Download Distortions Generated by Welding Process Using ANSYS-FEA

Distortions Generated by Welding Process Using
ANSYS-FEA
Cristian Simion, Corneliu Manu, Saleh Baset, Julian Millard
Atomic Energy of Canada Limited
Abstract
The objective of this paper is to present both a methodology of detailed axisymmetric simulation of a small
scale welding test using the ANSYS general purpose finite element computer code and a comparison with
test data. To simulate this manufacturing process, an APDL (ANSYS Parametric Design Language) macro
was developed and implemented. The methodology implemented deals with the major phenomena of the
welding process from transient heat analysis to structural thermal-plastic analysis. The methodology is
based on an axisymmetric approach and accounts for nonlinearities due to temperature-dependent thermal
and mechanical material properties as well as surface contacts. It predicts, with reasonable accuracy, the
real test measurements.
CANDU* nuclear reactors use horizontal fuel channels housed in a large fabricated horizontal vessel
called the calandria vessel. This calandria vessel has two end shields (each consists of double endplates),
which provide support for the fuel channels as well as defining the basic reactor geometry. Each pair of
double endplates is joined by tubes called lattice tubes. The lattice tubes, which are welded to the endplates,
provide passages for the fuel channels and stiffness to the calandria vessel.
To ensure that the distortions of the endplates due to welding satisfy the design requirements, small scale
and large scale welding tests are performed. The initial small scale welding test consisted of welding a
lattice tube to the associated section of an endplate. A larger scale welding test consists of welding a 2x6
array of lattice tubes to assess the accumulated distortion on the end plate. In order to develop a deep
understanding of the distortions generated by welding, these welding tests are simulated using detailed
finite element analyses. The detailed simulations include heat flow and elastic plastic analysis. Numerical
simulations of the complete assembly, using coarse finite element models and elastic-only analyses of the
endplate assembly, are also included as part of the design qualification effort.
Introduction
Welding has been for many years a big part of the manufacturing process in many industries around the
world. The advantages of welding do not need further exemplification. Unfortunately the welding process
induces also few problems that need to be more accurately identified and after that minimized as much as
possible. Among the welding typical problems and most important are the residual stress/strain and the
induced distortions in structures. In order to better understand the welding process and its effects on
structures, engineers and researchers around the world, covering a large number of industries, have been
trying to create algorithms and methodologies to simulate the complete process or just individual phases
(e.g. the cooling phase). In recent years, due to the high expansion of computers computations possibilities,
many researchers identified the Finite Element Analysis (FEA) as a reliable method for this purpose.
Following this path and using the commercial code ANSYS [1], this document shows how the FEA, can be
used in the welding simulation.
By its nature, the welding process is a complicated transient procedure typically occurring in a 3-D (3dimensional) structure. A typical welding simulation consists of two phases: a transient heat flow analysis
phase and a quasistatic plastic structural analysis phase. A reasonably good welding simulation has to have
a fine enough mesh to accommodate the high thermal gradients developed (in time and in space) during the
transient heat flow analysis phase. At the same time, the mesh has to be able to solve the plastic-structural
phase of the problem. Now, due to the big temperature range to be covered (from melting temperature to
room temperature, 1400-20°C for the present case), the plastic-structural analysis phase requires a
considerable number of steps. Recalling that completion of a weld requires several welding passes, it can
be easily acknowledged that the simulation problem becomes very demanding from a computational point
of view.
After a careful review of a comprehensive set of open literature papers, it became clear that welding
simulation requires some simplifications. One common simplification is the use of a 2-D (2-dimensional)
approach. For this purpose a 2-D model represents a section of the weld perpendicular to the welding
direction. It is a good approximation because the welding process in spite of being a transient 3-D from a
thermal point of view, the stress/strain field developed in one section perpendicular on the welding
direction is quasi stationary and therefore, the process can be captured by a 2-D approach. Goldak and his
co-workers provide eloquent assessments regarding the merits and efforts associated with 2-D and 3-D
simulations of welding processes in a long series of papers summarized in References 2 and 3. Recently,
detailed 3-D welding simulations are more often presented, for example Sarkani et al. [4], and X. Lu et al.
[5]. However, a 2-D simulation approach is adopted in this paper in order to develop an understanding of
the phenomena governing the welding process and to assist us in developing an effective 3-D model when
required. Assessment of welding behaviour of large 3-D structures can then be addressed by applying
equivalent loadings based on the knowledge accumulated from detailed welding simulations. References
[6, 7 and 11] provide good examples of such approaches.
Procedure
The main phenomena identified in a welding process can be summarized as follows:
a) heat input;
• amount of added welding material per unit time;
• heat generated by the welding process;
• speed of the welding machine; and
• number of welding passes.
b) heat flow;
• latent heat;
• heat lost by convection and radiation;
• heat conducted in the welded structure.
c) solidification;
• temperature change in the weld during phase change.
d) microstructure changing; and
• volumetric change of the material during cooling
e) plastic deformation and stress/strain accumulation.
Majority of the authors neglect the solidification and the microstructure changes considering these
phenomena to be less important in developing stress and strains. The addition of the new weld material is
usually modeled by using a capability called “death and birth” of finite elements.
The phenomena of plastic deformation and stress/strain accumulation are most critical in the simulation of
a welding process.
The methodology recently developed and presented in this paper is already implemented in a FEA
simulation algorithm. It is based on a specially developed APDL (ANSYS Parametric Design Language)
macro and consists of the following two main uncoupled analysis steps:
Transient thermal analysis:
• Heat generated in the weld material in 2 steps (as a 2 pass weld). The amount of heat defined is
based on the heat generated in the test. The heat is defined as heat generated rate; a body force
load.
• Natural convection to the surrounding environment. Convection only is defined to the surrounding
environment; the radiation is considered negligible since high temperatures occur for very short
time (Figure 4).
•
•
Transient heat flow analysis to room temperature. In order to reproduce the temperature profiles of
the real welding process, the transient heat flow analysis is extended long enough for the model to
reach room temperature.
Having simulated a 2 pass weld type, the “death and birth” feature of the elements is used for
adding the second weld pass.
Structural - thermal stress analysis:
• Elastic-plastic structural thermal stress analysis is performed using the thermal loads provided by
the thermal profiles resulted from the transient thermal analysis
• “Death and birth” feature is used to bring the welding material into the analysis based on a
temperature criterion, similar to the real welding phenomenon.
• Especially dedicated APDL macro is developed and implemented for handling all these analysis
sequences.
Materials
Both welded parts and weld material are made of stainless steel 304L. Most of the thermal and mechanical
material properties are entered as function of temperature. The stress-strain curves are represented as
bilinear kinematic hardening curves.
Analysis
In our real problem we are dealing with a big horizontal vessel, called calandria vessel, which houses the
horizontal fuel channels. This vessel has double endplates at each side, called end shield. The end plates of
each end shield are linked by the lattice tubes, providing strength for the entire calandria vessel and access
for the fuel channel that are passing through (Refer to Figure 1). The lattice tubes are welded to each
endplate. Now, noticing that the diameter of the calandria vessel (and implicit of the end shield) is in the
order of a few meters and that the fuel channels number in a few hundreds, it can be easily appreciated that
dealing with the full structure from the welding simulation point of view is not a trivial task. A test at a full
scale is also prohibitively costly and also not very useful in term of gaining experience to be implemented
in the design process.
Figure 1. Section through the Calandria Vessel (Sketch) and the End Shield Half (Isometric
View)
As mentioned before, when it comes to simulating the welding process for any purpose, deformations only,
stresses only or both, the “piece of the structure” that can be modeled has to be reasonably small. In the real
problem, it can be observed that hundreds of welds are similar. As a first approximation, it can be
considered that the welds for one channel are identically reproduced from all points of view in every other
channel. Both the weld and the lattice tube to are axisymmetric. However the end plate is not because it has
hundreds of holes. Careful examination of the end plate shows axial symmetry region with a radius half of
the ligament around the hole as shown in Figure 2. This represents the basis for the axisymmetric FEA
model shown in Figure 3.
Figure 2. One channel test piece and the axisymmetric area considered in FEA
Figure 3. Geometry Modeled, Weld Region Mesh and the 6 Points for Temperature Graphs
The axisymmetric model developed for this analysis and mesh around the weld are shown in Figure 3. The
parts of the joint and the nodes considered for the post-processing of the temperature results (Refer to
Figure 4) are also indicated in Figure 3.
Figure 4. Temperature Variation at the 6 Points Shown in Figure 3, [°C]
Analysis Results & Discussion
The following results are discussed:
•
The temperature variations for the 6 points indicated in Figure 3 are shown in the Figure 4. It can
be easily noticed that a big thermal gradients between those points is developed. This is a typical
characteristic of a welding process.
•
The plastic strain developed in the welding process is shown in Figure 5. As it can be seen, plastic
strain is developed in the entire weld area and into the base material. It should be noted that the
plastic zone extends in a sufficiently large surrounding area to generate the fusion of the base
material with the weld.
•
The hoop stress SZ (see Figure 6) shows significant tension in the weld and in the HAZ (Heat
Affected Zone). During welding, the weld material is liquid and the structure material in the
adjacent area is very soft because of high temperature. This allows the joint to expand without
developing stress. In the cooling process, these areas (the weld and the area adjacent to the weld)
shrink. These areas develop a tensile state of hoop stress, due to the strength of the surrounding
structure.
•
The equivalent von Mises stress (see Figure 7) provides information about the area under high
stress, as affected by the welding. The stressed region is the same as for the hoop stress.
•
The radial deformation developed due to the welding is shown in the Figure 8. The average
displacement of the tube sheet especially in the rolled (lower) part in Figure 8 is of particular
interest from a manufacturing point of view. A comparison of FEA results versus the test results
shows a reasonable agreement.
•
The axial deformation induced by the welding process is shown in the Figure 9, where the
simulation results are compared with the test results. As it can be seen, from this figure, there is
good agreement between the FEA and the test results. The axial deformation is one order of
magnitude higher than the radial deformation and it is more important for the design requirements.
Figure 5. Equivalent (von Mises) Plastic Strain after the Welding
Figure 6. Hoop Stress Developed during Welding
Figure 7. Equivalent (von Misses) Stress Developed during Welding
Figure 8. Radial Deformation Developed during Welding (scaled 50x)
Figure 9. Axial Deformation (Out of Plane) Developed during Welding (scaled 50x)
Additional analyses have been carried out to investigate the effect of some design characteristics on the
induced welding distortions. The following over all observations can be drawn from these analyses:
•
The ratio of the pitch length to channel diameter (P/D) is shown in Figure 1. This ratio is an inplan characteristic of the end plates. The bigger this ratio is, the lower the induced deformations
are.
•
The ratio of weld position to thickness end plate (t/T) is shown in Figure 3. This ratio is a
characteristic of the weld position on the thickness of the end plate. The closer the weld to the
middle plane of the thickness is, the lower the induced distortions are.
Conclusions
The analysis results help us understand the phenomena governing the welding of a joint, offering insight on
the mechanisms and mechanical aspect particular to the welding process. Having understood the welding
mechanism, the effects of the welding can be better quantified and therefore can be better addressed in the
early stages of the design.
Specific knowledge gained from this detailed welding simulation is implemented in a 3-D methodology to
evaluate the welding induced distortions of the calandria vessel endplates. This 3-D methodology makes
the subject of a different paper, Reference 11.
References
1.
ANSYS 8.1, ANSYS Inc.
2.
J. Goldak, P. Balvantrai, B. Malcolm and J. Moore, "Computational Weld Mechanics", AGARD
Pannel Meeting, June 1985
3.
J. Goldak, "Modeling Thermal Stresses and Distortions in Welds", Second International Conference on
Trends in Welding Research, Gatlinburg, Tennesse, USA, May 1989, pp. 71-82
4.
S. Sarkani, V. Tritchkov, G. Michaelov, “An Efficient Approach for Computing Residual Stresses in
Welded Joints”, Finite Elements in Analysis and Design, v.35, n.3, June 2000, p.247-268
5.
X. Lu and T. Hassan, "Residual Stresses in Butt and Socket Welded Joints", 16th International
Conference on Structural Mechanics in Reactor Technology, Washington DC, August 2001
6.
P. Michaleris, A. DeBiccari, “Prediction of Welding Distortion”, Welding Journal, v.76, n.4, April
1997, p. 172s-181s
7.
C. L. Tsai. S. C. Park, W. T. Cheng, “Welding distortion of a thin-plate panel structure” Welding
Journal, v.78, n.5, May 1999, p. 156s-165s
8.
F. P. Incropera, D. P. Dewitt, “Fundamentals of Heat and Mass Transfer”, third edition, 1990,
Published by John Wiley & Sons (New York)
9.
N. Bagshaw, E. Olden and D. Bersato, “Numerical Simulation and Experimental Validation of Low
Stress No Distortion (LSND) Welding in 304 Stainless Steel”, The Welding Institute, United
Kingdom, Report 780/2003.
10. ASME Boiler and Pressure Vessel Code, Materials, Section II, Part D, Subpart 2, Physical Properties
Table, 2001
11. C. Manu, C. Simion, S. Baset and J. Millard, “Methodology for Evaluation of Welding Induced
Distortions of Large a Structure”, Proceedings of the Canadian Society for Mechanical Engineering
Forum, May 21-23, Calgary, Alberta, 2006.
∗
CANDU - CANada Deuterium Uranium is a registered trademark of Atomic Energy of Canada Limited
(AECL).