Finite element thermal analysis of the fusion welding of a P92 steel

Mech. Sci., 3, 33–42, 2012www.mech-sci.net/3/33/2012/doi:10.5194/ms-3-33-2012© Author(s) 2012. CC Attribution 3.0 License.

Mechanical Sciences

Open Access

Finite element thermal analysis of the fusion weldingof a P92 steel pipe

A. H. Yaghi, D. W. J. Tanner, T. H. Hyde, A. A. Becker, and W. Sun

Materials, Mechanics and Structures Research Division, Faculty of Engineering, University of Nottingham,Nottingham NG7 2RD, UK

Correspondence to:A. H. Yaghi ([email protected])

Received: 22 November 2011 – Revised: 20 February 2012 – Accepted: 12 April 2012 – Published: 9 May 2012

Abstract. Fusion welding is common in steel pipeline construction in fossil-fuel power generation plants.Steel pipes in service carry steam at high temperature and pressure, undergoing creep during years of service;their integrity is critical for the safe operation of a plant. The high-grade martensitic P92 steel is suitable forplant pipes for its enhanced creep strength. P92 steel pipes are usually joined together with a similar weldmetal. Martensitic pipes are sometimes joined to austenitic steel pipes using nickel based weld consumables.Welding involves severe thermal cycles, inducing residual stresses in the welded structure, which, without postweld heat treatment (PWHT), can be detrimental to the integrity of the pipes. Welding residual stresses canbe numerically simulated by applying the finite element (FE) method in Abaqus. The simulation consists ofa thermal analysis, determining the temperature history of the FE model, followed by a sequentially-coupledstructural analysis, predicting residual stresses from the temperature history.

In this paper, the FE thermal analysis of the arc welding of a typical P92 pipe is presented. The two parts of theP92 steel pipe are joined together using a dissimilar material, made of Inconel weld consumables, producinga multi-pass butt weld from 36 circumferential weld beads. Following the generation of the FE model, theFE mesh is controlled using Model Change in Abaqus to activate the weld elements for each bead at a timecorresponding to weld deposition. The thermal analysis is simulated by applying a distributed heat flux to themodel, the accuracy of which is judged by considering the fusion zones in both the parent pipe as well as thedeposited weld metal. For realistic fusion zones, the heat flux must be prescribed in the deposited weld pass andalso the adjacent pipe elements. The FE thermal results are validated by comparing experimental temperaturesmeasured by five thermocouples on the pipe outside surface with the FE temperature history at correspondingnodal points.

1 Introduction

The numerical simulation of the process of fusion weldingof steel pipes in power generation plants has been the subjectof research and publication for a few decades. Steel pipesare an essential component in fossil-fuel and other powerplants, delivering steam at high temperature and pressure.The construction of the pipes requires joining them by apply-ing fusion welding, involving intense thermal cycles, causingrapid heating and cooling of the welded metal. This inducesresidual stresses in the weld material and the heat affectedzone (HAZ) of the pipes, which usually exceed the material

yield stress at certain locations. The large magnitude of thewelding-induced residual stresses can be detrimental to themechanical performance of the welded components duringservice. This problem is usually overcome by the post-weldheat treatment (PWHT) of the welded pipes, which is costlyand can be technically challenging; PWHT significantly re-duces the magnitude of residual stresses but it cannot elim-inate them. Therefore, the determination of residual stressesthroughout the welded pipes can be valuable for decidinghow to apply PWHT or indeed how to modify welding pro-cedures to mitigate the ill-effects of residual stresses.

Published by Copernicus Publications.

34 A. H. Yaghi et al.: Finite element thermal analysis

The determination of residual stresses in welded pipescan be achieved by experimental procedures, such as tech-niques based on centre-hole drilling, deep-hole drilling, X-ray diffraction and neutron diffraction, which are costlyand difficult at times, and can involve destructive or semi-destructive procedures, otherwise the revealed stress field islimited to the surface or near the surface (Yaghi et al., 2010).The experimentally-determined residual stress field is mea-sured at specific locations only and the experimental meth-ods involve averaging over an area, possibly missing out onsharp changes in the residual stress field which are typicalof what is induced by the process of welding. The finite ele-ment (FE) numerical method, however, can provide residualstress predictions throughout the welded component, captur-ing the sharp peaks of the stress field, depending on the re-finement of the generated FE mesh. The challenging part ofproviding accurate numerical results lies in acquiring a set ofmaterial properties which truly reflect the behaviour of themodelled material. If the required set of material propertiesfor the numerical simulation is available and the generatedFE mesh has sufficient refinement, the residual stress fielddue to welding can be accurately obtained throughout the FEmodel, making the FE method highly effective at predictingwelding residual stresses.

The FE method comprises two parts: a thermal analy-sis, simulating the welding thermal cycles and delivering atemperature history throughout the welding process, and asequentially-coupled structural analysis, processing the tem-perature history as input data and determining the developingthermal stresses and the residual stress field induced by thethermal cycles. Evidently, the accuracy of residual stressesis dependent on the accuracy of the temperature history de-termined by the thermal analysis. In this paper, the thermalanalysis of an arc welded steel pipe, typically found in powergeneration plants, is presented. The pipe is made of P92 steeland is dissimilar in the sense that the weld metal is differ-ent from the two P92 parts being joined together. P92 steelpipes are usually joined together with a similar weld metal.Martensitic pipes are sometimes joined to austenitic steelpipes using nickel based weld consumables. In this study,the P92 steel pipe has been welded with a dissimilar weldmetal for research purposes. The weld is a circumferentialbutt-weld consisting of 36 beads. The first bead or weld passis made of Inconel 82 (IN82) and the remaining 35 beadsor weld passes are made of Inconel 625 (IN625). P92, alsoknown as NF616 in Japan, is a relatively newly-developed9–12 % Cr ferritic steel that can be used in high tempera-ture applications, such as in fossil-fuel and nuclear powerstations as well as chemical plants. P92 is a modified form ofits predecessor P91. Molybdenum, Mo, in P91 (9Cr1Mo) hasbeen partially replaced by Tungsten, W, for P92 (9Cr2W).P92 has creep strength approximately 30 % higher than thecurrently widely used steel P91 (Brozda, 2005), and it hasproved suitable for piping and headers at temperatures upto around 625◦C (Richardot et al., 2000). Hence, in modern

super-critical steam power plants, P92 allows higher operat-ing parameters and therefore higher efficiencies.

In general, an FE simulation of a welded pipe canbe three-dimensional (3-D) or, when appropriate, can betwo-dimensional (2-D) axisymmetric (Deng and Murakawa,2006). A 3-D simulation is desirable when effects that can-not be otherwise modelled are revealed, such as stresses atwelding starting and stopping locations. A 2-D axisymmetricsimulation, however, can be more suitable, when such effectsare to be ignored, and when the more uniform residual stressfield is of interest. This is because a 2-D axisymmetric simu-lation can have a much finer FE mesh for the same computa-tional processing time in comparison with a 3-D simulationof the same model, which can lead to significantly more ac-curate results for large models with numerous weld passes.3-D models of welds with many passes can overcome the ne-cessity of running prohibitively large analyses by employingtechniques such as lumping of the weld passes together andkeeping the FE mesh relatively coarse, which would reducethe accuracy of the results.

The reported thermal analysis is based on a 2-D axisym-metric model of the welded pipe with the FE mesh con-trolled using the facility named Model Change in Abaqus(ABAQUS User Manual, 2009). Model Change allows theweld material for each weld pass to become active during thenumerical simulation at the time corresponding to the depo-sition of the pass. The heat delivered by the welding arc hasbeen modelled by prescribing a uniformly distributed heatflux, which is a triangular function against time, correspond-ing to the approaching and then departing welding arc. Thedistributed heat flux has been applied to each weld pass whenit is deposited and also to the surrounding pipe (parent) mate-rial in order to obtain the desired thermal contours in the FEmodel. In the FE thermal study, consideration has been givento fusion zones in the weld and parent material, peak tem-perature contours throughout the HAZ and the temperaturehistory at locations corresponding to a set of attached ther-mocouples. The actual welded pipe had five thermocouplesattached at different locations, making it possible to validatethe temperature history determined by the reported FE ther-mal analysis.

2 Dissimilar welded pipe

The FE thermal analysis has been conducted to model thethermal behaviour of a welded steel pipe. Prior to welding,the P92 pipe was cut into two halves and the ends preparedfor joining, a photograph of which is shown in Fig. 1. Thetwo halves were axisymmetrically aligned and held in largemetal vices and then welded together using 36 weld beads(or passes) of dissimilar weld materials. The right-hand sidewas released after the third weld pass had been completed,and the left-hand side remained fixed throughout the processof welding – please see Fig. 2.

Mech. Sci., 3, 33–42, 2012 www.mech-sci.net/3/33/2012/

A. H. Yaghi et al.: Finite element thermal analysis 35

Table 1. Chemical composition of the pipe and weld materials.

Fe C Mn P S Si Cr W Mo V Nb N B Al Ni

P92 Bal 0.10 0.45 0.015 0.002 0.45 8.62 1.86 0.33 0.21 0.076 0.047 0.003 0.019 0.27IN625 1.48 0.03 0.76 0.005 0.005 0.33 21.9 – 8.67 – 3.340 – – – BalIN82 0.80 0.01 3.00 – – 0.10 20.0 – – – 2.700 – – – Bal

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is a circumferential butt-weld consisting of 36 beads. The first bead or weld pass is made ofInconel 82 (IN82) and the remaining 35 beads or weld passes are made of Inconel 625 (IN625).P92, also known as NF616 in Japan, is a relatively newly-developed 9-12% Cr ferritic steel thatcan be used in high temperature applications, such as in fossil-fuel and nuclear power stations aswell as chemical plants. P92 is a modified form of its predecessor P91. Molybdenum, Mo, in P91(9Cr1Mo) has been partially replaced by Tungsten, W, for P92 (9Cr2W). P92 has creep strengthapproximately 30% higher than the currently widely used steel P91 (Brózda, 2005), and it hasproved suitable for piping and headers at temperatures up to around 625°C (Richardot, 2000).Hence, in modern super-critical steam power plants, P92 allows higher operating parameters andtherefore higher efficiencies.

In general, an FE simulation of a welded pipe can be three-dimensional (3D) or, when appropriate,can be two-dimensional (2D) axisymmetric (Deng, 2006). A 3D simulation is desirable wheneffects that cannot be otherwise modelled are revealed, such as stresses at welding starting andstopping locations. A 2D axisymmetric simulation, however, can be more suitable, when sucheffects are to be ignored, and when the more uniform residual stress field is of interest. This isbecause a 2D axisymmetric simulation can have a much finer FE mesh for the same computationalprocessing time in comparison with a 3D simulation of the same model, which can lead tosignificantly more accurate results for large models with numerous weld passes. 3D models ofwelds with many passes can overcome the necessity of running prohibitively large analyses byemploying techniques such as lumping of the weld passes together and keeping the FE meshrelatively coarse, which would reduce the accuracy of the results.

Figure 1. P92 pipe before welding.

The reported thermal analysis is based on a 2D axisymmetric model of the welded pipe with theFE mesh controlled using the facility named Model Change in Abaqus (ABAQUS User Manual,2009). Model Change allows the weld material for each weld pass to become active during thenumerical simulation at the time corresponding to the deposition of the pass. The heat deliveredby the welding arc has been modelled by prescribing a uniformly distributed heat flux, which is a

Figure 1. P92 pipe before welding.

2.1 Pipe geometry, weld details and materialspecifications

The geometry and dimensions of the welded P92 pipe, aswell as the welding sequence, are shown in Fig. 3. Thewelded pipe has an outside diameter of 355 mm and a to-tal length of 700 mm. The first weld pass was deposited bytungsten inert gas (TIG) welding using the nickel based weldmaterial Inconel 82 (IN82), having the manufacturer’s brandname UTP A068HH, with a rod diameter of 2.4 mm, dur-ing which argon gas was used to shield the weld from atmo-spheric gases. Weld passes 2 to 36 were deposited by manualmetal arc (MMA) welding using the nickel based weld ma-terial Inconel 625 (IN625), having the manufacturer’s brandname NIMROD 625 KS, with the filler rod for weld passes 2and 3 having a diameter of 3.2 mm and the remaining passeshaving a diameter of 4.0 mm. The chemical compositions forthe parent and weld materials are shown in Table 1. The ma-terial properties are discussed in Sect. 3.2.

2.2 Thermocouples and weld procedure

The temperature of the pipe was monitored during weldingby attaching five thermocouples to the surface of the pipeand recording the temperatures with a chart plotter through-out the welding process. The thermocouples measured thesurface temperature at five different locations with varyingdistance from the weld centre line (WCL) and also at differ-ent angular positions around the pipe circumference. Sincethe FE analysis is axisymmetric, the angular position of the

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triangular function against time, corresponding to the approaching and then departing welding arc.The distributed heat flux has been applied to each weld pass when it is deposited and also to thesurrounding pipe (parent) material in order to obtain the desired thermal contours in the FE model.In the FE thermal study, consideration has been given to fusion zones in the weld and parentmaterial, peak temperature contours throughout the HAZ and the temperature history at locationscorresponding to a set of attached thermocouples. The actual welded pipe had five thermocouplesattached at different locations, making it possible to validate the temperature history determinedby the reported FE thermal analysis.

2. Dissimilar Welded Pipe

The FE thermal analysis has been conducted to model the thermal behaviour of a welded steelpipe. Prior to welding, the P92 pipe was cut into two halves and the ends prepared for joining, aphotograph of which is shown in Figure 1. The two halves were axisymmetrically aligned andheld in large metal vices and then welded together using 36 weld beads (or passes) of dissimilarweld materials. The right-hand side was released after the third weld pass had been completed,and the left-hand side remained fixed throughout the process of welding – please see Figure 2.

Figure 2. P92 pipe during welding.

2.1 Pipe Geometry, Weld Details and Material Specifications

The geometry and dimensions of the welded P92 pipe, as well as the welding sequence, are shownin Figure 3. The welded pipe has an outside diameter of 355mm and a total length of 700mm.The first weld pass was deposited by tungsten inert gas (TIG) welding using the nickel based weldmaterial Inconel 82 (IN82), having the manufacturer’s brand name UTP A068HH, with a roddiameter of 2.4mm, during which argon gas was used to shield the weld from atmospheric gases.Weld passes 2 to 36 were deposited by manual metal arc (MMA) welding using the nickel basedweld material Inconel 625 (IN625), having the manufacturer’s brand name NIMROD 625 KS,

Figure 2. P92 pipe during welding.

thermocouples around the circumference is irrelevant to thereported study. The fact that the angular position varies forthe thermocouples, however, is relevant in explaining someof the scatter noticed in the temperature results as well assome of the discrepancies observed between the experimen-tally measured and the FE determined temperatures, as willbe discussed later in the paper. Thermocouples 1 and 2 (TC1and TC2) are closest to the weld and are located on the left-hand pipe section, both at an axial distance of 32.4 mm fromthe WCL. The other thermocouples, TC3, TC4 and TC5, arelocated on the right-hand pipe section at axial distances of35.0 mm, 40.5 mm and 70.3 mm from the WCL, respectively.All the distances were measured after depositing the thirdweld pass and before the rest of the passes were deposited,i.e. before any significant deformation took place. It is worthnoting that the distances relating to the thermocouples are ac-curate to within±0.5 mm; they are, however, quoted here tothe stated precision to correspond to the matching nodal po-sitions in the FE mesh, where temperature comparisons aremade.

The thermocouples were used to monitor the temperatureof the pipe to maintain the required preheat and interpasstemperature, necessary in the case of P92 steel for a weldingprocedure which would likely be free from the risk of microor macro-cracking during welding or just after, as the pipecooled to room temperature. The temperature history ob-tained from the thermocouples has also been used to validatethe thermal behaviour determined by the FE simulation. Thepipe was heated by an electric blanket on both sides as can be

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with the filler rod for weld passes 2 and 3 having a diameter of 3.2mm and the remaining passeshaving a diameter of 4.0mm. The chemical compositions for the parent and weld materials areshown in Table 1. The material properties are discussed in section 3.2.

Figure 3. Sketch of weld bead sequence showing overall weld dimensions.

Table 1. Chemical composition of the pipe and weld materials.

Fe C Mn P S Si Cr W Mo V Nb N B Al Ni

P92 Bal 0.10 0.45 0.015 0.002 0.45 8.62 1.86 0.33 0.21 0.076 0.047 0.003 0.019 0.27

IN625 1.48 0.03 0.76 0.005 0.005 0.33 21.9 - 8.67 - 3.340 - - - Bal

IN82 0.80 0.01 3.00 - - 0.10 20.0 - - - 2.700 - - - Bal

2.2 Thermocouples and Weld Procedure

The temperature of the pipe was monitored during welding by attaching five thermocouples to thesurface of the pipe and recording the temperatures with a chart plotter throughout the weldingprocess. The thermocouples measured the surface temperature at five different locations withvarying distance from the weld centre line (WCL) and also at different angular positions aroundthe pipe circumference. Since the FE analysis is axisymmetric, the angular position of thethermocouples around the circumference is irrelevant to the reported study. The fact that theangular position varies for the thermocouples, however, is relevant in explaining some of thescatter noticed in the temperature results as well as some of the discrepancies observed betweenthe experimentally measured and the FE determined temperatures, as will be discussed later in thepaper. Thermocouples 1 and 2 (TC1 and TC2) are closest to the weld and are located on the left-hand pipe section, both at an axial distance of 32.4mm from the WCL. The other thermocouples,

32mm

28mm

2.0-2.5mm

15°

30°

30mm

1mm

3mm

4.35mm

1.5mm

1

7654

32

8 910

11 1213

14 151617 1819

20 2122 2324 2526 27

28 2930 3132 3334 35 36

Figure 3. Sketch of weld bead sequence showing overall weld di-mensions.

seen in Fig. 2. Although the blanket would go on and off andwould reach relatively high temperatures when switched on,the temperatures at the weld region and the HAZ were rela-tively steady, and therefore the FE modelling of the blanketeffect was achieved by prescribing a sink temperature at thesurface where the blanket was in contact with the pipe. Thesink temperature was varied from one weld pass to the other(from 105◦C for Pass 1 to 251◦C for Pass 26), during the FEsimulation, to emulate the experimentally measured temper-atures during the interpass periods. During the actual weld-ing of the pipe, as recommended by literature, it was aimedto keep the preheat and interpass temperatures for the TIGwelding of the first pass between 100◦C and 150◦C (Hol-loway et al., 2008) and for the MMA welding of all the otherpasses between 200◦C and 250◦C (Richardot et al., 2000).

3 FE thermal analysis

The FE thermal analysis reported in this paper is part of amore complete FE simulation intended by the authors to de-termine welding residual stresses for the dissimilar weldedP92 pipe. The FE thermal analysis, which forms the first partof the simulation, is reported here, whereas the sequentiallycoupled structural analysis, which forms the second part ofthe simulation, is to be reported in a future publication. Thetype of simulation adopted by the authors is described assolid-mechanical, modelling the heat flux delivered to thepipe by the heat source and allowing for the thermo-physicalbehaviour, such as conductivity, and then translating the ther-mal effects into structural mechanical effects, such as volumeexpansions and plasticity, without allowing for any fluid ef-fects of the molten regions. The solid mechanics approachis justified in ignoring the fluid effects, since stresses be-come significant only when the material has solidified andis relatively cool. When the material is molten or close tobeing molten, it is soft enough not to sustain any signifi-cant stresses. As long as the mechanical properties used in

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(ABAQUS User Manual, 2009). A complete FE mesh has been generated from the start, whichincludes the pipe and the weld region.

The weld pass sequence in the FE model is identical to that in the actual weld, as shown in Figure4. The shape of the weld passes in the FE model does not have to accurately match the actualshape of the weld beads to produce representative and realistic thermal contours (Yaghi, 2010).Although the shapes of the FE passes are rather square compared to the actual weld, the resultingthermal contours emerge rounded and realistic. Nonetheless, it is believed that the final layer ofweld passes has the most significant effect on residual stresses, and therefore an attempt has beenmade to make the final layer closer in shape to the actual weld beads. The shape of the final layerof beads can easily be adjusted without unduly complicating the FE model, which is not the casefor the other beads. The actual FE mesh which has been generated for the model is shown inFigures 5 and 6. It can be seen in Figure 5 that the mesh is refined in the weld region and HAZand that it becomes gradually coarser as it moves away from the weld, as shown in Figure 6. Thecomplete 2D axisymmetric FE mesh comprises 9022 nodes and 2919 elements. The element typeused throughout the FE model is an eight-node continuum solid quadratic axisymmetric diffusiveheat transfer quadrilateral, given the name DCAX8 in Abaqus.

Figure 4. Weld pass sequence in the FE model.

The elements in the FE mesh forming each weld pass are assigned a group name so that each passcan be deposited independently during the simulation. At the inception of the simulation, theelements of all the weld passes are made to become inactive, rendering them thermally dormant,while still keeping all the elements of the mesh attached together. This is achieved in Abaqus byusing the command “Model Change, Type=Element, Remove” at the beginning of the first step inthe input file of the thermal analysis. Each weld pass is deposited in the corresponding step in thethermal analysis by using the command “Model Change, Type=Element, Add”, which reactivatesthe corresponding elements in the FE mesh.

Figure 4. Weld pass sequence in the FE model.

the simulation represent the actual behaviour of the material,fluid effects do not have to be taken into consideration.

3.1 FE model of the welded pipe

The FE simulation of the fusion welding of the dissimilarwelded P92 pipe starts with generating an FE model by firstlycreating an FE mesh. The commercial package used for thispurpose and indeed for performing the complete FE thermalanalysis, reported in this paper, is Abaqus (ABAQUS UserManual, 2009). A complete FE mesh has been generatedfrom the start, which includes the pipe and the weld region.

The weld pass sequence in the FE model is identical to thatin the actual weld, as shown in Fig. 4. The shape of the weldpasses in the FE model does not have to accurately match theactual shape of the weld beads to produce representative andrealistic thermal contours (Yaghi et al., 2010). Although theshapes of the FE passes are rather square compared to theactual weld, the resulting thermal contours emerge roundedand realistic. Nonetheless, it is believed that the final layerof weld passes has the most significant effect on residualstresses, and therefore an attempt has been made to makethe final layer closer in shape to the actual weld beads. Theshape of the final layer of beads can easily be adjusted with-out unduly complicating the FE model, which is not the casefor the other beads. The actual FE mesh which has been gen-erated for the model is shown in Figs. 5 and 6. It can be seenin Fig. 5 that the mesh is refined in the weld region and HAZand that it becomes gradually coarser as it moves away fromthe weld, as shown in Fig. 6. The complete 2-D axisymmet-ric FE mesh comprises 9022 nodes and 2919 elements. Theelement type used throughout the FE model is an eight-nodecontinuum solid quadratic axisymmetric diffusive heat trans-fer quadrilateral, given the name DCAX8 in Abaqus.

The elements in the FE mesh forming each weld passare assigned a group name so that each pass can be de-posited independently during the simulation. At the incep-tion of the simulation, the elements of all the weld passes



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Figure 5. FE mesh showing the weld, HAZ and part of the pipe.

The requirement of the FE model to exchange heat, at its surface, with the surroundingenvironment is facilitated using the option “sfilm” in the input file. A surface film or “sfilm” isgenerated by selecting the outside surface of relevant elements which are expected to exchangeheat with the surroundings by convection and radiation or due to direct contact with a solidmaterial as in the case of the electric blanket. The surface film is specified and the film propertiesare assigned values to allow appropriate amount of heat to be exchanged at the surface. Thetemperature of the environment surrounding the model is specified by prescribing a constant sinktemperature of 20°C at the relevant surfaces. Therefore, by specifying sets of surface films andfilm properties, the FE model can exchange heat with the surroundings, simulating the heat flowbetween the surface of the model and the environment, whether the surface belongs to the weld orpipe, and also between the pipe and the electric blanket. The surface film is made to follow theevolution of the outside surface as the deposition of each weld pass is simulated.

Figure 6. Complete 2D axisymmetric FE mesh.

3.2 Material Properties

The material property data required for the FE thermal analysis have been obtained for threedifferent materials, namely, P92 steel, Inconel 82 (IN82) and Inconel 625 (IN625). Some of thedata are available in literature, but some have been derived either by extrapolation to highertemperatures or by referring to other similar materials with known material property data. This


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The requirement of the FE model to exchange heat, at its surface, with the surroundingenvironment is facilitated using the option “sfilm” in the input file. A surface film or “sfilm” isgenerated by selecting the outside surface of relevant elements which are expected to exchangeheat with the surroundings by convection and radiation or due to direct contact with a solidmaterial as in the case of the electric blanket. The surface film is specified and the film propertiesare assigned values to allow appropriate amount of heat to be exchanged at the surface. Thetemperature of the environment surrounding the model is specified by prescribing a constant sinktemperature of 20°C at the relevant surfaces. Therefore, by specifying sets of surface films andfilm properties, the FE model can exchange heat with the surroundings, simulating the heat flowbetween the surface of the model and the environment, whether the surface belongs to the weld orpipe, and also between the pipe and the electric blanket. The surface film is made to follow theevolution of the outside surface as the deposition of each weld pass is simulated.

Figure 6. Complete 2D axisymmetric FE mesh.

3.2 Material Properties

The material property data required for the FE thermal analysis have been obtained for threedifferent materials, namely, P92 steel, Inconel 82 (IN82) and Inconel 625 (IN625). Some of thedata are available in literature, but some have been derived either by extrapolation to highertemperatures or by referring to other similar materials with known material property data. This

Figure 6. Complete 2-D axisymmetric FE mesh.

are made to become inactive, rendering them thermally dor-mant, while still keeping all the elements of the mesh at-tached together. This is achieved in Abaqus by using thecommand “Model Change, Type=Element, Remove” at thebeginning of the first step in the input file of the thermal anal-ysis. Each weld pass is deposited in the corresponding step inthe thermal analysis by using the command “Model Change,Type=Element, Add”, which reactivates the correspondingelements in the FE mesh.

The requirement of the FE model to exchange heat, at itssurface, with the surrounding environment is facilitated usingthe option “sfilm” in the input file. A surface film or “sfilm”is generated by selecting the outside surface of relevant el-ements which are expected to exchange heat with the sur-roundings by convection and radiation or due to direct con-tact with a solid material as in the case of the electric blan-ket. The surface film is specified and the film properties areassigned values to allow appropriate amount of heat to be ex-changed at the surface. The temperature of the environmentsurrounding the model is specified by prescribing a constantsink temperature of 20◦C at the relevant surfaces. Therefore,by specifying sets of surface films and film properties, theFE model can exchange heat with the surroundings, simu-lating the heat flow between the surface of the model andthe environment, whether the surface belongs to the weld orpipe, and also between the pipe and the electric blanket. Thesurface film is made to follow the evolution of the outsidesurface as the deposition of each weld pass is simulated.

3.2 Material properties

The material property data required for the FE thermal anal-ysis have been obtained for three different materials, namely,P92 steel, Inconel 82 (IN82) and Inconel 625 (IN625). Someof the data are available in literature, but some have been de-

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approach has been adopted, since parametric analyses by the authors of welding residual stressesfor different materials have indicated that welding residual stresses in general are mostly sensitiveto the value of yield stress as well as that of the coefficient of linear thermal expansion (which arepart of the FE structural analysis); they are significantly less sensitive to changes in thermalproperties; moreover, heat fluxes would have to be adjusted to obtain realistic molten zones, whichwould in turn compensate for inaccuracies in the thermal properties. It has also been possible tocalculate melting temperatures from thermodynamic software packages. The material propertiesthat need to be specified in the thermal analysis are the thermal conductivity, density, specific heatcapacity, latent heat capacity, solidus and liquidus temperatures and heat transfer coefficients.

Figure 7. Thermal conductivity and specific heat capacity in the FE model.

The FE model uses the thermal conductivity of P91 steel (Yaghi, 2010). The specific heatcapacity of P92 steel has been obtained from literature (Richardot, 2000) from room temperatureup to 650°C, above which it has been assumed to remain unchanged. Both material properties areshown in Figure 7. The solidus and liquidus temperatures for P92 steel have been calculated as1420°C and 1500°C by using MTDATA software developed at the National Physical Laboratory(NPL) in the UK and they have been corroborated by the same values published for P91 steel(Yaghi, 2010). The density for P92 steel is specified in literature (Richardot, 2000) as 7850kg/m3.The latent heat capacity for P92 steel has been assumed to be the same as that for P91 steel,specified as 260kJ/kg (Yaghi, 2010).

The thermal conductivity and specific heat capacity of IN625 are provided in literature (SpecialMetals, 2006) for temperatures up to 982°C and 1093°C respectively. The same values have beenassumed for the weld material NIMROD 625 KS for the given temperature range, above which thetwo thermal properties are assumed to remain constant up to the melting point. As the materialgoes from the solidus to the liquidus temperature, the thermal conductivity is doubled inmagnitude to compensate for the stirring effects in the molten state (Brickstad, 1998). The samethermal conductivity and specific heat capacity have been assumed for the weld material UTP

Figure 7. Thermal conductivity and specific heat capacity in the FEmodel.

rived either by extrapolation to higher temperatures or by re-ferring to other similar materials with known material prop-erty data. This approach has been adopted, since parametricanalyses by the authors of welding residual stresses for dif-ferent materials have indicated that welding residual stressesin general are mostly sensitive to the value of yield stressas well as that of the coefficient of linear thermal expansion(which are part of the FE structural analysis); they are signif-icantly less sensitive to changes in thermal properties; more-over, heat fluxes would have to be adjusted to obtain realisticmolten zones, which would in turn compensate for inaccura-cies in the thermal properties. It has also been possible to cal-culate melting temperatures from thermodynamic softwarepackages. The material properties that need to be specifiedin the thermal analysis are the thermal conductivity, density,specific heat capacity, latent heat capacity, solidus and liq-uidus temperatures and heat transfer coefficients.

The FE model uses the thermal conductivity of P91 steel(Yaghi et al., 2010). The specific heat capacity of P92 steelhas been obtained from literature (Richardot et al., 2000)from room temperature up to 650◦C, above which it has beenassumed to remain unchanged. Both material properties areshown in Fig. 7. The solidus and liquidus temperatures forP92 steel have been calculated as 1420◦C and 1500◦C byusing MTDATA software developed at the National PhysicalLaboratory (NPL) in the UK and they have been corrobo-rated by the same values published for P91 steel (Yaghi etal., 2010). The density for P92 steel is specified in literature(Richardot et al., 2000) as 7850 kg m−3. The latent heat ca-pacity for P92 steel has been assumed to be the same as thatfor P91 steel, specified as 260 kJ kg−1 (Yaghi et al., 2010).

The thermal conductivity and specific heat capacity ofIN625 are provided in literature (Special Metals, 2006) fortemperatures up to 982◦C and 1093◦C, respectively. Thesame values have been assumed for the weld material NIM-ROD 625 KS for the given temperature range, above whichthe two thermal properties are assumed to remain constant



up to the melting point. As the material goes from the solidusto the liquidus temperature, the thermal conductivity is dou-bled in magnitude to compensate for the stirring effects inthe molten state (Brickstad and Josefson, 1998). The samethermal conductivity and specific heat capacity have beenassumed for the weld material UTP A068HH used for thefirst pass due to the lack of data for IN82. It is worth not-ing that the effect of the first weld pass on residual stressesnear the outside surface of the pipe, where most interestlies for such wall thickness, is expected to be negligible.The thermal conductivity and specific heat capacity for theweld materials are shown in Fig. 7. The density for IN625is given as 8440 kg m−3 (Special Metals, 2006). This valuehas been assumed for both weld materials. The latent heatcapacity for pure nickel is listed as 297 kJ kg−1 (The Engi-neering ToolBox, 2011). Therefore, an approximate and in-termediate value between those of P92 steel and pure nickelof 280 kJ kg−1 has been assumed for the weld materials. Thesolidus and liquidus temperatures for UTP A068HH havebeen calculated to be 1301◦C and 1368◦C and for NIM-ROD 625 KS to be 1250◦C and 1345◦C, respectively, usingthe software ThermoCalc Classic Version S (TCC), devel-oped by the Foundation of Computational Thermodynamicsin Stockholm of Sweden, and utilising Thermotech NickelsDatabase Version 7 (TTNi7) (Saunders et al., 1996).

Heat losses at the surface of the pipe have been mod-elled by prescribing appropriate values for the coefficient ofheat transfer under the Abaqus command Film Property. Heatloss coefficients for P92 steel and also the Inconel weld ma-terial have been assumed to be the same as those for P91steel, given in literature (Yaghi et al., 2005) to be 0.0668T(W m−2 K) when the temperatureT is below 500◦C and0.231T–82.1 (W m−2 K) whenT is above 500◦C. A similarapproach has been adopted to model the effect of the elec-tric blanket on the pipe. Recognising that the blanket wouldfluctuate in temperature substantially and would be switchingon and off to control the interpass temperature during weld-ing, the modelling has been simplified to having the blanketat a constant temperature during the deposition of any oneweld pass, prescribing a very high value to the coefficient ofheat transfer at the surface with which the blanket has con-tact, and controlling the sink temperature at that surface (be-tween 105◦C for Pass 1 and 251◦C for Pass 26) to obtain thedesired temperature history at the five thermocouples. Thevalue prescribed for the coefficient of heat transfer for thispurpose is 200T (W m−2 K), whereT is the temperature ofthe pipe.

3.3 Heat fluxes and fusion zones in the FE model

The main part of the thermal analysis is to model the ef-fect of the heat source on the pipe during fusion welding, i.e.the heat delivered by the welding arc into the weld and sur-rounding material. This has been covered in detail for similarwelded pipes in a previous publication (Yaghi et al., 2005).

The modelling method, however, needs to be modified forthe thermal analysis of a dissimilar welded pipe, as is thecase here.

In general, the heat effect of the welding arc on a pipe canbe modelled by prescribing a uniformly distributed heat flux(Dflux in Abaqus) for each of the weld passes as they aredeposited. In the reported work, the flux is assumed to fol-low a triangular function (Yaghi et al., 2005), which startsfrom zero and rises linearly to reach a peak at half time, tosignify the approach of the weld arc, and then it decreaseslinearly back to zero at full time, to signify the departureof the arc. The welding speed was determined to be around50 mm min−1 for the first pass and ranged between 152 and207 mm min−1 for the remaining passes. The welding currentranged between 90 and 130 A, and the DC voltage ranged be-tween 9 and 10 for the first pass and between 24.3 and 26.5 Vfor passes 2 to 36. The heat flux in the FE model lasted for10.7 s for the first pass, 5.9 s for each of the second and thirdpasses, and between 6.6 s and 7.6 s for each of the remainingpasses. The method of deriving the heat flux from the weld-ing parameters for a 2-D axisymmetric FE model has beendescribed in detail elsewhere (Yaghi et al., 2005). When theflux for each pass had been estimated in the reported study,the magnitudes for the 36 passes were specified in a tabularform in the Abaqus input file. The tabular form comes underthe Abaqus command Amplitude. Initially, the same conven-tional method was followed of prescribing the flux for eachpass at the elements forming the weld pass material. If thewelded pipe were not dissimilar, that would usually producethe expected thermal contours and fusion zones. In the re-ported study, however, the welded pipe is dissimilar, i.e. theweld material is different from that of the pipe; also, the melt-ing temperatures for the materials are substantially different.This caused the fusion zones to be unrealistic when the initialanalysis was conducted; fusion zones in the weld developedas expected, but the pipe material, having a higher meltingtemperature range, remained solid. An example of an unre-alistic fusion zone is shown in Fig. 8 for weld pass 14. Thefigure shows fusion into the pre-deposited weld material, sur-rounding the current pass, to a depth of 2.2 mm and no fusionat all in the pipe (parent) material.

The conventional method has been modified by prescrib-ing the heat flux for each weld pass in the elements formingthe weld pass and also in the pipe material surrounding thesame pass. This is consistent with the actual welding prac-tice, as the arc delivers heat to the pipe material as well asthe weld. This modification has resulted in obtaining real-istic fusion zones. Fusion into the weld material surround-ing the current pass is expected to be typically between 1.5and 2.5 mm and into the pipe material to be between 1.0 and2.0 mm. If the fusion zone for any weld pass is not as ex-pected, the heat flux can be adjusted until realistic results areobtained. The heat flux in the input file can be adjusted eitherby modifying the numbers in the Amplitude table or by in-cluding a modifying factor referred to as the Reference Flux



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deriving the heat flux from the welding parameters for a 2D axisymmetric FE model has beendescribed in detail elsewhere (Yaghi, 2005). When the flux for each pass had been estimated inthe reported study, the magnitudes for the 36 passes were specified in a tabular form in the Abaqusinput file. The tabular form comes under the Abaqus command Amplitude. Initially, the sameconventional method was followed of prescribing the flux for each pass at the elements formingthe weld pass material. If the welded pipe were not dissimilar, that would usually produce theexpected thermal contours and fusion zones. In the reported study, however, the welded pipe isdissimilar, i.e. the weld material is different from that of the pipe; also, the melting temperaturesfor the materials are substantially different. This caused the fusion zones to be unrealistic whenthe initial analysis was conducted; fusion zones in the weld developed as expected, but the pipematerial, having a higher melting temperature range, remained solid. An example of an unrealisticfusion zone is shown in Figure 8 for weld pass 14. The figure shows fusion into the pre-depositedweld material, surrounding the current pass, to a depth of 2.2mm and no fusion at all in the pipe(parent) material.

Figure 8. Fusion zone when heat flux is applied only to weld elements.

The conventional method has been modified by prescribing the heat flux for each weld pass in theelements forming the weld pass and also in the pipe material surrounding the same pass. This isconsistent with the actual welding practice, as the arc delivers heat to the pipe material as well asthe weld. This modification has resulted in obtaining realistic fusion zones. Fusion into the weldmaterial surrounding the current pass is expected to be typically between 1.5 and 2.5mm and intothe pipe material to be between 1.0 and 2.0mm. If the fusion zone for any weld pass is not asexpected, the heat flux can be adjusted until realistic results are obtained. The heat flux in theinput file can be adjusted either by modifying the numbers in the Amplitude table or by includinga modifying factor referred to as the Reference Flux Magnitude, the value of which comes underthe Abaqus command Dflux, within the corresponding step in the input file. An example of arealistic fusion zone is shown in Figure 9 for weld pass 14. The figure shows a fusion of 2.2mminto the weld material and 1.2mm into the pipe (parent) material. It also shows that the weld pass

Figure 8. Fusion zone when heat flux is applied only to weld ele-ments.

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causes melting throughout the surface it comes into contact with, otherwise the quality of the weldwould be compromised. The average Dflux for the whole welding process is 9.15GW/m3 and theaverage time span is 7.2s.

Figure 9. Fusion zones (FZ) when heat flux is applied to pipe and weld elements.

3.4 Comparison of FE Temperatures and Thermocouple Measurements

The FE thermal results have been validated by comparing them to the temperature historymeasured by five thermocouples (TC1, TC2, …, TC5). The temperature history at the locations ofthe thermocouples predicted by the FE thermal analysis can be made to vary significantly (tens ofdegrees) by altering the magnitude of the heat fluxes and the corresponding time spans over whichthe fluxes last. The heat fluxes and the time spans have to be changed together so as to keep thefusion zones unchanged. This way, the FE temperature history at the thermocouple locations canbe changed without disrupting the fusion zones which are consistent with realistic expectations.The final set of heat fluxes and time spans have been reached and their average values are reportedin the previous subsection.

Comparisons between the FE predicted temperatures and the measured temperatures by thethermocouples are depicted in Figures 10 to14. Figure 10 shows measured temperatures by TC1and TC2 and corresponding FE temperatures at 32.4mm to the left-hand side of the WCL.Although both thermocouples are attached at an equal distance from the WCL (at different angularlocations around the pipe), they exhibit a slightly different temperature history. This willevidently be missed out in a 2D axisymmetric FE analysis due to its 2D nature. Also, thisprovides a justification for the discrepancies between the FE results and the experimentallymeasured temperatures, with the discrepancies varying from one pass to another depending onstarting and stopping positions relative to the measurement locations, bearing in mind that for eachweld pass (once around the circumference) there were six sets of starting and stopping points. Theresults from TC1 and TC2 are averaged and compared again to the FE temperatures at 32.4mm to

Figure 9. Fusion zones (FZ) when heat flux is applied to pipe andweld elements.

Magnitude, the value of which comes under the Abaqus com-mand Dflux, within the corresponding step in the input file.An example of a realistic fusion zone is shown in Fig. 9 forweld pass 14. The figure shows a fusion of 2.2 mm into theweld material and 1.2 mm into the pipe (parent) material. Italso shows that the weld pass causes melting throughout thesurface it comes into contact with, otherwise the quality ofthe weld would be compromised. The average Dflux for thewhole welding process is 9.15 GW m−3 and the average timespan is 7.2 s.

3.4 Comparison of FE temperatures and thermocouplemeasurements

The FE thermal results have been validated by comparingthem to the temperature history measured by five thermo-couples (TC1, TC2, . . . , TC5). The temperature history atthe locations of the thermocouples predicted by the FE ther-

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the left-hand side of the WCL in Figure 11. Figures 12, 13 and 14 compare the results from TC3,TC4 and TC5, respectively, with the FE temperatures at the right-hand side of the WCL atcorresponding axial distances of 35.0mm, 40.5mm and 70.3mm from the WCL respectively. Thefive figures each depict the thermal cycles corresponding to the 36 passes. The variety in the fullexperimental time span between depositing any two weld passes, observed from the thermocouplechart plotter, is not shown, since it does not add value to the presented information. It is worthnoting, however, that the welding procedure took place over a period of four days, throughoutwhich the interpass temperature was maintained and recorded. The figures also show that the FEinterpass temperatures, i.e. the minimum FE cyclic temperatures, vary from one pass to the other.This has been effected in the FE simulation by changing the sink temperature, associated with theelectric blanket, in each step (i.e. for each pass) in the input file, in order to achieve betteragreement with the experimental results.

Figure 10. Temperature measurements by TC1 & TC2 and FE temperature history.

Figure 11. Averaged TC1 & TC2 measurements and FE temperature history.

Figure 10. Temperature measurements by TC1 and TC2 and FEtemperature history.

mal analysis can be made to vary significantly (tens of de-grees) by altering the magnitude of the heat fluxes and thecorresponding time spans over which the fluxes last. Theheat fluxes and the time spans have to be changed togetherso as to keep the fusion zones unchanged. This way, theFE temperature history at the thermocouple locations can bechanged without disrupting the fusion zones which are con-sistent with realistic expectations. The final set of heat fluxesand time spans have been reached and their average valuesare reported in the previous subsection.

Comparisons between the FE predicted temperatures andthe measured temperatures by the thermocouples are de-picted in Figs. 10 to 14. Figure 10 shows measured tempera-tures by TC1 and TC2 and corresponding FE temperatures at32.4 mm to the left-hand side of the WCL. Although boththermocouples are attached at an equal distance from theWCL (at different angular locations around the pipe), theyexhibit a slightly different temperature history. This will evi-dently be missed out in a 2-D axisymmetric FE analysis dueto its 2-D nature. Also, this provides a justification for thediscrepancies between the FE results and the experimentallymeasured temperatures, with the discrepancies varying fromone pass to another depending on starting and stopping posi-tions relative to the measurement locations, bearing in mindthat for each weld pass (once around the circumference) therewere six sets of starting and stopping points. The results fromTC1 and TC2 are averaged and compared again to the FEtemperatures at 32.4 mm to the left-hand side of the WCLin Fig. 11. Figures 12, 13 and 14 compare the results fromTC3, TC4 and TC5, respectively, with the FE temperaturesat the right-hand side of the WCL at corresponding axial dis-tances of 35.0 mm, 40.5 mm and 70.3 mm from the WCL,respectively. The five figures each depict the thermal cyclescorresponding to the 36 passes. The variety in the full exper-imental time span between depositing any two weld passes,observed from the thermocouple chart plotter, is not shown,since it does not add value to the presented information. It is



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the left-hand side of the WCL in Figure 11. Figures 12, 13 and 14 compare the results from TC3,TC4 and TC5, respectively, with the FE temperatures at the right-hand side of the WCL atcorresponding axial distances of 35.0mm, 40.5mm and 70.3mm from the WCL respectively. Thefive figures each depict the thermal cycles corresponding to the 36 passes. The variety in the fullexperimental time span between depositing any two weld passes, observed from the thermocouplechart plotter, is not shown, since it does not add value to the presented information. It is worthnoting, however, that the welding procedure took place over a period of four days, throughoutwhich the interpass temperature was maintained and recorded. The figures also show that the FEinterpass temperatures, i.e. the minimum FE cyclic temperatures, vary from one pass to the other.This has been effected in the FE simulation by changing the sink temperature, associated with theelectric blanket, in each step (i.e. for each pass) in the input file, in order to achieve betteragreement with the experimental results.

Figure 10. Temperature measurements by TC1 & TC2 and FE temperature history.

Figure 11. Averaged TC1 & TC2 measurements and FE temperature history.Figure 11. Temperature measurements by TC1 and TC2 and FEtemperature history.

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Figure 12. Temperature measurements by TC3 and FE temperature history.



Figure 12. Temperature measurements by TC3 and FE temperaturehistory.

worth noting, however, that the welding procedure took placeover a period of four days, throughout which the interpasstemperature was maintained and recorded. The figures alsoshow that the FE interpass temperatures, i.e. the minimumFE cyclic temperatures, vary from one pass to the other. Thishas been effected in the FE simulation by changing the sinktemperature, associated with the electric blanket, in each step(i.e. for each pass) in the input file, in order to achieve betteragreement with the experimental results.

3.5 Temperature history of the FE model

The thermal cycles due to welding induce metallurgicalchanges in the weld region and HAZ, which lead to the fi-nal microstructure of the welded pipe, which is significantsince it is desired that the pipe has good resistance to creepand fracture under the abrasive conditions during service.Usually, the HAZ is the most vulnerable part of the pipe,risking the accumulation of creep damage at the intercriticalzone, near the edge of the HAZ where the peak tempera-ture is 830◦C (which is the Ac1 of the P92 steel, at whichaustenitic transformation starts), and eventually leading to

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Figure 13. Temperature measurements by TC4 and FE temperaturehistory.

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Figure 14. Temperature measurements by TC5 and FE temperature history.Figure 14. Temperature measurements by TC5 and FE temperaturehistory.

creep failure in the same region. Therefore it is useful to iden-tify the peak temperatures throughout the FE model, to tryto relate them to any mechanical features, such as residualstresses or microstructurally-distinct regions in the weld ma-terial or HAZ. For that reason, the peak temperatures for theFE model have been identified and plotted in Fig. 15 in theform of contours. It can be seen from the figure that the peaktemperature in some of the weld passes can exceed 2000◦C.This has not been verified experimentally, so until then, itwould be difficult to establish whether it is typical of weld-ing or whether it is peculiar to the way the heat fluxes areprescribed in the FE simulation. Peak temperatures, however,in the fusion welding of such materials, as the ones reportedhere, are expected to be hundreds of degrees higher than themelting point of the materials involved. The peak tempera-tures in the FE thermal analysis can be obtained by runninga simple User Defined Field (USDFLD) subroutine togetherwith the input file. The subroutine can keep a record of thepeak temperatures by equating a State Field Variable in thesubroutine to the temperature whenever the latter is on therise. The State Field Variable can then be plotted from the



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3.5 Temperature History of the FE Model

The thermal cycles due to welding induce metallurgical changes in the weld region and HAZ,which lead to the final microstructure of the welded pipe, which is significant since it is desiredthat the pipe has good resistance to creep and fracture under the abrasive conditions duringservice. Usually, the HAZ is the most vulnerable part of the pipe, risking the accumulation ofcreep damage at the intercritical zone, near the edge of the HAZ where the peak temperature is830°C (which is the Ac1 of the P92 steel, at which austenitic transformation starts), and eventuallyleading to creep failure in the same region. Therefore it is useful to identify the peak temperaturesthroughout the FE model, to try to relate them to any mechanical features, such as residual stressesor microstructurally-distinct regions in the weld material or HAZ. For that reason, the peaktemperatures for the FE model have been identified and plotted in Figure 15 in the form ofcontours. It can be seen from the figure that the peak temperature in some of the weld passes canexceed 2000°C. This has not been verified experimentally, so until then, it would be difficult toestablish whether it is typical of welding or whether it is peculiar to the way the heat fluxes areprescribed in the FE simulation. Peak temperatures, however, in the fusion welding of suchmaterials, as the ones reported here, are expected to be hundreds of degrees higher than themelting point of the materials involved. The peak temperatures in the FE thermal analysis can beobtained by running a simple User Defined Field (USDFLD) subroutine together with the inputfile. The subroutine can keep a record of the peak temperatures by equating a State Field Variablein the subroutine to the temperature whenever the latter is on the rise. The State Field Variablecan then be plotted from the results file as the peak temperature throughout the process of welding.

Figure 15. Peak temperature contours.Figure 15. Peak temperature contours.

results file as the peak temperature throughout the process ofwelding.

Certain areas of interest have been plotted in Fig. 16using the same FE peak temperature results. Referring tothe solidus and liquidus temperatures of the P92, IN82 andIN625, four distinct regions in the FE model have been iden-tified and plotted in Fig. 16: fully-melted zone, partially-melted zone, HAZ, and pipe region always below 830◦C(which is the Ac1 of the P92 steel). It can be seen that thepipe material has experienced full or partial melting wher-ever it comes into contact with the weld material and that theHAZ, away from the pipe bore, is typically around 3 mm inwidth.

4 Discussion and concluding remarks

The FE thermal analysis of a dissimilar welded pipe has beenpresented. The pipe is made of P92 steel and the weld ismade of IN82 for the first weld pass and IN625 for the other35 passes. Residual stresses induced by the welding thermalcycles can be detrimental to the mechanical integrity of thepipes and their performance in service, and therefore theywill be the subject of a future paper by the authors, deter-mined by performing an FE sequentially-coupled structuralanalysis, based on the reported thermal analysis, using thesame FE mesh, comprising 2919 axisymmetric (2-D) ele-ments and 9022 nodes. The total number of increments inthe thermal analysis was 5856 and the total number of itera-tions was 16 887. The run-time (wallclock time in the Abaqusmessage file) was almost one hour, using Microsoft WindowsXP Professional×64 Edition operating system and a com-puter (PC) having Intel Core i5-660 (dual 3.33 GHz proces-sor cores) and 7.86 GB 1066 MHz RAM.

The elements of the weld as well as the pipe have been cre-ated as part of the FE mesh right from the start. The weld el-ements are rendered inactive at the beginning and they are inturn activated to simulate their deposition. The step in the in-put file in which each weld pass is activated must be separatefrom the subsequent step prescribing the heat flux associatedwith the pass deposition. This is necessary because in Abaqusany heat flux prescribed during the activation of elements

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Certain areas of interest have been plotted in Figure 16 using the same FE peak temperatureresults. Referring to the solidus and liquidus temperatures of the P92, IN82 and IN625, fourdistinct regions in the FE model have been identified and plotted in Figure 16: fully-melted zone,partially-melted zone, HAZ, and pipe region always below 830°C (which is the Ac1 of the P92steel). It can be seen that the pipe material has experienced full or partial melting wherever itcomes into contact with the weld material and that the HAZ, away from the pipe bore, is typicallyaround 3mm in width.

Figure 16. Fusion and thermal zones.

4. Discussion and Concluding Remarks

The FE thermal analysis of a dissimilar welded pipe has been presented. The pipe is made of P92steel and the weld is made of IN82 for the first weld pass and IN625 for the other 35 passes.Residual stresses induced by the welding thermal cycles can be detrimental to the mechanicalintegrity of the pipes and their performance in service, and therefore they will be the subject of afuture paper by the authors, determined by performing an FE sequentially-coupled structuralanalysis, based on the reported thermal analysis, using the same FE mesh, comprising 2919axisymmetric (2D) elements and 9022 nodes. The total number of increments in the thermalanalysis was 5856 and the total number of iterations was 16887. The run-time (wallclock time inthe Abaqus message file) was almost one hour, using Microsoft Windows XP Professional ×64

Figure 16. Fusion and thermal zones.

within the same step would be linearly ramped up from zeroto the prescribed magnitude over the step time, which wouldcompletely jeopardise the heat flux. Therefore, keeping theactivation of the weld elements and the prescription of theheat fluxes in separate steps in the input file is essential for athermally correct simulation. Another numerical considera-tion which is worth noting is that the weld elements becomethermally active when they are activated during the thermalanalysis. If they were significantly different in temperaturefrom the surrounding elements, they would act as a heat sinkand would disrupt the temperature history in the region. Toavoid this from happening, it has been necessary to revisitthe input file and adjust the initial temperature prescribed toall the weld elements, so that when they are activated, theymatch the temperature of the surrounding elements. For thatpurpose, initial temperatures of 20◦C and 210◦C have beenfinally assigned to the pipe and weld elements, respectively.

The FE thermal analysis of welding requires the adjust-ment of the heat flux associated with the deposition of eachweld pass to obtain the expected molten zones throughoutthe pipe thickness. Extensive parametric analyses by the au-thors have indicated that when the material thermal proper-ties used in the FE simulation are slightly changed, the heatfluxes have to be adjusted to maintain the size and shape ofthe molten zones, cancelling out the effect of changing thevalue of the properties. This explains why the temperaturehistory obtained at the weld region and HAZ can still beaccurate when approximate thermal material properties areused in the FE thermal analysis. This argument applies onlyto the thermal part of the simulation. The structural analysispart has to have accurate material property data, otherwisethe accuracy of residual stresses becomes compromised.

The results from the FE thermal analysis have been vali-dated by comparing them to experimentally measured tem-peratures at five locations on the outer surface of the pipe.Although the FE simulation is axisymmetric, it still providesa temperature history which is considered to be sufficientlyaccurate for the purpose. Peak temperature contours, whichdetermine fusion zones and the HAZ, have been obtainedthrough a user defined subroutine and depicted for the FEmodel.



Acknowledgements. We would like to acknowledge the supportof The Energy Programme, which is a Research Councils UKcross council initiative led by EPSRC and contributed to by ESRC,NERC, BBSRC and STFC, and specifically the Supergen initiative(Grants GR/S86334/01 and EP/F029748) and the followingcompanies; Alstom Power Ltd., Doosan Babcock, E.ON, NationalPhysical Laboratory, Praxair Surface Technologies Ltd, QinetiQ,Rolls-Royce plc, RWE npower, Siemens Industrial Turbomachin-ery Ltd. and Tata Steel, for their valuable contributions to theproject.

Edited by: A. BarariReviewed by: D. S. Simandjuntak and another anonymous referee

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The Engineering ToolBox: Metals and Latent Heat ofFusion, available at: www.engineeringtoolbox.com/fusion-heat-metals-d1266.html, cited on 31 January 2011.

Yaghi, A. H., Hyde, T. H., Becker, A. A., Williams, J. A., and Sun,W.: Residual stress simulation in welded sections of P91 pipes,J. Mater. Process. Tech., 167, 480–487, 2005.

Yaghi, A. H., Hyde, T. H., Becker, A. A., Sun, W., Hilson, G.,Simandjuntak, S., Flewitt, P. E. J., Pavier, M. J., and Smith, D. J.:A comparison between measured and modelled residual stressesin a circumferentially butt-welded P91 steel pipe, J. Press. Vess.-T. ASME, 132, 011206-1–011206-10, 2010.


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Finite element thermal analysis of the fusion welding of a P92 steel