Abstract
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 for plant pipes for its enhanced creep strength. P92 steel pipes are usually joined together with a similar weld metal. 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 post weld heat treatment (PWHT), can be detrimental to the integrity of the pipes. Welding residual stresses can be numerically simulated by applying the finite element (FE) method in Abaqus. The simulation consists of a thermal analysis, determining the temperature history of the FE model, followed by a sequentially-coupled structural 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 the P92 steel pipe are joined together using a dissimilar material, made of Inconel weld consumables, producing a multi-pass butt weld from 36 circumferential weld beads. Following the generation of the FE model, the FE mesh is controlled using Model Change in Abaqus to activate the weld elements for each bead at a time corresponding to weld deposition. The thermal analysis is simulated by applying a distributed heat flux to the model, the accuracy of which is judged by considering the fusion zones in both the parent pipe as well as the deposited weld metal. For realistic fusion zones, the heat flux must be prescribed in the deposited weld pass and also the adjacent pipe elements. The FE thermal results are validated by comparing experimental temperatures measured by five thermocouples on the pipe outside surface with the FE temperature history at corresponding nodal points.
Highlights
The numerical simulation of the process of fusion welding of steel pipes in power generation plants has been the subject of research and publication for a few decades
This problem is usually overcome by the post-weld heat treatment (PWHT) of the welded pipes, which is costly and can be technically challenging; PWHT significantly reduces the magnitude of residual stresses but it cannot eliminate them
The determination of residual stresses throughout the welded pipes can be valuable for deciding how to apply PWHT or how to modify welding procedures to mitigate the ill-effects of residual stresses
Summary
The numerical simulation of the process of fusion welding of steel pipes in power generation plants has been the subject of research and publication for a few decades. If the required set of material properties for the numerical simulation is available and the generated FE mesh has sufficient refinement, the residual stress field due to welding can be accurately obtained throughout the FE model, making the FE method highly effective at predicting welding residual stresses. A 2-D axisymmetric simulation, can be more suitable, when such effects are to be ignored, and when the more uniform residual stress field is of interest This is because a 2-D axisymmetric simulation can have a much finer FE mesh for the same computational processing time in comparison with a 3-D simulation of the same model, which can lead to significantly more accurate results for large models with numerous weld passes. The actual welded pipe had five thermocouples attached at different locations, making it possible to validate the temperature history determined by the reported FE thermal analysis
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