrfor th input nrgy, which is a function of wlding volum, is incrasd.thn, it can b concludd that th lvl of inducd rsidual strsss is incrasd. at th last linar wlding spd, i.. (mm/s), can b sn on th outr surfac of th pip. Similarly, th minimum tnsil strss at th most linar wlding spd, that is 4 (mm/s), can b sn on th outr surfac of th pip. Th lss wlding spd, incrass input hat pr volum unit which causs th mlting and hat-affctd zon largr. So th lvl of th inducd rsidual strsss incras Th ffct of th amprag changs on th lvl of rsidual strss Fig.14 shows th axial tnsion in th outr surfac of pip for thr diffrnt currnts of 150, 00 and 300 amp, holding all othr variabls constant. Incrasing of th amprag causs th ovrall input hat pr volum unit incras. Th ffcts of input hat pr volum unit is dirctly on th tmpratur distribution and th rsulting rsidual strss graph in wldd structurs, in th condition that th linar wlding spd and all othr variabls ar hld constant. Fig. 1 Changs in nvironmntal strsss on th innr surfac of th tub along th lngth of th tub by incrasing th numbr of passs Fig. 14 Rsidual axial strss on th outr surfac of th tub at 150 dgrs from th starting point for th diffrnt wlding brazing 4 CONCLUTION Fig. 13 Axial strss by changing th linar vlocity changs 3.. Th ffct of linar vlocity on th surfac of wlding rsidual strss Th axial rsidual strss filds for wlding linar spds qual to, 3 and 4 (mm/s), which ar usd in th simulations, is shown in Fig. 13. Maximum tnsil strsss In this articl, th rsidual strsss in multi-pass wlding that was don circumfrntially on stainlss stl pips wr studid. Comparing th rsults of numrical simulation with practical masurmnts, th accptabl accuracy of simulations is prsntd. Th rsults of dtaild study of nvironmntal rsidual strsss in th wlding part and th ara affctd by th hat ar shown in Figs. 7 and 8 in which th strss gradint in 1mm from th wlding lin is vry high. In addition, th rsults of analyzing th finit lmnt show that th maximum tnsil strsss and th

7 Int J Advancd Dsign and Manufacturing Tchnology, Vol. 9/ No. 3/ Sptmbr maximum comprssiv strsss ar cratd in th cntral wlding lin and th ara affctd by th hat rspctivly. As it is clar, th maximum tnsil and comprssiv strsss ar cratd in th innr surfac of th pip; thrfor, this surfac is suscptibl of cracks rsultd from wlding rsidual strss. In all mchanical mthods of masuring rsidual strsss, a part of matrial is rmovd or liminatd. Now rgarding th fact that a part of matrial is rmovd, th strsss rlatd to that ara will b rmovd too, th formr rsidual strss distribution that was tnding to quilibrat will b changd. In othr words, it dos not rmain in an quilibratd stat any mor. Thus, th objct is transformd to achiv quilibrium and this transformation is rgistrd in masurmnt mthods. Thn, using th rlations givn for th cntral drilling mthod, th rsidual strss of th pic will b calculatd. Th problm confronting th mchanical mthods or th mthods that rmov a part of matrial is th plasticity ffct in th objct. Until th objct is in th lastic form, th pic will b rturnd to its original stat by rmoving th xtrnal load. Th transformations mad in th objct ar not prmannt, but as th objct is transformd to plastic, th mad transformations ar not rvrsibl anymor and rmain in th objct. Sinc th cntral hol drilling mthod is basd on rlasing th lastic strss, th plasticity during strain rlasing procss can lad to mak rror in masurmnt. On th othr hand, du to th inhrnt and computational rrors of th finit lmnt mthod, thr is a dviation btwn numrical solution and th practical mthod. 5 REFRENCES [1] Hibbitt H. D., Marcal P. V., 1973, A numrical thrmomchanical modl for th wlding and subsqunt loading of a fabricatd structur, Company and Structur, Vol. 3, pp [] Fridman, E., 1978, Analysis of wld puddl distortion, Wlding J Rsarch Suppl., pp [3] Wstby O., 1968, Tmpratur distribution in th workpic by wlding, Dpartmnt of Mtallurgy and Mtals Working, Th Tchnical Univrsity, Trondhim Norway. 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