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.