RESIDUAL STRESS DISTRIBUTION FOR A SINGLE PASS WELD IN PIPE

Transcription

1 Number1 Vlume 16 march 010 Jurnal f Engineering RESIDUAL STRESS DISTRIBUTION FOR A SINGLE PASS WELD IN PIPE Dr. Adnan N. Jameel Ass. Prf University f Baghdad Dr.Nabeel K. Abid Al-Sahib Ass. Prf. University f Baghdad Dr. Osama F. Abd Al Latteef Lecturer University f Baghdad ABSTRACT Heat input due t the welding f mild steel pipe causes a temperature gradient in the parent metal. After welding and temperature cling dwn, residual stresses appear arund welding zne which reduces the weld strength. Residual stresses are a result f the temperature gradient and the dependency f material prperties n the temperature, such as yield strength, elasticity mdulus, and thermal expansin cefficient. In this study, a typical flat jint f a single pass weld in a thin pipe was studied analytically and numerically. Analytical apprach is perfrmed by explring a simple methd t calculate the magnitude f residual stress in terms f the weld shrinkage behavir. Numerical analysis is perfrmed by applying nn-linear transient heat transfer analysis using welding parameters, such as heat generatin, free r frce cnvectin with ambient, are perfrmed using a general purpse FE package ANSYS 8.0 in rder t btain the temperature distributin in the welded parts. A nn-linear thermal-elastic-plastic stress analysis is then perfrmed using the same package t predict the stress fields during and after welding.. بعبد درجبات الحبرارة للمعبدن اصلب الخالصة أن الحرارة الناتجة عن لحام اإلنبوب الفوالذي يسب ببب تغيرفب منققبة اللحبام ممبا تب دي البخ أنخفباض اللحام و نتيجة صنخفاض درجات الحرارة تظهر إجهاد متبقية حبو المبادة درجبات الحبرارة وتبعيةخلباص ف مقاومة المعبدن. أن اإلجهبادات المتبقيبة تظهبر نتيجبة التغيرفب التمبدد الحبراري. تبم فب ذبذا البحب المرونة ومعامب مقاو بمة الخضوع معام علخ درجة الحرارة مث تبم فيبأ أسبتخدام قريقبة دراسة اصجهادات الناتجة عن لحام اصنابيب الرقيقة تحليليا وعدديا. المبنه التحليلب بسيقة لحساب مقدار اإلجهاد المتبق من ناحيبة سبلوإ إنامبال اللحبام. أمبا المبنه العبددي تبم فيبأ إسبتعما تحليب نقب الحبرارة العبابر ال خ بقب مبع عوامب اللحبام المختلفبة مثب الحبرارة المتولبدة و معامب اصنتقبا نحلب برنام التحلي العددي متعدد اصغراض ANSYS 8.0 لا الحراري مع الجو المحيق بإستعما علببخ توعيببع درجببة الحببرارة فبب اصجببعا الملحومببة. ثببم أسببتخدام تحليبب اإلجهبباد الب سببتيا الحببراري ال خ بق بإستعما نفس البرنام لتو بقع اإلجهاد أثنا وبعد عملية اللحام.

2 A. N. Jameel Residual Stress Distributin fr N. K. Abid Al-Sahib A Single Pass Weld In Pipe O. F. Abd Al Latteef KEYWORDS: Residual Stress, Weld Pipe, Therm-elast-plasticity. INTRODUCTION: In fusin welding, a weldment is lcally heated by the welding heat surce. Due t the nnunifrm temperature distributin during the thermal cycle, incmpatible strains lead t thermal stresses. These incmpatible strains due t dimensinal changes assciated with slidificatin f the weld metal, metallurgical transfrmatin, and plastic defrmatin, are the surces f residual stresses and distrtin. Welding-induced residual stresses and distrtin can play a very imprtant rle in the reliable design f welded jints and welded structures. Hwever, the welding prcess itself is a very cmplex phenmenn, which has nt been fully understd, s that distributin and magnitude f residual stress is nt readily available frm the literature [Wu, 001]. Residual stress distributin and distrtin in a welded pipe are strngly affected by many parameters and by their interactin. In particular, there are structural, material, and welding factrs. The structural parameters include gemetry f the pipes, thickness and diameter and jint type. Amng the material parameters mechanical and physical prperties and type f prcess emplyed, welding prcedure, current, vltage, arc travel speed, and arc efficiency. As a cnsequence f the nn-unifrm temperature distributin, parts f materials clse t the weld are subject t different rates f expansin and cntractin develping a three dimensinal cmplex residual stress state. The stresses at hp welds in pipes have been investigated by many previus wrkers. The mst recent researches implemented a welding simulatin methdlgy and experimentally validated t predict residual stresses n multipass welds. The simulatin invlves perfrming therm-elastplastic analysis using a cnsistent element activatin apprach in the mechanical analysis [Panagitis, 1997]. In this paper, mathematical mdel and finite element simulatin f the single pass welding prcess yielding the welding-induced residual stresses in a welded pipe is presented. RESIDUAL STRESS DUE TO PIPE WELDING: Residual stresses are frmed by the plastic defrmatins during the thermal cycle f welding. Befre welding, the structure is assumed t be at rm temperature and free f stresses. During the weld thermal cycle, material mechanical prperties change drastically, especially when material appraches melting pint temperature. Therefre, due t the temperature dependence f material prperties and the large defrmatin in fusin welding, material and gemetrical nn-linearity have t be taken int accunt. The initial expansin material due t the temperature increase is cnstrained by material placed away frm the heat surce, therefre generating cmpressive stress. At a temperature higher than material critical temperature, the material starts exhibiting thermal sftening where heating results in decrease f flw stress. As phase change ccurs deviatry stress becmes zer and cnsiderable plastic defrmatin ccurs in the weld metal and the base metal regins near the weld. As temperature decreases during the cling phase, the stress in the slidifying material increases, and becmes tensile due t the psitive temperature gradient. The regin placed away frm the weld line, will therefre, be in cmpressin since the resultant frce and the resultant mment induced by residual stress evaluated in any plane sectin must satisfy translatin and rtatinal equilibrium. The plastic strains resulting frm the heating causes stress, which in turn prduce internal frces that may cause buckling, bending, and rtatin. These displacements are in general called distrtin. The residual stress cmbined with distrtin and

3 Number1 Vlume 16 march 010 Jurnal f Engineering degradatin f the material mechanical prperties influence the buckling strength and fatigue life f welded structure [Me, 003]. MATHEMATICAL MODEL: In a girth-welded pipe, shrinkage f the weld in the circumferential directin induces shearing frce, Q, and bending mments, M, t the pipe as shwn in Fig. (1). the angular distrtin caused by butt welding als induces bending mment. Distributin f residual stresses is affected by the fllwing [Kichi, 1980]: 1-The diameter and wall thickness f the pipe. -The jint design (square, butt, vee, x.etc). 3-The welding prcedure and sequence (welded n utside nly, welded n bth sides, utside first, r welded n bth sides, inside first). 4-The welding parameters (current, vltage, speed, electrde diameter, type f electrde etc). 5-Material prperties. Fig. 1 Residual stresses in a welded pipe [Nrman, 1984] The tendn frce lcked int a weld is defined as the imaginary line lad which wuld prduce the bserved shrinkage parallel t the weld, and is apprximately independent f restraint. It may therefre be estimated n the basis f the assumptin f infinite restraint against any hp wise expansin f the heat material during welding. Then, the residual hp stress at any pint is a functin f the maximum temperature reached, and the hp tendn frce is btained by integrating the stress distributin. Fr a surface weld bead, the maximum temperature as a functin f the distance r frm the weld centerline is given apprximately by [Nrman, 1984]: max. ζq = eπ vr ρc T (1) p Where ζ is the arc efficiency, q the weld pwer, v the weld travel speed, and ρc p the vlumetric specific heat. Inside the regin where the thermal strain αtmax. is greater than twice the yield strain σ y /E, yield ccurs in cmpressin during heating, and then again in tensin during cling. Fr thermal strains between σ y /E and σy/e, the material yields in cmpressin during heating, but unlads t a sub-yield tensile strain during cling. Material whse maximum thermal strain is less than σ y /E des nt yield, and ends at zer stress fr the idealized cnditins f infinite hp wise restraint.

4 A. N. Jameel Residual Stress Distributin fr N. K. Abid Al-Sahib A Single Pass Weld In Pipe O. F. Abd Al Latteef The width f the zne cntaining yield magnitude tensile stresses (Fig. ()) can be fund by substituting T max. = σ y /Eα in eq. (1): r = Eα eπσ ρc y p ζq v () Where E mdulus f elasticity, α thermal expansin cefficient, and σy yield strength f the pipe material. Fig. Hp tensile zne at girth weld The ttal frce lcked int the tensile zne is given by: F=A w σ yw + (1.17*t r-a w ) σ y (3) Where A w and σ yw are the area and yield strength f the weld, and the factr 1.17 allws fr the subyield tensile stresses between r and r. The inward radial deflectin f a circular cylindrical shell laded symmetrically with respect t its axis [Timshenk, 1959]: y = -βz - e 3 β D [ ( ) ] βm csβz sin βz + Q csβz (4) And hence: dy dz = e β -βz D [ ( )] βm csβz + Q csβz + sin βz (5) d y dz = -βz - e βd [ ( ) ] βm csβz + sin βz + Q sin βz (6) 8166

5 Number1 Vlume 16 march 010 Jurnal f Engineering Where β 1/ υ, R t Et D =, and R radius f the pipe. ( ) υ Fig. 3 (a) Shrinkage parameters Ω and F (b) Crrespnding lads at weld centerline In rder t mdel the deflectins due t a hp weld centered at z=0, the applied frce Q and mment M are related t the angular distrtin Ω and tendn frce F which characterize the shrinkages f the weld. The radial shear frce is related t the tendn frce F by cnsidering radial equilibrium at z=0 [Timshenk, 1959]: F Q (7) R - Ω The bending mment M is fund by setting dy w at z=0 in eq. (5). Hence: dz -1 M = ( β DΩ + Q ) w (8) β Substitutin fr Q and M in eqs. (4), (5) and (6) gives the radial deflectin and its derivatives at any distance frm the weld. The axial bending stresses σ zb n the inner and uter surfaces at any sectin inside r utside the tensile zne are a functin f the curvature: Et d y σ = (9) zb ( 1 υ ) dz = FINITE ELEMENT MODELING PROCEDURES: The FE analysis was carried ut in tw steps. A nn- linear transient thermal analysis was cnducted first t btain the glbal temperature histry generated during and after welding prcess. 8166

6 A. N. Jameel Residual Stress Distributin fr N. K. Abid Al-Sahib A Single Pass Weld In Pipe O. F. Abd Al Latteef A stress analysis was then develped with the temperatures btained frm the thermal analysis used as lading t the stress mdel. The general purpse FE package ANSYS 8.0 was used fr bth thermal and stress analysis perfrmed sequentially. The mesh used in the stress analysis was cmpatible t that in the thermal analysis. The material prperties f the weld meld and base metal are assumed t be the same in this analysis and it is temperature dependent. The thermal and mechanical material prperties are listed in table (1) & () respectively. In the thermal analysis, the heat was input in fur lad steps as shwn in figure (4).The ndal temperature slutin btained frm the thermal analysis were read as lading int the stress analysis. In rder t capture the residual stress induced due t the heating and cling cycle, the temperature histry needed t be read at a sufficient large number f time pints, especially where the temperature gradient is large. Hwever, the greater the number f the thermal slutin steps used, the greater the cmputatinal time and the larger stre space required. Fig. 4 Heat functin input during welding [Wu, 001] (t 1 =.5 sec, t =3 sec, t 3 =.5 sec, t 4 =396 sec) Table 1 Thermal material prperties used in mdel [Brwn, 199] NO. T Temperature C K Thermal Cnductivity W/m C Cp Specific heat J/Kg C h Cnvectin heat Transfer cefficient W/m.C

7 Number1 Vlume 16 march 010 Jurnal f Engineering Table Mechanical prperties used in mdel [Brwn, 199] NO. T Temperature C σy Yield stress MPa ν Pissn rati E Yung ' s Mdulus GPa α Thermal Expansin 10-6 /º C RESULTS AND DISCUSSIONS: The sample f calculatins was made n a single span ASTM14-71 mild steel pipe with (1 m) length, (5.4 mm) radius, (1.5 mm) thickness. The welded pipe was frmed by jining tw (0.5 m) pipes by single pass fusin arc welding. The amunt f heat input was fund as the prduct f arc efficiency, vltage, and current (Pwer arc welding =f 1 f VI), Where: f 1 =heat transfer efficiency, f =melting efficiency, V=vltage, and I=current. We take f 1 =0.9, f =0.6, V=30 vlt, I=300 Amp, using an electrde type E7010-G t make a straight pipe 1m length with welding n its mid span. Fig. (5) Shws the temperature time histry at a pint in the center f welding. Fig. (6) Shws the temperature time histry at a path alng the length f the pipe at times during and after welding predicted by FE analysis. Fig. (7) Shws the ndal slutin f the temperature distributin alng the pipe. It was seen that max. Temperature in a center line f welding equal (1566 ºC) at the mment f end welding then decreased and distributed alng the pipe. During the welding prcess, the temperature is lcally increased t the melting pint f mild steel. Mrever, in the heat affected zne f the weld, the temperature reaches high values. The mechanical prperties f mild steel such as yield strength and elasticity mdulus are highly depending n the temperature. After welding, the heat energy is dissipated due t cnvectin with ambient air causing the steel t cl dwn and cnsequently gain its riginal yield strength. During this prcess f increasing and decreasing the temperature, residual stresses and plastic strains are develped. Fig. (8) Shws the lngitudinal stress time histry at a path alng the length f the pipe at times during and after welding predicted by FE analysis. The ndal slutin f residual stress distributin 8168

8 A. N. Jameel Residual Stress Distributin fr N. K. Abid Al-Sahib A Single Pass Weld In Pipe O. F. Abd Al Latteef in Z-directin during and after welding alng the tp surface predicted by FE analysis is shwn in fig. (9). It can be seen frm these figures that the stress is changed frm cmpressin at the centerline f weld and tensin away frm the weld centerline at the end f welding (t=3.5 sec) t tensin at the centerline f weld and cmpressin away frm the weld centerline when it cled (t=400 sec), which are a gd agreement between the analytical and numerical results with the published results [Wu, 001; Erika, 00]. The Vn Mises residual stress distributin is shwn in fig. (10) at time equal 400 sec. Fig. (11) Illustrates the results f residual stress distributin alng the pipe due t welding. It is clear that the predicted residual stress by FE analysis is very clse t the analytical analysis at the center f welding. It shws a tensin peak f σz=5 MPa at weld line and adjacent parent metal ging int cmpressive at abut Z=10 mm frm weld centerline and then blending int the backgrund elastic distributin at abut Z=30 mm frm weld centerline. At psitin remte frm the weld, the stresses didn't g t zer, but tend twards a steady value f abut 10 MPa cmpressins. These results agree well with theretical studies f residual stresses, at girth welds in pipes [Nrman, 1984]. Fig. (1) Shws time variatin f plastic strain at a centerline f welding. It is clear that the plastic strain increases rapidly during welding and then still cnstant during cling. The average values f thermal prperties ver the applied temperature distributin were used t study the effect f temperature dependent material prperties n the final residual stress results. The residual stress btained frm the FE analysis using cnstant thermal prperties shw a significant difference frm the btained using temperature dependent thermal and mechanical prperties as shwn in fig. (13), especially in the heat affected zne due t the fact that the yield strength variatin at higher temperature has less effect. This suggested that care must be taken t identity crrectly the temperature-dependent yield strength f material which is ften nt easily determined. Fig. 5 Temperature time histry at a pint in the center f welding

9 Temperature ( C) Number1 Vlume 16 march 010 Jurnal f Engineering time 3.5 sec time 4 sec time 50 sec time 100 sec time 400 sec Distance (m) Fig. 6 Temperature distributins alng welded pipe at different times Fig. 7 Temperature distributin during and after welding 8161

10 Stress (MPa) A. N. Jameel Residual Stress Distributin fr N. K. Abid Al-Sahib A Single Pass Weld In Pipe O. F. Abd Al Latteef time3.5sec time4sec time50sec time100sec time400sec Distance (m) Fig. 8 Residual stress distributins alng welded pipe at different times Fig. 9 Residual stress distributin alng welded pipe during and after welding

11 Plastic strain Residual Stress ( MPa ) Number1 Vlume 16 march 010 Jurnal f Engineering Fig. 10 Vn Mises residual stress distributin in a welded pipe FE Analytical Distance ( m ) Fig. 11 Residual stress distributin alng welded pipe Time ( sec ) Fig. 1 Plastic strain in a centerline f weld

12 Residual stress (MPa) A. N. Jameel Residual Stress Distributin fr N. K. Abid Al-Sahib A Single Pass Weld In Pipe O. F. Abd Al Latteef Temp. dependency MP Cnstant MP Distance (m) Fig. 13 Effect f temperature dependency material prperties n the residual stress alng a weld pipe CONCLUSIONS: Analytical and finite element analysis are used t determine the residual stresses resulting frm the single pass welding f mild steel pipe f 5.4 mm radius, 1.5 mm thickness. Fllwing the main summarized cnclusins raised by this paper: 1- Welding f a pipe induced residual stresses. It has been seen that these stresses changed frm cmpressin at the centerline f welding and tensin away frm the weld centerline at the end f welding t tensin at the centerline f weld and cmpressin away frm the weld centerline when it cled. - The residual stress btained by using cnstant thermal and mechanical prperties shw significant difference frm that btained using temperature dependent thermal and mechanical prperties especially in the heat affected zne. REFERENCES: Brwn S. and Sng H., "Finite element simulatin f welding f large structures", Vl.114, Nvember 199. Erika Hedblm, " Multipass welding f nuclear cmpnents-cmputatins ", Lulea University f Technlgy, Department f Applied Physics and Mechanical Engineering, Divisin f Cmputer Aided Design, 00. Kichi Masubuchi, " Analysis f welded structures (residual stresses, distrtin, and their cnsequences), Me M. and Vignjevic R., "Welding simulatin using FEA ", Cllege f Aernautics, Cranfield University, Bedfrd, UK, MK430AL,003.

13 Number1 Vlume 16 march 010 Jurnal f Engineering Nrman Eatn, "Welding in energy-related prjects ", Welding Institute f Canada, Panagitis Michaleris, "Residual stress distributins fr multi-pass welds in pressure vessel and piping cmpnents ", Edisn Welding Institute, Clumbus, Timshenk S. and Winwsky-Krieger S., "Thery f Plates and Shell", secnd editin, McGraw-Hill, Wu A., Syngellakis S., and Mellr B.G., "Finite Element Analysis f Residual Stresses in a Butt Weld", University f Suthamptn, High field, U.K, 001. NOMENCLATURE Symbls Units A w Area f the weld m C P Specific heat f pipe material J/Kg C E Mdulus f elasticity N/m F Tendn frce N/m M Bending mments Nm Q Shear frce N q Weld pwer watt R Radius f the pipe mm Tmax Maximum temperature at the weld t Thickness f the pipe mm v Weld travel speed m/sec ζ Arc weld efficiency ρ Density f pipe material Kg/m 3 α Thermal expansin / C σ y Yield stress f a pipe metal N/m σ yw Yield stress f the weld N/m σ zb Axial bending stress N/m Ω w Angular distrtin C 8144