Residual stress distribution depending on welding sequence in multi-pass welded joints with x-shaped

Masahito Mochizuki

Department of Manufacturing Science, Graduate School of Engineering,

Osaka University,

2-1,Yamada-oka,

Suita,Osaka565-0871,Japan;

formerly,Hitachi,Ltd. e-mail:mmochi@mapse.eng.osaka-u.ac.jp

Makoto Hayashi

Toshio Hattori

Hitachi,Ltd., Tsuchiura,Ibaraki300-0013,Japan Residual Stress Distribution Depending on Welding Sequence in Multi-Pass Welded Joints

With X-Shaped Groove

Residual stress in a large-diameter multi-pass butt-welded pipe joint was calculated for various welding pass sequences by thermal elastic-plastic analysis using the?nite element method.The pipe joint used had an X-shaped groove,and the sequences of welding passes were changed.The distribution of residual stress depends on the welding pass sequences. The mechanism that produces residual stress in the welded pipe joint was studied in detail by using a simple prediction model.An optimum welding sequence for preventing stress-corrosion cracking was determined from the residual stress distribution.

?S0094-9930?00?00701-0

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Introduction

Residual stress caused by welding has various kinds of in?u-ence,eventually leading to fatigue-fracture,stress-corrosion cracking,or brittle fracture in welded structures.In particular,it is important to know the distribution of residual stress,including that in the through-thickness direction,for strength design or crack propagation evaluation of welded structures.Even though residual stress can be measured precisely and easily,it is desirable to establish an analytical method for estimating residual stress commonly as well as a measurement technique.This is because an analytical method can be used to evaluate designs before fabricat-ing a specimen or actual products.

There are two methods of calculating welding residual stress: inherent strain analysis?1–6?and thermal elastic-plastic analysis ?3,7–17?.Inherent strain analysis can simply evaluate residual

stress by elastic calculation.Therefore,its range of application will expand when a database of inherent strain values for various welding conditions and structure shapes is prepared?4?.Thermal elastic-plastic analysis,on the other hand,requires an enormous amount of computation time,but it can precisely evaluate residual stress in a speci?c object.Improvements in computer power have made it feasible to perform thermal elastic-plastic analysis on a two-dimensional modeled object without much dif?culty?16,3?. Various evaluations of welding residual stress have been done using these techniques,but most of them were done under speci?c welding conditions.It is well known that welding residual stress varies depending on the welding conditions,but the relationship between residual stress and welding conditions has not always been clear.In this paper,the welding pass sequences were varied as a parameter of generating residual stress,and residual stress in large-diameter multi-pass welded pipe joints with an X-shaped groove were calculated by thermal elastic-plastic analysis.The production mechanism of residual stress was studied in detail us-ing a simple hand-calculated prediction model.The simple model was validated by comparing its calculated values of residual stress with computed results.Considering the operating conditions of welded pipe joints,an optimum welding sequence for preventing stress-corrosion cracking at the heat-affected zone of the inner surface in a multi-pass butt-welded pipe joint is proposed from the residual stress distribution.Pipe Joint Con?guration

The con?guration of a large-diameter multi-pass butt-welded pipe joint is shown in Fig.1.The outside diameter is5600mm and the wall thickness is50mm.The cross section in Fig.1shows the X-shaped groove for multi-pass welding on both inner and outer sides.The weld metal and the base metal are both austenitic stainless steels.Welding is done with gas tungsten arc. Residual Stress Analysis

Residual stress was calculated by thermal elastic-plastic analy-sis using the?nite element method as an axisymmetric model. Welding residual stress in a butt-welded pipe joint actually varies around the circumference.However,various axisymmetric analy-ses by thermal elastic-plastic analysis have been conducted for calculating it.These analytical results show that thermal elastic-plastic analysis is effective for obtaining the precise residual stress in multi-pass butt-welded pipe joints for an axisymmetric model, once reliable material properties have been obtained,because the analytical results agree well with measurements using strain gages and neutron diffraction?8,16,18?.

Residual stress in a multi-pass butt-welded pipe joint was thus calculated using an axisymmetric model by using the conventional method of thermal elastic-plastic analysis.The temperature-dependent material properties at various temperatures from room temperature to the melting point must be known for the materials used in the analysis.The temperature distribution at each time due

Contributed by the Pressure Vessels and Piping Division for publication in the J OURNAL OF P RESSURE V ESSEL T ECHNOLOGY.Manuscript received by the PVP Division,September16,1998;revised manuscript received October11,1999.Asso-ciate Technical Editor:R.

Seshadri.Fig.1Con?guration of a large-diameter multi-pass butt-welded pipe joint and its cross section

to the heat input of welding during the welding procedure was computed by transient heat conduction analysis.Transient stress during the welding steps was sequentially calculated from the thermal loads obtained by heat conduction analysis,which was done by thermal elastic-plastic analysis taking into account the nonlinearity of materials.Residual stress is obtained as the stress when the material is cooled down to room temperature after welding ?16,3?.

The residual stress distribution depends on the welding pass sequence since the joint is fabricated by multiple welding passes.In order to study the dependence of residual stress on the welding sequence,residual stress was analyzed for six welding sequences,which are shown in Fig.2.In these sequences,the welding ma-chine position was changed once in Cases 1and 2,twice in Cases 3and 4,and three times in Cases 5and 6.The welding sequences show in detail the following:

Case 1:welding the whole inside groove before the whole outside

groove;

Case 2:welding the whole outside groove before the whole inside

groove;

Case 3:welding half the inside groove,the whole outside groove,

and then the remaining inside groove;

Case 4:welding half the outside groove,the whole inside groove,

and then the remaining outside groove;

Case 5:welding half the inside groove,half the outside groove,

the remaining inside groove,and the remaining outside groove;

Case 6:welding half the outside groove,half the inside groove,

the remaining outside groove,and the remaining inside groove.The lamination number of welding passes is dozens for actual welding of a multi-pass butt-welded pipe joint.Welding lamina-tion in the analytical model was treated as 10layers for the X -shaped groove.Half the ?rst part of the groove was divided into three layers and the remaining part was divided into two layers each for the inside and outside grooves.The effectiveness in this simpli?ed modeling of welding pass has been validated by the authors ?16?.

Results of Residual Stress Analysis

Six analytical results for circumferential residual stresses on the inner and outer surface in multi-pass butt-welded pipe joints are compared in Figs.3and 4,respectively.The horizontal axes show the axial distance from the center of the weld metal on the surface.Through-thickness circumferential residual stresses along the heat-affected zone are compared in Fig.5,where the horizontal axis shows the radial distance from the inner surface to the outer surface along the heat-affected zone.In Figs.3–5,the circumfer-ential residual stresses all had a similar tendency for the six se-quences.Tensile circumferential stress distributes near the weld-ing deposit on the inner and outer surfaces.The stress is maxmum near the weld metal and decreases with distance from the weld metal,and ?nally it becomes compressive about 40mm from the center of weld metal.In each analytical result,tensile stress of about 300MPa occurs on the surface of the heat-affected

zone.

Fig.2Welding sequences of a multi-pass welded pipe

joint

Fig.3Comparison of circumferential residual stress on the inner surface in multi-pass welded pipe

joints

Fig.4Comparison of circumferential residual stress on the outer surface in multi-pass welded pipe

joints

Fig.5Comparison of circumferential residual stress across through-thickness along the heat-affected zone in multi-pass welded pipe joints

Axial residual stresses on the inner and outer surfaces of multi-pass butt-welded pipe joints are shown in Figs.6and 7,respec-tively,and through-thickness axial residual stresses along the heat-affected zone are compared in Fig.8.In Figs.6–8,axial residual stresses near the welding deposit differ,depending on the welding sequences.Those in the heat-affected zone vary with the welding sequence both on the surface and in through the thickness direction,but the axial residual stress distribution at parts remote from the weld metal is not affected by the welding sequence.

Production Mechanism of Residual Stress

A stress-corrosion crack,which is the easiest type of crack to generate and propagate,can be assumed to exist in a large-diameter multi-pass welded pipe joint by considering three condi-tions that induce the stress-corrosion cracking:the magnitude and direction of residual stress,environment of a welded joint,and sensitivity of materials.Considering the residual stress distribu-tion and the area where the material is sensitized,a surface crack along the heat-affected zone is the easiest location of crack to generate and propagate in a multi-pass welded pipe joint ?19?.This crack is located on the vertical cross section in the axial direction of the welded pipe joint.Therefore,the axial residual stress distribution has a large in?uence on its generation and propagation.In practical use,it is also effective to establish a simple method of predicting the residual stress distribution in a welded pipe joint,in the same way as in thermal elastic-plastic analysis.A simple hand-calculated model for predicting axial re-sidual stress through the thickness direction in the weld metal of a large-diameter multi-pass butt-welded pipe joint is proposed.The production mechanism of residual stress from welding pass lami-nation is studied by using the simple prediction model.

The distributions of the axial residual stress in the weld metal and that in the heat-affected zone have the same tendency.There-fore,the mechanism of producing axial residual stress in the weld metal can be directly applied to that in the heat-affected zone.Residual stress caused by welding is actually generated as the result of the complex elastic-plastic behavior of the materials,and it has a complicated distribution.But it can be approximated as being elastically generated and can be superposed in our simple model ?20,16?.The following theoretical study was conducted with a wall thickness much smaller than the pipe diameter in the multi-pass welded pipe joint.

Axial residual stress in a large-diameter multi-pass butt-welded pipe joint can be considered to be generated mainly by the sum of the stress caused by the axial shrinkage of the welding deposit and the stress caused by axial bending deformation of the pipe.As an example of a simple prediction method,two kinds of production mechanism of residual stress for the welding sequence of Case 6are shown in Figs.9and 10.Figure 9shows the axial stress generated by the axial shrinkage of the welding deposit,and Fig.10shows that caused by the axial bending deformation of the pipe.Residual stress at each welded part is calculated as the sum of these stresses,as shown in Fig.11.These ?gures show the production mechanism of axial residual stress in order of welding sequence as follows:half the outside groove,half the inside groove,the remaining outside groove,and the remaining inside groove.

In the ?rst pass of the welding sequence ?welding half the out-side groove ?,hardly any axial stress is generated in the welding deposit.One reason for this is that no stress is caused by axial shrinkage of the welding deposit because the axial shrinkage be-comes a rigid deformation due to the restriction-free condition.The other reason is that no stress is caused by axial bending de-formation either,because there is no part which restricts the axial bending deformation near the welding deposit.Thus,hardly any axial stress is created in the welding deposit by these two factors in the ?rst pass of the welding sequence.

In the second welding pass,half the inside groove is welded.The part that is already welded ?half the outside groove by the ?rst welding sequence ?restricts the axial shrinkage of this inner weld-ing deposit.Therefore,tensile axial stress caused by the axial shrinkage of the deposit occurs in the inner welding deposit,and compressive stress occurs in the outer latest welding deposit,as shown in Fig.9.The maximum absolute values of the axial stress are assumed to be 300MPa,which are around the yield stress of the material.And axial bending stress is caused by the

bending

Fig.6Comparison of axial residual stress on the inner sur-face in multi-pass welded pipe

joints

Fig.7Comparison of axial residual stress on the outer sur-face in multi-pass welded pipe

joints

Fig.8Comparison of axial residual stress across through-thickness along the heat-affected zone in multi-pass welded pipe joints

deformation due to welding half the inside groove,as shown in https://www.sodocs.net/doc/1a5404982.html,pressive and tensile bending stresses occur on the surface of the inner and outer welding deposits,respectively.Their maximum and minimum values are also assumed to be ?300MPa.Residual stress after the second weld of the sequence can be calculated as the sum of the stresses caused by axial shrinkage and radial deformation,as shown in Fig.11.

In the third pass,the remaining part of the outside groove is welded and axial residual stresses caused by axial shrinkage of the welding deposit and by axial bending deformation of the pipe are added.Tensile axial stress of ?300MPa caused by the deposit shrinkage occurs on the latest outer welding deposit,and compres-sive stress appears on the previous deposits,as shown in Fig.9.The compressive stress is ?150MPa as the result of the stress balance because the existing deposit has twice the area of the latest one.Bending stress occurs on the outer surface of the latest welding deposit as ?300MPa and on the inner surface of the previous welding deposit as ?300MPa,as shown in Fig.10.And the stresses generated by this third welding pass are superposed on the stress generated before.Residual stress after the third welding sequence is obtained as the sum of the stress caused by axial shrinkage and the stress caused by axial bending deformation,as shown in Fig.11.

Finally,in the fourth welding pass,the remainder of the inner groove is welded.Axial stress caused by axial shrinkage of the welding deposit occurs in the latest welding deposit as ?300MPa,and compressive stress of ?300MPa occurs in the whole of the previous welding deposit,distributing equally throughout ?100MPa as a result of the stress balance,as shown in Fig.9.Bending stress of ?300MPa is loaded on the ?nal outer surface and ?300MPa on the ?nal inner surface,as shown in Fig.10.The axial stresses generated by axial shrinkage and by bending deformation during ?nal welding are superposed on the stress pro-duced in the previous three welding steps.And the ?nal residual stress distribution is generated as the sum of both stresses.

The ?nal through-thickness residual stress predicted by the pro-posed simple model ?Fig.11?agrees well with the results from thermal elastic-plastic analysis ?Fig.8?.And stress distributions in the previous three welding steps were also con?rmed to agree with the thermal elastic-plastic analytical results.Furthermore,the mechanism of producing axial residual stress by the welding se-quence of Case 1?welding the whole inside groove,then welding the whole outside groove ?is also shown in Fig.12,which super-poses the stresses caused by the axial shrinkage of the deposit

and

Fig.9Production mechanism of axial residual stress by axial shrinkage of the welding deposit in a multi-pass welded pipe joint …Case 6sequence in Fig.2

…Fig.10Production mechanism of axial residual stress by bending deformation of the pipe in a multi-pass welded pipe joint …Case 6sequence in Fig.2

…

Fig.11Production mechanism of axial residual stress as the results of axial shrinkage and bending deformation in a multi-pass welded pipe joint …Case 6sequence in Fig.2…

by bending deformation of the pipe.The stress distribution pre-dicted by the simple method also agrees well with the results of thermal elastic-plastic analysis.Moreover,this prediction model was also veri?ed by the other welding sequences.Therefore,through-thickness residual stress in a welded pipe joint can be simply predicted by using the proposed method.

The simple prediction model is used when the thickness is much smaller than the diameter in the multi-pass welded pipe joint.Therefore,the residual stress distribution should be symmet-ric in the thickness direction when the welding sequences are symmetric in the thickness direction ?e.g.,Case 1versus Case 2?.But the results from thermal elastic-plastic analysis show that these stress distributions are not necessarily symmetric in the thickness direction,as shown in the comparison of Figs.4and 6or in Fig.8.This nonsymmetry occurs because the welded pipe joint is curved and the wall thickness is not much smaller than the diameter.However,the simple prediction model of residual stress is considered to be effective for evaluating residual stress rapidly and simply in a large-diameter multi-pass butt-welded pipe joint because its results agree with measurements fairly well.

Optimum Welding Sequence for Preventing Stress-Corrosion Cracking

Considering the possibility of stress-corrosion cracking in a multi-pass butt-welded pipe joint,a surface crack along the heat-affected zone is easiest to generate and propagate ?19?.This crack distributes on the vertical cross section in the axial direction of the welded pipe joint.An optimum welding sequence for suppressing stress-corrosion cracking can be selected by considering the re-sidual stress distribution,which affects its generation and propa-gation.Therefore,through-thickness axial residual stress along the heat-affected zone is very important in evaluating this.In the ac-tual environment of a welded joint,the inner surface is exposed to a more severe environment than the outer surface because the pipe may contain corrosive liquid.The crack is therefore likely to gen-erate from the inner surface along the heat-affected zone in the welded pipe joint.

There are two steps in selecting an optimum welding sequence for preventing stress-corrosion cracking:?i ?lowering the axial re-sidual stress on the inner surface along the heat-affected zone because crack generation should be prevented ?rst;and ?ii ?low-ering the through-thickness axial stress near the inner surface so that the crack propagation rate is lower or zero,even if a crack does begin to propagate by chance.

The through-thickness distribution of axial residual stress along the heat-affected zone in Fig.8reveals the optimum welding se-quence.According to Fig.8,Cases 2,3,and 6are candidates since they produce lower stress on the inner surface of the heat-affected zone.Among these three cases,the welding sequence of Case 6is best because axial through-thickness stress near the in-ner surface is almost zero until a depth of 6mm.Therefore,the optimum welding sequence is as follows:half the outside groove,half the inside groove,the remaining outside groove,then the remaining inside groove.This welding sequence should have the lowest possibility of generating and propagating stress-corrosion cracking.Moreover,a welding sequence that welds inside and outside alternately is also suitable for preventing welding distortion.

Summary

Residual stress in a large-diameter multi-pass butt-welded pipe joint was calculated for six welding sequences by thermal elastic-plastic analysis using the ?nite element method.The pipe joint used had an X-shaped groove and the sequences of welding passes were changed.The distribution of residual stress depends on the differences in the sequence of welding passes.The mechanism that produces welding residual stress was studied in detail using a simple prediction model,and it was veri?ed by comparing the predicted results with the computed results.An optimum welding sequence was determined from the residual stress distribution,which is the most reliable means of preventing stress-corrosion cracking,considering the operating conditions of actual welded pipe joints.

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受，而是试图延缓这一进程，或者通过与其他国家联合的方式，制约中国影响力的上升。这就在一定时期内增大了双方发生摩擦的可能性。类似的情况，也在中印关系、中韩关系等双边关系中不同程度地存在。 四是美国在东亚地区，存在着一个同盟体系。中国周边的日本、韩国、都是美国的盟国，此外，美国与菲律宾、泰国、新加坡、印度等保持着紧密的安全合作关系。特别是，日美同盟关系在很大程度上是直接针对中国的。美国的战略是维持在全球事务中的领导地位，并试图排除任何一个大国对美国领导地位可能发起的挑战。因此，近来美国的战略东移，试图牵制中国崛起的意图更加明显。 不过，中国的周边环境也存在一些有利因素，首先是中国对周边地区的影响力逐渐增大。中国与周边国家的贸易额巨大，是许多周边国家最重要的贸易伙伴。同时，中国在周边国家的投资也在迅速增长。周边国家对中国经济影响力的扩大，持一种多少有些矛盾的心态，一方面由于中国经济高速增长所带来的经济机会；另一方面，也有一些防范心理，担心从经济上受制于中国。但不管怎么说，中国与周边国家的经济关系，是双边关系中的一个重要稳定因素，这在几年前中日关系的“政冷经热”中已经有清晰的体现。 其次是国际社会对中国战略意图的认知。随着中国的发展，很多周边国家对“中国威胁”感到焦虑。对此，中国提出了一系列政策理念和政策举措，来缓解这方面的焦虑，包括新安全观、和平崛起、负责任大国、建立一系列伙伴关系、提出睦邻、安邻、富邻的外交政策，

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