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Title of Thesis

Muhammad Siddique
Institute/University/Department Details
Faculty of Mechanical Engineering GIK Institute of Engineering Sciences and Technology
Mechanical Engineering
Number of Pages
Keywords (Extracted from title, table of contents and abstract of thesis)
stresses, distortions, welded pipe-flange joints, residual stresses, welding distortions, finite element, welding parameters, welding deformations

Use of bolted flanged pipe joints is very common in petrochemical, nuclear and process industry. Welding of pipe to flange results in residual stress built up and distortions which provide negative effects on the structural integrity and sealing capability of the pipe flange joint. The effects include stress corrosion cracking, brittle fracture, reduced fatigue and creep strengths, poor sealing performance and reduced buckling strength etc. The objective of this study is to investigate the extent and distribution of residual stress built up and distortions in the welded pipe-flange joints. The effects of welding parameters, welding procedure, applied mechanical constraints on residual stress built-up and distortions are evaluated. Effort are made to suggest the preventive or corrective measures for the reduction of welding distortions and residual stresses to improve the performance and service life of welded pipe-flange joints.

This thesis is organized in three phases. In the first phase, preliminary studies for implementation of numerical technique for welding simulation and selection of appropriate models are performed. In the second phase, welding residual stresses are investigated including their dependence on welding parameters and their mitigation technique. The third phase is associated with the welding deformations.

In Chapter-1, a general introduction of welding deformation and residual stresses and their effect in pipe-flange joints in presented. In Chapter-2, a comprehensive literature review of experimental and finite element simulation of welding in general and its application in circumferential joint in particular is discussed. Chapter-3, describes the general finite element formulation and its application technique for material modeling, heat source modeling, modeling of filler material and analysis procedure etc.

Chapter-4, presents a comparative study of two and three-dimensional FE models of the pipe-flange joint subjected to a welding process. The aim of this study was to select an appropriate FE model for forth coming parametric numerical studies with the objective to investigate residual stresses and deformations. In Chapter-5, experimental validation procedure has been described and a few selective validations are also presented. The objective of this chapter is to establish the reader confidence over the accuracy of numerical results.

Chapter-6, describes the effect of welding (Current and speed) and geometrical (pipe diameter and thickness) parameters on residual stress profile. In Chapter-7, an analytical study regarding mitigation of residual stresses using mechanical stress relieving is presented and the effects of internal pressure, external pressure and axial pull on the residual stresses are determined. The objective of this study is to explore the most effective mode of mechanical stress relieving.

In Chapter-8, the issues related to welding procedure specifications are addressed. In this chapter a numerical procedure for modeling of tack welds is suggested. In addition, the effect of tack weld locations on welding deformations and residual stresses are investigated and appropriate position for the tack welds is suggested. The effect of root gap is also analyzed and presented.

Chapter-9, consists of experimental and numerical studies describing the effects of external constraints during welding on the flange face distortions and over all residual stress profile. Overall conclusions and further work recommendations are summarized in Chapter-10.

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2784.44 KB
S. No. Chapter Title of the Chapters Page Size (KB)
1 0 Contents
112.26 KB
2 1 Welding Distortions and Residual Stresses 1
211.06 KB
  1.1 Introduction 1
  1.2 Effects of Residual Stresses 4
  1.3 Effects of Welding Distortions 11
  1.4 Need and Scope of Present Work 16
3 2 An Overview of Past Research 18
622.39 KB
  2.1 Introductions 18
  2.2 Computer Simulation of Welding 19
  2.3 Computational/Experimental Work in Girth Welding 22
  2.4 Comments on Previous Work 51
  2.5 Experimental Determination of Residual Stresses 53
  2.6 Hole-Drilling Stress Measurement Technique 58
4 3 FE Simulation of Welding: Fundamentals 67
326.29 KB
  3.1 Introduction 67
  3.2 Mathematical Model of Welding Simulation 67
  3.3 Finite Element Formulation 68
  3.4 Physics of Arc Welding 72
  3.5 FE Modelling of Arc Welding 73
  3.6 References 94
5 4 Finite Element Model Selection 96
287.46 KB
  4.1 Introduction 96
  4.2 Finite Element Models 98
  4.3 Other Modelling Aspects 100
  4.4 Temperature History 101
  4.5 Computational Time and Data Storage Requirements 103
  4.6 Welding Deformations 104
  4.7 Residual Stresses 110
  4.8 Conclusion 116
  4.9 References 117
6 5 Experimental Validation 121
242.59 KB
  5.1 Introduction 121
  5.2 Experimental Setup for Welding 122
  5.3 Thermal Model Validation 123
  5.4 Structural Model Validation 125
  5.5 Conclusion 135
  5.6 References 136
7 6 Effects of Welding and Geometrical Parameters on Residual Stresses 139
237.14 KB
  6.1 Introduction 139
  6.2 Present study 140
  6.3 FE Models and Meshing 142
  6.4 Heat Source Modelling and Analysis Procedure 144
  6.5 Effect of Welding Speed 147
  6.6 Effect of welding Current 150
  6.7 Effect of Pipe Diameter 152
  6.8 Effect of Wall Thickness 155
  6.9 Conclusion 157
  6.10 References 158
8 7 Mechanical Stress Relieving 160
466.55 KB
  7.1 Introduction 160
  7.2 Present Work 162
  7.3 Multi-Pass GTAW 163
  7.4 Simulation of MSR Treatment 170
  7.5 Results of MSR Treatment 174
  7.6 Results Comparison 188
  7.7 Conclusion 194
  7.8 References 195
9 8 Tack Modelling and Effects of Tack Welds Orientation and Root Gap 198
244.72 KB
  8.1 Introduction 198
  8.2 Tack Modelling 199
  8.3 Tacks Orientation and Root Gap 208
  8.4 References 220
10 9 Effects of Mechanical Constraints on Welding Deformations And Residual Stresses 221
290.84 KB
  9.1 Introduction 221
  9.2 Present Study 222
  9.3 FE Modelling and Meshing 223
  9.4 Boundary Conditions 225
  9.5 Welding Deformations 227
  9.6 Residual Stresses 238
  9.7 Conclusion 241
  9.8 References 243
11 10 Conclusions and Recommendations 245
49.83 KB
  10.1 Two Vs. Three Dimensional FE Model 245
  10.2 Effects of Welding and Geometrical Parameters 246
  10.3 MSR Treatment 247
  10.4 Tack Modelling and Tack Effects 248
  10.5 Effects of Constraints 249