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Effect of forming and calibration operations on the final shape of large diameter welded tubes
Journal of Materials Processing Technology 164–165 (2005) 1089–1098
Effect of forming and calibration operations on the final shape of large diameter welded tubes G. Palumbo ∗ , L. Tricarico Department of Management and Mechanical Engineering, Division of Production Technologies, Politecnico of Bari, Viale Japigia, 182 70126 Bari, Italy
Abstract Large welded tubes are primarily used in oil pipelines and offshore platforms. They are actually produced by the steelmaker industry ILVA-Laminati Piani, Taranto (Italy). The production process is characterised by three phases: (1) forming; (2) welding and (3) calibration. In addition, the first phase is composed by three steps: the forming of the longitudinal border of the blank (C-forming); the air bending of the C-formed blank (U-bending) and the forming inside a circular shaped die of the U-formed blank (O-forming). After the welding phase, the final forming operation (calibration) is performed to correct the tube distortion due to the thermal input using a special purpose machine (expanding machine). In the present work, the calibration phase is investigated using the finite element (FE) method. Both 2D and 3D FE models have been used. The 2D analysis has been aimed to obtain the tube profile after the forming phase (simulation of the C–U–O forming steps); in addition the simulation of the calibration phase has also performed under the hypothesis of plane strain condition, but not accurate results have been obtained. On the contrary, the 3D FE analysis has allowed to accurately investigate the calibration phase; two different numerical models have been used: the first one considering a cylindrical tube (the tube profile obtained by the previous 2D simulation was simply extruded along a straight line); the second one modelling the tube inflections due to residual stress after welding (the real tube section has been extruded along a circular path, according to experimental data). The aim of this search is to investigate stress and strain state in the tube; in particular the action of the expanding machine is simulated and the effect on the tube section end profiles and on the tube lengthwise inflection have been evaluated. The possibility of both obtaining a good circular shape of the tube section and correcting the lengthwise inflection is investigated. The FE model is defined according to data coming from the workshop: the sheet material is X70 steel and initial blank dimensions are 14,000 mm × 3000 mm × 27.5 mm. © 2005 Elsevier B.V. All rights reserved. Keywords: Sheet metal forming; Finite element method; Large diameter welded tube
1. Introduction Large welded tubes are primarily used in oil pipelines and offshore platforms. Main difficulties in the production come from sheet dimensions (12,000 mm × 3000 mm–16,000 mm × 4500 mm, 20–50 mm thick), which determine problems in material handling and forming. High costs are related to this production activity since each part is quite expensive. In addition, high quality and strict standard requirements are needed for offshore tubes; thus an optimal circular shape must be obtained. Three main phases may be individuated in the large welded tubes production cycle: (1) ∗
Corresponding author. E-mail address: [email protected] (G. Palumbo).
0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2005.02.150
the forming phase, (2) the welding phase and the (3) calibration phase; in addition, as shown in Fig. 1, the first phase is composed by three steps: the C-forming, the U-bending and the O-forming. After a preliminary chamfering operation, the C-bending process forms the blank borders region using two parabolic dies. In the U-bending step the whole blank profile is changed from a flat one into a U-shaped one; at first an air bending operation is performed followed by an over-bending by moving the support rollers towards the punch axis. In the O-bending step, the upper part of the U-shaped profile is changed into a circular one using two cylindrical dies. As regarding the welding phase it needs a fixture to clamp the tube and to bring its endings near. The process is completely automated and the submerged arc welding technique is adopted. Both the
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Fig. 1. Large welded tubes production cycle.
oval section due to the forming steps and distortions caused by the welding process can be corrected in the third phase using an expanding machine. It is composed by an internal head equipped with expandable sectors (they have a radial stroke). The head allows both to obtain a good circular shape of the tube section and to correct the lengthwise inflection due to welding. A deep knowledge of each forming step involved is necessary to obtain high quality products. C-forming, U-bending and O-forming operations are strictly connected and each of them affects the final shape of the tube. In addition, the welding phase evidently produces further deformations in the tube, modifying both the end profiles and the body of the tube. The output of each process step is influenced by differences in sheet characteristics (material behaviour, thickness, final tube diameter) and by changes in process parameters (set up of tools). According to various material types and to new production items, process parameters are usually varied using a “trial and error” technique to find the optimal process set up. But this approach is not cost effective when the final product is quite expensive and maintenance costs (machines and tools) are also high. Thus the adoption of numerical techniques is preferred [1,2]. The three main steps of the forming phase were investigated by authors in a previous work [3,4]; the effect of process parameters and the process window for each operation were evaluated using a 2D finite element (FE) model. In this work the authors adopt a FE approach for investigating the calibration phase; in particular the stress–strain distributions in different tube sections have been evaluated and the possibility of obtaining both a circular tube shape
and also correcting the lengthwise inflection after welding have been simulated. Even if the attention has been focused on the calibration phase, also the forming phase has been simulated (using a 2D model) to define the tube shape after the three forming steps (C–U–O operations). A 3D analysis was necessary for modelling the calibration steps: in fact the plane strain condition is far from the real one since the calibration process is performed in more than one step. Two different 3D FE models have been used: the first one considering a cylindrical tube (the tube profile obtained by the previous 2D simulation was simply extruded along a straight line); the second one modelling the tube inflections due to residual stress after welding (the real tube section has been extruded along a circular path, according to real production data). Experimental data concerning the production of tubes using X70 steel and blanks with initial dimensions equal to 14,000 mm × 3000 mm × 27.5 mm have been provided by the workshop of the steelmaker industry ILVA-Laminati Piani, Taranto, Italy (see Fig. 2). Using the numerical approach instead of the “trial and error” one, experimental tests have been drastically reduced avoiding waste of material and tools ruptures.
2. Description of the calibration phase The main concept of the calibration phase is quite simple: the expanding machine is equipped with an expanding device which has expandable tools along the periphery characterised by a circular external shape. They can move in the radial direction, thus stretching the tube in the hoop direction and making the tube section circular (see Fig. 3). The head of the expanding machine is assembled on a long beam which can move it in the longitudinal direction; a hydraulic piston is used to avoid inflections and to set the vertical position. After the welding phase, the tube is internally washed to remove residuals from previous operations but also to lubricate the surface which will be in contact with the expanding device; also the expandable tools are lubricated before the process. The tube is positioned on supporting rollers which allow its correct position with respect to the
Fig. 2. The U-bending (a) and the O-forming (b) steps of the forming phase and the welding phase (c) of the large diamters welded tubes production process (ILVA-Laminati Piani, Taranto, Italy).
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tion steps along the tube, since both the supporting and the copying roller exert a constraint on the calibrating tube. The third problem is related to the position of the head of the expanding machine inside the tube internal section, which affects the amount of deformation both in the section and on the whole tube.
3. Bi-dimensional finite element analysis 3.1. 2D finite element model Fig. 3. The expanding machine.
head of the expanding machine (this process parameter plays an important role in the final shape of the tube, as discussed further). Since the tube is very long, two symmetrical expanding machines perform the calibration, each of them working half a tube, even if not simultaneously. In addition, since each expanding device is about 800 mm long, the calibration phase is subdivided in many steps. Each calibration step can be subdivided into two parts: the first one characterised by the radial stroke of the expandable tools; the second one by the moving back of the tools (tube spring-back occurs). After the first two steps, a support is needed inside the tube, thus, the hydraulic piston is substituted by a roller (the “copying roller”), as shown in Fig. 4. The copying roller allows the head of the expanding machine both to be supported and to be driven in the following steps according to the tube profile calibrated in the previous ones. In such a way a final straight tube can be produced. Even if the calibrating process is basically simple, many problems exist and make the process quite complicated. The first problem is the spring-back effect; the real stroke of the tools of the device must be set according to the contraction resulting when they are moved back. The second problem is the need of subdividing the calibration phase in more than one step, because of the large tube length (up to 14 m); the final section shape of the tube end sections (which are the most important because of the connections with other tubes) is influenced by the calibra-
Fig. 4. Machine configuration after the second calibration step.
The implicit ABAQUS/Standard solver has been used since it is more suitable in the spring-back calculation [5]. Because of the complexity of the large diameter welded tube production process (many phases and a lot of steps), an initial “cost effective” approach has been chosen (in terms of both computational and human costs). In fact the aim was to study the whole process and thus simulating all the operations performed on the blank to evaluate the tube section profile after the forming phase (C–U–O steps). For this reason, assuming the plane strain condition, a 2D model has been defined [3] for simulating the three steps which characterize the forming phase: the C-forming, the U-bending and the O-forming. Since the blank is very long (12 m) and the tools act along the whole length, the plane strain hypothesis is not far from the real condition. In addition, only half width of the blank has been modelled, due to the symmetry of geometry, boundary and loading condition. The blank is deformable, meshed by four-node, bi-dimensional elements with a plane strain formulation. All tools have been modelled using analytical rigid surfaces which strongly simplify the contact algorithm and thus the calculation time. Blank material has been modelled as elastic/plastic. Elastic properties are necessary for spring-back calculation, while an isotropic hardening (Von Mises yield criterion) has been adopted for the simulation of the plastic behaviour; the linear work hardening law plotted in Fig. 5 has been used, based on results obtained performing standard tensile tests on some specimens cut by the tube blank.
Fig. 5. X70 material characteristics.
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Fig. 6. The 2D (plane strain) FE model.
The Coulomb friction model (µ = 0.05) has been assumed. In Fig. 6 the complete 2D model is shown. The numerical analysis of the forming phase has been divided into eight steps: (1) blank clamping and C-bending tools positioning; (2) border deformation by C-forming lower tool; (3) C-forming tools removal and U-bending tools activation; (4) downward punch motion; (5) support rollers horizontal movement (over-bending); (6) U-bending tools removal and O-forming tools activation; (7) downward movement of the upper circular die and (8) O-forming tools removal. The FE model was calibrated using experimental data concerning the production of the 42 in. tube diameter (thickness: 27.5 mm; material: steel X70). In particular, different blanks were marked with a grid before their production cycle. The grid was impressed in one of the two transversal sections of the blank to evaluate the blank deformation at the end of each forming step. In Fig. 7, a comparison between experimental and numerical profiles after the U-bending step is shown. As can be noted, the numerical model allowed to correctly predict the blank shape.
Fig. 7. Comparison between experimental and numerical blank profiles after the U-bending step.
The U-bending operation is an air bending process whose aim is to give to the blank a U-shaped profile; this is the intermediate blank shape before obtaining the final circular profile. It is realised through a circular punch and two supporting rollers. The supporting rollers can move along the horizontal direction: in this way the vertical punch stroke is reduced and an over-bending (beyond 90◦ ) can be also realised for compensating spring-back effects. A good U-shaped profile is necessary to perform correctly the O-forming operation. In
3.2. Analysis of the forming phase (C–U–O forming steps) The first forming step is crimping. In this operation a couple of parabolic dies act on the blank borders; the blank is clamped by two holders. The aim is to realize in the blank borders region a curvature to avoid interferences between the chamfered edges of the blank and the O-bending cylindrical dies. The proper relative position of upper and lower crimping tool has analysed using the FE approach. In fact, by changing the relative positions of the crimping dies, the extension of a Non-deformed region and the curvature of the tube edge are modified. It is clearly shown in Fig. 8, where the map of the equivalent plastic strain (PEEQ) in the blank borders region after the C-forming step is plotted.
Fig. 8. PEEQ map when changing the position of the dies.
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Fig. 9. Results concerning the U-bending step.
addition, the circular shape of the bottom part of the tube is realized in the U-bending step, because in the next O-forming operation no plastic deformation will be impressed to this region. Fig. 9 highlights the region where maximum plastic strain values are experienced by the sheet. In the O-forming step, the U-shaped blank is deformed using two hemi-circular dies. At the end of the stroke of the upper die, the gap between the two hemi-circular die centres can be zero, positive (dies not in contact) or negative (there is an over-closure of the two dies). The over-closure may be necessary to reduce spring-backs at the stamp opening. In fact the main target of this process is to obtain a perfect circular profile without unformed regions or excessive borders distance, which would make difficult the welding phase. Fig. 10 shows that plastic strains are involved only in few zones; in particular in the bottom part of the U-formed blank there are only elastic strains. Thus, at stamp opening, the initial profile will be again acquired. On the contrary, the
Fig. 10. Results concerning the O-forming step.
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upper part of the U-formed blank is interested by large plastic strains since during the process the curvature value is changed from a non-finite value to a finite one. As a consequence of the absence of plastic strains in the bottom part of the blank, it is possible to note that the O-forming operation is not able to modify the curvature in this region. This is very important because the final circular shape is not completely dependent by O-forming process parameters, but also by U-bending and C-forming ones. The bottom radius of the U-shaped blank influences also the closure of longitudinal borders at the end of the O-bending operation: when for example, the bottom is greater then the die radius, longitudinal borders will be far from each others. To increase the total plastic strain amount in the bottom part of the U-shaped blank, a negative gap can be adopted; thus, a smaller bottom part radius and a negative gap are generally used for reducing the sheet borders distance. O-forming process is also influenced by C-forming one. In fact also the upper part region located near the chamfered zone has small plastic strains during the O-forming process. The occurrence of some common defects is shown in Fig. 11, the flat zone (curvature absence) and the peaking (tangent singularity in the welded zone). The peaking is a consequence of the difficulty of parabolic dies to form the region near the sheet chamfer. Such defects can be reduced or avoided with the crimping operation. Moreover, the flat zone is reduced increasing the curvature of the formed region. 3.3. Analysis of the calibration phase. The joining of the O-shaped blank borders resulting from the welding phase has been simulated before performing the calibration phase. In particular nodes lying along the border section have been moved towards the symmetry axis and blocked in the horizontal direction. The calibration phase has been simulated considering two steps: in the first one the expandable tools move in the radial direction; in the second one the contact pairs concerning the interaction between the tools and the internal tube surface are deactivated (for the spring-back calculation). The head of the expanding machine has been positioned in the middle of the tube section resulting from the O-forming operation; the radial stroke of the tools has been set according to experimental data (25 mm); in addition the expandable tools have been modelled according to the production parameters acquired in the workshop. Results obtained by the bi-dimensional analysis have been summarised in Fig. 12. It may be noted that, after the O-forming operation, the blank is characterised by some of the defects previously highlighted (non-deformed region). At the end of the first part of the calibration phase, the tube profile radius is near to the optimal value of 508 mm; it is also possible to note the (small) oscillations in the radius values due to the action of distinct expandable tools. The final profile is very similar to the one during the calibration phase: it is simply translated 1.5 mm downward because of the spring-back occurrence. After the calibration, the tube profile is quite improved if compared to
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Fig. 11. Main defects in the tube profiles obtained after the forming phase.
lation of the C–U–O forming steps. Only half tube has been modelled due to symmetry: the nodes lying along the symmetry line have been bonded in the horizontal direction. The expandable tools and the copying roller have been modelled as discrete rigid bodies, while the supporting roller, since working outside the tube, has been modelled as an analytical rigid surface (see Fig. 13). To reduce the computational time shell elements (five integration points across the thickness) have been used for meshing the tube; in addition a tube of 6 m long has been modelled (even if its real length is 12 m). During the simulation of the calibration process, full constraint boundary conditions has been imposed at the tube end opposite to the head of the expanding machine. The same Fig. 12. Tube profiles obtained by the 2D analysis.
the one at the end of the O-forming step: the final shape can be considered very near to be a circular one. Small oscillations can be recognised in the final part of the plot (the region near to the symmetry axis), probably due to the symmetry boundary conditions. However, the calibration phase shows to be effective in correcting the shape defects due to the previous forming phase.
4. FE analysis modelling a cylindrical tube 4.1. First 3D finite element model The tube has been modelled extruding along a straight line the middle section of the tube shape obtained by 2D simu-
Fig. 13. Three-dimensional ABAQUS/Standard model.
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Fig. 16. Tube section profiles (end and middle of the calibration region) during and after the first step. Fig. 14. PEEQ map at the end of the first part of the first calibration step.
material behaviour, yield criterion and friction coefficient adopted for the bi-dimensional model have been assumed. 4.2. Results The strain distribution (PEEQ) map during the first step of the calibration process (when expandable tools of the head are working) is shown in Fig. 14. The strain values are not uniform along the longitudinal direction since the head of the expanding machine can work on a small portion of the whole tube length; the PEEQ decrease up to zero in the region beside the one deformed by the expandable tools. In addition, the expandable tools act on different sectors of the whole section circumference, thus, determining a discontinuous strain field. As highlighted in Fig. 15, the critical region at the end of the first step is located near the one just deformed by
Fig. 15. MISES stress distributions after the first and the second calibration step.
the tools, since it is characterised by the highest stress values. During the second calibration step the average stress value is about 520 MPa in the whole deformed region, the same which characterised the first calibration step. However, after the second step of the calibration phase the previous transition region is deformed by the action of the expandable tools, thus resulting a uniform plastic strain which redistribute the local strain peaks and reduces the stress values (see Fig. 15). For this reason, after the first step tube end profiles are modified. In Fig. 16, the radial position of nodes belonging to both the tube end section and the tube middle section of the calibrated region have been plotted at the end of the first step of the calibration phase; in addition also the tube profiles obtained by the previous 2D analysis (tube section profiles after the first part and at the end of the calibration phase) have been plotted. The tube profiles while performing the calibration (that is when expandable tools are working) are quite similar both in the 2D and 3D simulation. On the contrary, comparing the results concerning the end and the middle section obtained at the end of the 3D simulations (first step) with the final shape obtained by the 2D analysis, it may be noted that they are quite different, thus confirming the large error when assuming the plane strain condition during the calibration process. The profile in the middle section of the calibrated region and the 2D one are partially overlapping (see nodes between 20 and 40) since in the middle section the material deformation is nearer to the plane strain condition. Remarkable differences exist also in the tube shape: the 3D simulation reveals that the section is more oval than estimated by the 2D simulation. Focusing the attention on the tube end section profiles obtained by 3D analysis, it may be noted that small differences in the radius values can be found in the upper part of the tube section (high node numbers) while large differences in the bottom part (small node numbers). Thus, an oval tube section results from the 3D analysis after the material spring back due to the tool moving back.
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Fig. 17. Map of displacements when using a positive eccentricity of the copying and supporting rollers.
In the steps following the second one, the hydraulic piston which supports the head of the expanding machine is substituted by a roller, which is always in contact with the internal profile of the tube; the tube, on the contrary, is supported by an external roller. Both the rollers can work at a constant level (copying the diameter of the tube obtained in the previous steps) or can be moved either down (we have defined the eccentricity “positive”) or up (we have defined the eccentricity “negative”). As concerning the stress and strain distribution maps, the values do not differ from the one resulting form the previous steps. On the contrary remarkable differences in the tube end section profile can be found. Since an additional boundary condition exists along the tube (due to the copying roller and the supporting one), the calibration on the middle region of the tube affects also those regions already worked by the expandable tools. In particular it is important to evaluate the tube end section profile deformation resulting from the calibration steps following the second, since this is the section which must guarantee the standard requirement for the assembly with other tubes. In Fig. 17, the displacement both in the horizontal and vertical direction have been mapped when a positive eccentricity (20 mm) is used. It may be noted that the tube end section is characterised by a diameter increase in the vertical direction and by a diameter reduction in the horizontal direction. On the contrary when using a negative eccentricity (copying and supporting rollers moved 20 mm upwards), opposite results have been obtained by the simulation. It can be thus concluded that the eccentricity can be used as a process parameter to reduce or to avoid the oval shape resulting from previous calibration steps and/or production phases (C–U–O forming and welding). In Fig. 18, the tube end profiles after the fourth calibration step performed using both a positive and a negative eccentricity equal to 20 mm have been compared with the tube end profile resulting after the first calibration step.
Fig. 18. Comparison of results obtained using a positive (left) and a negative (right) eccentricity.
5. FE analysis modelling the tube distortion after welding 5.1. Second 3D finite element model In this model a real tube section, acquired in the workshop, has been extruded along a circular line whose radius Rc is equal to 603 m, according to experimental data (measured inflection in the middle section of a tube 12 m long: 120 mm). Other assumptions and main hypothesis adopted in the previous 3D model have been again assumed (about tools, material, boundary conditions). Two different simulations have been performed with the aim of analyzing the effect of the expandable tools on both the tube section and the lengthwise tube inflection: 1. the head of expanding machine is positioned in the middle of the tube end section while the copying roller and the supporting roller have a positive eccentricity (they both are moved 20 mm downward to bend the tube in the opposite direction with respect to its inflection); 2. the head of expanding machine is positioned with an initial eccentricity (20 mm) with respect to the centre of the tube end section while the copying roller and the supporting roller have a positive eccentricity (they both are moved 20 mm downward). 5.2. Results When the head of the expanding machine is positioned in the middle of the tube end section, the downward movement of both the copying and the supporting rollers make the head position not centred with respect to the tube internal profile. For this reason largest strain values are in the upper part and thus the initial inflection of the tube cannot be corrected, but it is increased, as shown in Fig. 19.
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Fig. 21. Longitudinal inflection using the second set-up.
Fig. 19. Deformed shape after the calibration phase (the undeformed tube has been superimposed).
This can be also confirmed by Fig. 20, which compares the tube inflection values along the longitudinal direction before and after the calibration phase. In fact during the calibration phase, when expandable tools are working to stretch the tube in the hoop direction, a length reduction occurs according to the volume constancy. Since the upper part of the tube is characterised by larger strains, those fibres are also characterised by larger contractions, thus resulting an increased tube inflection. The second process condition has been simulated positioning downward (20 mm) the head of the expanding machine with respect to the centre of the tube end section. Fig. 21 shows that in this case the tube inflection is opposite if compared to the previous case; inflections acquired along the longitudinal direction are smaller, revealing that the initial tube bending can be corrected. The process parameter which has been varied in the workshop is the spacer thickness positioned between the copying
Fig. 22. Displacements when using different head positions.
roller and the tube internal profile. The effects produced by different values of this process parameter are put in evidence in Fig. 22 using displacement vectors proportional to the displacement magnitude along the tube. It is important to underline that the numerical results which have been obtained using the 3D FE model showed a good agreement with experimental data from workshop. Even if detailed tube profile measurements have still not performed, qualitative comparison showed a correspondence between the effects produced by the process parameter variation both in the workshop and in the simulation.
6. Conclusions
Fig. 20. Longitudinal inflection obtained with the first set-up.
The complete production process of large diameter welded tube has been investigated adopting a FE approach. The final section (oval after the forming phase) has been evaluated through a bi-dimensional analysis based on the assumption of a plane strain condition. The final circular shape revealed to be not completely dependent by O-forming pro-
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cess parameters, but also by U-bending and C-forming ones. In addition a strong influence on the tube shape is exerted by the C-forming step. The occurrence of some common defects (the flat zone and the peaking) can be reduced or avoided with the crimping operation. Two 3D FE analyses have also been performed. The first one, modelling a cylindrical tube, has highlighted that during the calibration phase the tube end sections are strongly affected by the action of the expandable tools, since they deform progressively the tube. The second one, modelling an inflected tube, has highlighted that the initial position of the head of the expanding machine plays a relevant role in the inflection correction. As a result, the control of the tube final shape, can be obtained not only through the proper evaluation of the process parameters on the forming steps; but it is also necessary to predict (for example by means of a simulation) the effect of expandable tools during the calibration phase since it strongly affects the tube end profiles (very important for the assembly requirements) and the tube longitudinal inflections. For this reason, the 3D FE model proposed by the authors is actually adopted in the workshop as process parameter designing tool when new tube have to be produced or different material type have to be used. Further developments of this research activity will be aimed to analyse the process window of the calibration phase
according to the tube final dimensions and blank characteristics (material, thickness, geometry). Acknowledgements Authors wish to thank Prof. Luigi Galantucci, Dr. Michele De Cosmo, Eng. Raffaele Cafarelli and Eng. Gennaro Pierro for their relevant contribute in this work. In addition, authors wish to thank Eng. Buffo from ILVA-Laminati Piani, Taranto. References [1] K. Shiro, Finite element analysis of plane-strain sheet bending, Int. J. Mech. Sci. 22 (1980) 583–594. [2] A. Makinouchi, H. Ogawa, Y. Tozawa, Simulation of sheet bending processes by elastic–plastic finite element method, Ann. CIRP 38 (1) (1989) 279–282. [3] A. Buffo, L.M. Galantucci, G. Palumbo, L. Tricarico, FEM analysis of the forming operations in the production of large diameter welded tubes, in: Proceedings of the V Convegno AITEM, Bari, 2001, pp. 439–448. [4] G. Palumbo, R. Spina, L. Tricarico, A numerical evaluation of sheet C-forming process for large tube production through neural network and finite element techniques, in: Proceedings of the International Conference on Competitive Manufacturing, Stellenbosh, South Africa, 2001, pp. 271–280. [5] ABAQUS User’s Manual, Version 6.4, ABAQUS Inc., 2004.
Effect of forming and calibration operations on the final shape of large diameter welded tubes G. Palumbo ∗ , L. Tricarico Department of Management and Mechanical Engineering, Division of Production Technologies, Politecnico of Bari, Viale Japigia, 182 70126 Bari, Italy
Abstract Large welded tubes are primarily used in oil pipelines and offshore platforms. They are actually produced by the steelmaker industry ILVA-Laminati Piani, Taranto (Italy). The production process is characterised by three phases: (1) forming; (2) welding and (3) calibration. In addition, the first phase is composed by three steps: the forming of the longitudinal border of the blank (C-forming); the air bending of the C-formed blank (U-bending) and the forming inside a circular shaped die of the U-formed blank (O-forming). After the welding phase, the final forming operation (calibration) is performed to correct the tube distortion due to the thermal input using a special purpose machine (expanding machine). In the present work, the calibration phase is investigated using the finite element (FE) method. Both 2D and 3D FE models have been used. The 2D analysis has been aimed to obtain the tube profile after the forming phase (simulation of the C–U–O forming steps); in addition the simulation of the calibration phase has also performed under the hypothesis of plane strain condition, but not accurate results have been obtained. On the contrary, the 3D FE analysis has allowed to accurately investigate the calibration phase; two different numerical models have been used: the first one considering a cylindrical tube (the tube profile obtained by the previous 2D simulation was simply extruded along a straight line); the second one modelling the tube inflections due to residual stress after welding (the real tube section has been extruded along a circular path, according to experimental data). The aim of this search is to investigate stress and strain state in the tube; in particular the action of the expanding machine is simulated and the effect on the tube section end profiles and on the tube lengthwise inflection have been evaluated. The possibility of both obtaining a good circular shape of the tube section and correcting the lengthwise inflection is investigated. The FE model is defined according to data coming from the workshop: the sheet material is X70 steel and initial blank dimensions are 14,000 mm × 3000 mm × 27.5 mm. © 2005 Elsevier B.V. All rights reserved. Keywords: Sheet metal forming; Finite element method; Large diameter welded tube
1. Introduction Large welded tubes are primarily used in oil pipelines and offshore platforms. Main difficulties in the production come from sheet dimensions (12,000 mm × 3000 mm–16,000 mm × 4500 mm, 20–50 mm thick), which determine problems in material handling and forming. High costs are related to this production activity since each part is quite expensive. In addition, high quality and strict standard requirements are needed for offshore tubes; thus an optimal circular shape must be obtained. Three main phases may be individuated in the large welded tubes production cycle: (1) ∗
Corresponding author. E-mail address: [email protected] (G. Palumbo).
0924-0136/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2005.02.150
the forming phase, (2) the welding phase and the (3) calibration phase; in addition, as shown in Fig. 1, the first phase is composed by three steps: the C-forming, the U-bending and the O-forming. After a preliminary chamfering operation, the C-bending process forms the blank borders region using two parabolic dies. In the U-bending step the whole blank profile is changed from a flat one into a U-shaped one; at first an air bending operation is performed followed by an over-bending by moving the support rollers towards the punch axis. In the O-bending step, the upper part of the U-shaped profile is changed into a circular one using two cylindrical dies. As regarding the welding phase it needs a fixture to clamp the tube and to bring its endings near. The process is completely automated and the submerged arc welding technique is adopted. Both the
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Fig. 1. Large welded tubes production cycle.
oval section due to the forming steps and distortions caused by the welding process can be corrected in the third phase using an expanding machine. It is composed by an internal head equipped with expandable sectors (they have a radial stroke). The head allows both to obtain a good circular shape of the tube section and to correct the lengthwise inflection due to welding. A deep knowledge of each forming step involved is necessary to obtain high quality products. C-forming, U-bending and O-forming operations are strictly connected and each of them affects the final shape of the tube. In addition, the welding phase evidently produces further deformations in the tube, modifying both the end profiles and the body of the tube. The output of each process step is influenced by differences in sheet characteristics (material behaviour, thickness, final tube diameter) and by changes in process parameters (set up of tools). According to various material types and to new production items, process parameters are usually varied using a “trial and error” technique to find the optimal process set up. But this approach is not cost effective when the final product is quite expensive and maintenance costs (machines and tools) are also high. Thus the adoption of numerical techniques is preferred [1,2]. The three main steps of the forming phase were investigated by authors in a previous work [3,4]; the effect of process parameters and the process window for each operation were evaluated using a 2D finite element (FE) model. In this work the authors adopt a FE approach for investigating the calibration phase; in particular the stress–strain distributions in different tube sections have been evaluated and the possibility of obtaining both a circular tube shape
and also correcting the lengthwise inflection after welding have been simulated. Even if the attention has been focused on the calibration phase, also the forming phase has been simulated (using a 2D model) to define the tube shape after the three forming steps (C–U–O operations). A 3D analysis was necessary for modelling the calibration steps: in fact the plane strain condition is far from the real one since the calibration process is performed in more than one step. Two different 3D FE models have been used: the first one considering a cylindrical tube (the tube profile obtained by the previous 2D simulation was simply extruded along a straight line); the second one modelling the tube inflections due to residual stress after welding (the real tube section has been extruded along a circular path, according to real production data). Experimental data concerning the production of tubes using X70 steel and blanks with initial dimensions equal to 14,000 mm × 3000 mm × 27.5 mm have been provided by the workshop of the steelmaker industry ILVA-Laminati Piani, Taranto, Italy (see Fig. 2). Using the numerical approach instead of the “trial and error” one, experimental tests have been drastically reduced avoiding waste of material and tools ruptures.
2. Description of the calibration phase The main concept of the calibration phase is quite simple: the expanding machine is equipped with an expanding device which has expandable tools along the periphery characterised by a circular external shape. They can move in the radial direction, thus stretching the tube in the hoop direction and making the tube section circular (see Fig. 3). The head of the expanding machine is assembled on a long beam which can move it in the longitudinal direction; a hydraulic piston is used to avoid inflections and to set the vertical position. After the welding phase, the tube is internally washed to remove residuals from previous operations but also to lubricate the surface which will be in contact with the expanding device; also the expandable tools are lubricated before the process. The tube is positioned on supporting rollers which allow its correct position with respect to the
Fig. 2. The U-bending (a) and the O-forming (b) steps of the forming phase and the welding phase (c) of the large diamters welded tubes production process (ILVA-Laminati Piani, Taranto, Italy).
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tion steps along the tube, since both the supporting and the copying roller exert a constraint on the calibrating tube. The third problem is related to the position of the head of the expanding machine inside the tube internal section, which affects the amount of deformation both in the section and on the whole tube.
3. Bi-dimensional finite element analysis 3.1. 2D finite element model Fig. 3. The expanding machine.
head of the expanding machine (this process parameter plays an important role in the final shape of the tube, as discussed further). Since the tube is very long, two symmetrical expanding machines perform the calibration, each of them working half a tube, even if not simultaneously. In addition, since each expanding device is about 800 mm long, the calibration phase is subdivided in many steps. Each calibration step can be subdivided into two parts: the first one characterised by the radial stroke of the expandable tools; the second one by the moving back of the tools (tube spring-back occurs). After the first two steps, a support is needed inside the tube, thus, the hydraulic piston is substituted by a roller (the “copying roller”), as shown in Fig. 4. The copying roller allows the head of the expanding machine both to be supported and to be driven in the following steps according to the tube profile calibrated in the previous ones. In such a way a final straight tube can be produced. Even if the calibrating process is basically simple, many problems exist and make the process quite complicated. The first problem is the spring-back effect; the real stroke of the tools of the device must be set according to the contraction resulting when they are moved back. The second problem is the need of subdividing the calibration phase in more than one step, because of the large tube length (up to 14 m); the final section shape of the tube end sections (which are the most important because of the connections with other tubes) is influenced by the calibra-
Fig. 4. Machine configuration after the second calibration step.
The implicit ABAQUS/Standard solver has been used since it is more suitable in the spring-back calculation [5]. Because of the complexity of the large diameter welded tube production process (many phases and a lot of steps), an initial “cost effective” approach has been chosen (in terms of both computational and human costs). In fact the aim was to study the whole process and thus simulating all the operations performed on the blank to evaluate the tube section profile after the forming phase (C–U–O steps). For this reason, assuming the plane strain condition, a 2D model has been defined [3] for simulating the three steps which characterize the forming phase: the C-forming, the U-bending and the O-forming. Since the blank is very long (12 m) and the tools act along the whole length, the plane strain hypothesis is not far from the real condition. In addition, only half width of the blank has been modelled, due to the symmetry of geometry, boundary and loading condition. The blank is deformable, meshed by four-node, bi-dimensional elements with a plane strain formulation. All tools have been modelled using analytical rigid surfaces which strongly simplify the contact algorithm and thus the calculation time. Blank material has been modelled as elastic/plastic. Elastic properties are necessary for spring-back calculation, while an isotropic hardening (Von Mises yield criterion) has been adopted for the simulation of the plastic behaviour; the linear work hardening law plotted in Fig. 5 has been used, based on results obtained performing standard tensile tests on some specimens cut by the tube blank.
Fig. 5. X70 material characteristics.
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Fig. 6. The 2D (plane strain) FE model.
The Coulomb friction model (µ = 0.05) has been assumed. In Fig. 6 the complete 2D model is shown. The numerical analysis of the forming phase has been divided into eight steps: (1) blank clamping and C-bending tools positioning; (2) border deformation by C-forming lower tool; (3) C-forming tools removal and U-bending tools activation; (4) downward punch motion; (5) support rollers horizontal movement (over-bending); (6) U-bending tools removal and O-forming tools activation; (7) downward movement of the upper circular die and (8) O-forming tools removal. The FE model was calibrated using experimental data concerning the production of the 42 in. tube diameter (thickness: 27.5 mm; material: steel X70). In particular, different blanks were marked with a grid before their production cycle. The grid was impressed in one of the two transversal sections of the blank to evaluate the blank deformation at the end of each forming step. In Fig. 7, a comparison between experimental and numerical profiles after the U-bending step is shown. As can be noted, the numerical model allowed to correctly predict the blank shape.
Fig. 7. Comparison between experimental and numerical blank profiles after the U-bending step.
The U-bending operation is an air bending process whose aim is to give to the blank a U-shaped profile; this is the intermediate blank shape before obtaining the final circular profile. It is realised through a circular punch and two supporting rollers. The supporting rollers can move along the horizontal direction: in this way the vertical punch stroke is reduced and an over-bending (beyond 90◦ ) can be also realised for compensating spring-back effects. A good U-shaped profile is necessary to perform correctly the O-forming operation. In
3.2. Analysis of the forming phase (C–U–O forming steps) The first forming step is crimping. In this operation a couple of parabolic dies act on the blank borders; the blank is clamped by two holders. The aim is to realize in the blank borders region a curvature to avoid interferences between the chamfered edges of the blank and the O-bending cylindrical dies. The proper relative position of upper and lower crimping tool has analysed using the FE approach. In fact, by changing the relative positions of the crimping dies, the extension of a Non-deformed region and the curvature of the tube edge are modified. It is clearly shown in Fig. 8, where the map of the equivalent plastic strain (PEEQ) in the blank borders region after the C-forming step is plotted.
Fig. 8. PEEQ map when changing the position of the dies.
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Fig. 9. Results concerning the U-bending step.
addition, the circular shape of the bottom part of the tube is realized in the U-bending step, because in the next O-forming operation no plastic deformation will be impressed to this region. Fig. 9 highlights the region where maximum plastic strain values are experienced by the sheet. In the O-forming step, the U-shaped blank is deformed using two hemi-circular dies. At the end of the stroke of the upper die, the gap between the two hemi-circular die centres can be zero, positive (dies not in contact) or negative (there is an over-closure of the two dies). The over-closure may be necessary to reduce spring-backs at the stamp opening. In fact the main target of this process is to obtain a perfect circular profile without unformed regions or excessive borders distance, which would make difficult the welding phase. Fig. 10 shows that plastic strains are involved only in few zones; in particular in the bottom part of the U-formed blank there are only elastic strains. Thus, at stamp opening, the initial profile will be again acquired. On the contrary, the
Fig. 10. Results concerning the O-forming step.
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upper part of the U-formed blank is interested by large plastic strains since during the process the curvature value is changed from a non-finite value to a finite one. As a consequence of the absence of plastic strains in the bottom part of the blank, it is possible to note that the O-forming operation is not able to modify the curvature in this region. This is very important because the final circular shape is not completely dependent by O-forming process parameters, but also by U-bending and C-forming ones. The bottom radius of the U-shaped blank influences also the closure of longitudinal borders at the end of the O-bending operation: when for example, the bottom is greater then the die radius, longitudinal borders will be far from each others. To increase the total plastic strain amount in the bottom part of the U-shaped blank, a negative gap can be adopted; thus, a smaller bottom part radius and a negative gap are generally used for reducing the sheet borders distance. O-forming process is also influenced by C-forming one. In fact also the upper part region located near the chamfered zone has small plastic strains during the O-forming process. The occurrence of some common defects is shown in Fig. 11, the flat zone (curvature absence) and the peaking (tangent singularity in the welded zone). The peaking is a consequence of the difficulty of parabolic dies to form the region near the sheet chamfer. Such defects can be reduced or avoided with the crimping operation. Moreover, the flat zone is reduced increasing the curvature of the formed region. 3.3. Analysis of the calibration phase. The joining of the O-shaped blank borders resulting from the welding phase has been simulated before performing the calibration phase. In particular nodes lying along the border section have been moved towards the symmetry axis and blocked in the horizontal direction. The calibration phase has been simulated considering two steps: in the first one the expandable tools move in the radial direction; in the second one the contact pairs concerning the interaction between the tools and the internal tube surface are deactivated (for the spring-back calculation). The head of the expanding machine has been positioned in the middle of the tube section resulting from the O-forming operation; the radial stroke of the tools has been set according to experimental data (25 mm); in addition the expandable tools have been modelled according to the production parameters acquired in the workshop. Results obtained by the bi-dimensional analysis have been summarised in Fig. 12. It may be noted that, after the O-forming operation, the blank is characterised by some of the defects previously highlighted (non-deformed region). At the end of the first part of the calibration phase, the tube profile radius is near to the optimal value of 508 mm; it is also possible to note the (small) oscillations in the radius values due to the action of distinct expandable tools. The final profile is very similar to the one during the calibration phase: it is simply translated 1.5 mm downward because of the spring-back occurrence. After the calibration, the tube profile is quite improved if compared to
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Fig. 11. Main defects in the tube profiles obtained after the forming phase.
lation of the C–U–O forming steps. Only half tube has been modelled due to symmetry: the nodes lying along the symmetry line have been bonded in the horizontal direction. The expandable tools and the copying roller have been modelled as discrete rigid bodies, while the supporting roller, since working outside the tube, has been modelled as an analytical rigid surface (see Fig. 13). To reduce the computational time shell elements (five integration points across the thickness) have been used for meshing the tube; in addition a tube of 6 m long has been modelled (even if its real length is 12 m). During the simulation of the calibration process, full constraint boundary conditions has been imposed at the tube end opposite to the head of the expanding machine. The same Fig. 12. Tube profiles obtained by the 2D analysis.
the one at the end of the O-forming step: the final shape can be considered very near to be a circular one. Small oscillations can be recognised in the final part of the plot (the region near to the symmetry axis), probably due to the symmetry boundary conditions. However, the calibration phase shows to be effective in correcting the shape defects due to the previous forming phase.
4. FE analysis modelling a cylindrical tube 4.1. First 3D finite element model The tube has been modelled extruding along a straight line the middle section of the tube shape obtained by 2D simu-
Fig. 13. Three-dimensional ABAQUS/Standard model.
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Fig. 16. Tube section profiles (end and middle of the calibration region) during and after the first step. Fig. 14. PEEQ map at the end of the first part of the first calibration step.
material behaviour, yield criterion and friction coefficient adopted for the bi-dimensional model have been assumed. 4.2. Results The strain distribution (PEEQ) map during the first step of the calibration process (when expandable tools of the head are working) is shown in Fig. 14. The strain values are not uniform along the longitudinal direction since the head of the expanding machine can work on a small portion of the whole tube length; the PEEQ decrease up to zero in the region beside the one deformed by the expandable tools. In addition, the expandable tools act on different sectors of the whole section circumference, thus, determining a discontinuous strain field. As highlighted in Fig. 15, the critical region at the end of the first step is located near the one just deformed by
Fig. 15. MISES stress distributions after the first and the second calibration step.
the tools, since it is characterised by the highest stress values. During the second calibration step the average stress value is about 520 MPa in the whole deformed region, the same which characterised the first calibration step. However, after the second step of the calibration phase the previous transition region is deformed by the action of the expandable tools, thus resulting a uniform plastic strain which redistribute the local strain peaks and reduces the stress values (see Fig. 15). For this reason, after the first step tube end profiles are modified. In Fig. 16, the radial position of nodes belonging to both the tube end section and the tube middle section of the calibrated region have been plotted at the end of the first step of the calibration phase; in addition also the tube profiles obtained by the previous 2D analysis (tube section profiles after the first part and at the end of the calibration phase) have been plotted. The tube profiles while performing the calibration (that is when expandable tools are working) are quite similar both in the 2D and 3D simulation. On the contrary, comparing the results concerning the end and the middle section obtained at the end of the 3D simulations (first step) with the final shape obtained by the 2D analysis, it may be noted that they are quite different, thus confirming the large error when assuming the plane strain condition during the calibration process. The profile in the middle section of the calibrated region and the 2D one are partially overlapping (see nodes between 20 and 40) since in the middle section the material deformation is nearer to the plane strain condition. Remarkable differences exist also in the tube shape: the 3D simulation reveals that the section is more oval than estimated by the 2D simulation. Focusing the attention on the tube end section profiles obtained by 3D analysis, it may be noted that small differences in the radius values can be found in the upper part of the tube section (high node numbers) while large differences in the bottom part (small node numbers). Thus, an oval tube section results from the 3D analysis after the material spring back due to the tool moving back.
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Fig. 17. Map of displacements when using a positive eccentricity of the copying and supporting rollers.
In the steps following the second one, the hydraulic piston which supports the head of the expanding machine is substituted by a roller, which is always in contact with the internal profile of the tube; the tube, on the contrary, is supported by an external roller. Both the rollers can work at a constant level (copying the diameter of the tube obtained in the previous steps) or can be moved either down (we have defined the eccentricity “positive”) or up (we have defined the eccentricity “negative”). As concerning the stress and strain distribution maps, the values do not differ from the one resulting form the previous steps. On the contrary remarkable differences in the tube end section profile can be found. Since an additional boundary condition exists along the tube (due to the copying roller and the supporting one), the calibration on the middle region of the tube affects also those regions already worked by the expandable tools. In particular it is important to evaluate the tube end section profile deformation resulting from the calibration steps following the second, since this is the section which must guarantee the standard requirement for the assembly with other tubes. In Fig. 17, the displacement both in the horizontal and vertical direction have been mapped when a positive eccentricity (20 mm) is used. It may be noted that the tube end section is characterised by a diameter increase in the vertical direction and by a diameter reduction in the horizontal direction. On the contrary when using a negative eccentricity (copying and supporting rollers moved 20 mm upwards), opposite results have been obtained by the simulation. It can be thus concluded that the eccentricity can be used as a process parameter to reduce or to avoid the oval shape resulting from previous calibration steps and/or production phases (C–U–O forming and welding). In Fig. 18, the tube end profiles after the fourth calibration step performed using both a positive and a negative eccentricity equal to 20 mm have been compared with the tube end profile resulting after the first calibration step.
Fig. 18. Comparison of results obtained using a positive (left) and a negative (right) eccentricity.
5. FE analysis modelling the tube distortion after welding 5.1. Second 3D finite element model In this model a real tube section, acquired in the workshop, has been extruded along a circular line whose radius Rc is equal to 603 m, according to experimental data (measured inflection in the middle section of a tube 12 m long: 120 mm). Other assumptions and main hypothesis adopted in the previous 3D model have been again assumed (about tools, material, boundary conditions). Two different simulations have been performed with the aim of analyzing the effect of the expandable tools on both the tube section and the lengthwise tube inflection: 1. the head of expanding machine is positioned in the middle of the tube end section while the copying roller and the supporting roller have a positive eccentricity (they both are moved 20 mm downward to bend the tube in the opposite direction with respect to its inflection); 2. the head of expanding machine is positioned with an initial eccentricity (20 mm) with respect to the centre of the tube end section while the copying roller and the supporting roller have a positive eccentricity (they both are moved 20 mm downward). 5.2. Results When the head of the expanding machine is positioned in the middle of the tube end section, the downward movement of both the copying and the supporting rollers make the head position not centred with respect to the tube internal profile. For this reason largest strain values are in the upper part and thus the initial inflection of the tube cannot be corrected, but it is increased, as shown in Fig. 19.
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Fig. 21. Longitudinal inflection using the second set-up.
Fig. 19. Deformed shape after the calibration phase (the undeformed tube has been superimposed).
This can be also confirmed by Fig. 20, which compares the tube inflection values along the longitudinal direction before and after the calibration phase. In fact during the calibration phase, when expandable tools are working to stretch the tube in the hoop direction, a length reduction occurs according to the volume constancy. Since the upper part of the tube is characterised by larger strains, those fibres are also characterised by larger contractions, thus resulting an increased tube inflection. The second process condition has been simulated positioning downward (20 mm) the head of the expanding machine with respect to the centre of the tube end section. Fig. 21 shows that in this case the tube inflection is opposite if compared to the previous case; inflections acquired along the longitudinal direction are smaller, revealing that the initial tube bending can be corrected. The process parameter which has been varied in the workshop is the spacer thickness positioned between the copying
Fig. 22. Displacements when using different head positions.
roller and the tube internal profile. The effects produced by different values of this process parameter are put in evidence in Fig. 22 using displacement vectors proportional to the displacement magnitude along the tube. It is important to underline that the numerical results which have been obtained using the 3D FE model showed a good agreement with experimental data from workshop. Even if detailed tube profile measurements have still not performed, qualitative comparison showed a correspondence between the effects produced by the process parameter variation both in the workshop and in the simulation.
6. Conclusions
Fig. 20. Longitudinal inflection obtained with the first set-up.
The complete production process of large diameter welded tube has been investigated adopting a FE approach. The final section (oval after the forming phase) has been evaluated through a bi-dimensional analysis based on the assumption of a plane strain condition. The final circular shape revealed to be not completely dependent by O-forming pro-
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cess parameters, but also by U-bending and C-forming ones. In addition a strong influence on the tube shape is exerted by the C-forming step. The occurrence of some common defects (the flat zone and the peaking) can be reduced or avoided with the crimping operation. Two 3D FE analyses have also been performed. The first one, modelling a cylindrical tube, has highlighted that during the calibration phase the tube end sections are strongly affected by the action of the expandable tools, since they deform progressively the tube. The second one, modelling an inflected tube, has highlighted that the initial position of the head of the expanding machine plays a relevant role in the inflection correction. As a result, the control of the tube final shape, can be obtained not only through the proper evaluation of the process parameters on the forming steps; but it is also necessary to predict (for example by means of a simulation) the effect of expandable tools during the calibration phase since it strongly affects the tube end profiles (very important for the assembly requirements) and the tube longitudinal inflections. For this reason, the 3D FE model proposed by the authors is actually adopted in the workshop as process parameter designing tool when new tube have to be produced or different material type have to be used. Further developments of this research activity will be aimed to analyse the process window of the calibration phase
according to the tube final dimensions and blank characteristics (material, thickness, geometry). Acknowledgements Authors wish to thank Prof. Luigi Galantucci, Dr. Michele De Cosmo, Eng. Raffaele Cafarelli and Eng. Gennaro Pierro for their relevant contribute in this work. In addition, authors wish to thank Eng. Buffo from ILVA-Laminati Piani, Taranto. References [1] K. Shiro, Finite element analysis of plane-strain sheet bending, Int. J. Mech. Sci. 22 (1980) 583–594. [2] A. Makinouchi, H. Ogawa, Y. Tozawa, Simulation of sheet bending processes by elastic–plastic finite element method, Ann. CIRP 38 (1) (1989) 279–282. [3] A. Buffo, L.M. Galantucci, G. Palumbo, L. Tricarico, FEM analysis of the forming operations in the production of large diameter welded tubes, in: Proceedings of the V Convegno AITEM, Bari, 2001, pp. 439–448. [4] G. Palumbo, R. Spina, L. Tricarico, A numerical evaluation of sheet C-forming process for large tube production through neural network and finite element techniques, in: Proceedings of the International Conference on Competitive Manufacturing, Stellenbosh, South Africa, 2001, pp. 271–280. [5] ABAQUS User’s Manual, Version 6.4, ABAQUS Inc., 2004.