Three-dimensional finite element simulation of residual stresses in circumferential welds of steel pipe including pipe diameter effects
Introduction
Steel pipes are commonly used in the piping systems of power plants, oil or water pipe transport systems and pressurized piping systems, etc. Owing to the long geometry relative to the diameter and the wall-thickness, fabrication of the pipe systems always involves joining process. The circumferential butt welding is a common type of joining process in pipe systems.
Welding is a reliable and efficient metal joining process in the production of many engineering and structural components. The advantage of welding as joining process includes high joint efficiency, simple set up and low fabrication cost. However, welding process induces undesired residua stresses and deformations which can play an important role in the integrity of the welded part. This is due to the highly localized, non-uniform, transient heating and subsequent cooling of the welded material, and the non-linearity of material properties. These stresses may lead to cracking just after welding and sometimes later, during the intended service life. Particularly, tensile residual stresses near the weld area generally have adverse effects, causing stress raising, fatigue failure, and brittle fracture [1]. Therefore, accurate estimation of the welding residual stresses would be of big help to assure the sound design and safety of the structure. However, accurate prediction of the welding residual stresses is very difficult because of the complexity of welding process which includes localized heating, temperature dependence of material properties and moving heat source, etc. Accordingly, finite element (FE) simulation has become a popular tool for the prediction of welding residual stresses [2], [3], [4].
Over the last two decades or so, a substantial amount of FE simulation focusing on circumferential welding with emphasis on pipe welding has been performed. However, most of them have employed the rotational symmetry condition (axisymmetric condition) in order to reduce computational power requirements [5], [6], [7], [8], [9], [10]. Indeed, in circumferentially butt-welded pipes, it can be seen that the axisymmetric model has provided a reasonable prediction of the welding residual stress distributions. Nevertheless, in general, a careful consideration of the axisymmetric model is required in view of its inherent limitation. The axisymmetric model can reduce the computational demand but make the problem over simplified by limiting the analysis to one section of the complete geometry. The underlying assumptions also imply that the entire circumferential weld is formed in a simultaneous manner. As such, the results obtained tend to reflect the residual stress distribution in circumferential welds in an average sense and cannot be interpreted, strictly speaking, as a representation of a cross section away from the welding start/stop positions. Furthermore, an axisymmetric weld formation hardly occurs in practice as far as arc-welding processes are concerned. As demonstrated in the work by Dong and Brust [11] and Dong [12], both the traveling arc and welding start/stop effects tend to violate the axisymmetric assumptions by introducing circumferential variations of welding residual stresses. Therefore, the axisymmetric model is not capable of predicting the three-dimensional effects in circumferential welding. Consequently, three-dimensional FE analysis is essential to closely approach a real circumstance, thereby accurately predicting the welding residual stresses in circumferentially butt-welded pipes.
In the available three-dimensional FE studies of pipe welding, limited models have been suggested. For example, Karlsson and Josefson [13] analyzed temperatures and residual stresses induced by single-pass circumferential butt welding of carbon-manganese pipe using the FE code ADINA, Dong and Brust [11] and Dong [12] employed the special shell element and moving welding arc to simulate welding residual stresses in stainless steel pipe, and Duranton et al. [14], Fricke et al. [15] and Deng and Murakawa [16] developed a three-dimensional FE model based on the SYSWELD software and the ABAQUS code respectively, to analyze temperature fields and residual stress states in multi-pass circumferential butt welds of stainless steel pipe. Therefore, further investigation on the three-dimensional FE analysis is then needed to establish exact numerical model for circumferentially butt-welded steel pipe. Moreover, studies on the effects of pipe diameter on residual stresses in three-dimensional model seem to be very lacking. Actually, Yaghi et al. [9] investigated the pipe diameter effects on residual stresses through the two-dimensional axisymmetric model. However, their study was limited to examine the residual stress variations through the thickness at one section of the whole geometry due to the inherent limitation of the axisymmetric model. Therefore, in their work, the effects of pipe diameter on spatial variations of residual stresses in circumferential welds could not be described.
In this study, three-dimensional thermo-mechanical FE model is developed in order to accurately capture the three-dimensional features in residual stress distribution in circumferential butt-welded steel pipe welds. Verification of the model is also implemented through the previously published experimental work. Based on the FE model, this paper presents parametric studies to investigate the effects of pipe geometry on residual stresses, especially the effects on circumferential variations of residual stresses, pointing out the main features of the residual stress fields.
Section snippets
FE modeling
The thermal and thermal–mechanical process associated with welding residual stress evolution during welding can be extremely complex. Rapid arc heating during welding produces a molten weld pool. The weld pool shape can be largely influenced by the weld metal transfer mode and corresponding fluid-flow dynamics. On cooling, both rapid solidification within the weld pool and solid-state phase transformation in the weld and heat affected zones (HAZ) occur, depending on both peak temperature and
Model geometry and material properties
FE simulation of the circumferential butt welding has been performed on a steel pipe with a length of 150 mm and a wall thickness of 7 mm, i.e. the geometry of the circumferential weld is similar to the one shown in Fig. 1 except for the pipe diameter. The welding arc travel direction and welding start/stop position (θ = 0°) are the same as those illustrated in Fig. 1. The uncoupled thermal–mechanical FE analysis has been conducted on a range of pipe radii, listed in Table 1, for the pipe wall
Conclusions
In this study, three-dimensional uncoupled thermo-mechanical FE analyses have been carried out to produce the residual stresses in circumferential welds of steel pipes with inside radius to wall thickness ratio ranging from 10.0 to 100.0, illustrating circumferential variations of residual stresses and the effects of diameter on residual stress distributions in circumferential welds of steel pipes. Based on the results in this work, we can draw the following conclusions.
- (a)
In circumferential
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