29-09-2014, 12:07 PM
ANALYSIS OF RESIDUAL STRESSES AND DISTORTIONS IN TIG-WELDED STAINLESS STEEL PIPE PROJECT REPORT
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INTRODUCTION
The distribution of residual stresses in a tig welded pipe is complex. Weld shrinkage in the
circumferential direction induces both shearing and bending that result in stress components in the
circumferential direction (hoop stress) and in the axial direction (meridian stress). Process and
geometric related factors that influence residual stresses include welding heat input, pipe diameter,
wall thickness and joint design [1-5].Brust, et al. [2], reported high tensile stresses in both axial and
hoop directions on the pipe inner surface using the axisymmetric, inherent shrinkage model.
FE ANALYSIS
2.1 FE model
For the tig welding of two pipe with "V" groove [23], a full 3D FE model along with finite
element statistics developed in ANSYS® is shown in Fig. 2. The element type in thermal analysis is
SOLID70 (linear 8-node brick element with one degree of freedom, i.e., temperature at each node)
and in structural analysis is SOLID45 (linear 8-node brick element with three degrees of freedom at
each node: translations in the nodal x, y, and z directions.). Further details about the selected
elements may be found in [17]. High temperature and flux gradients are anticipated in and around the
fusion zone (FZ) and heat affected zone (HAZ); therefore, a relatively fine mesh is used within a
distance of 10 mm on both sides of the weld line (WL). Away from the HAZ the element size
increases with an increase in the distance from WL. In the weld direction, the element size is kept
constant equal to 1.96 mm. Within the anticipated HAZ dimension of 10 mm on each side of the WL
in transverse direction, the element size of 1 mm is used. The element size away from the weld
region increases with the increase in distance. In the thickness direction there are total three
elements, 1 mm each to facilitate for “V” groove modeling. Two tack welds on the start, i.e., 0º and
middle, i.e., 180º of the weld are modeled, each of which is comprised of 4 elements (7.85 mm) in
circumferential direction and 4 and 2 elements (4mm and 2 mm, respectively) in two layers in the
thickness direction. The used mesh is based on a mesh sensitivity analysis performed for successive
mesh refinements.
Thermal analysis technique
A high non-uniform temperature field is generated during the welding process resulting in
residual stresses in the welds. The transient temperature distribution is a function of total heat applied
and heat distribution patterns within the domain and is highly sensitive to weld induced residual
stresses. A detailed and accurate thermal analysis with appropriate boundary conditions such as heat
transfer by conduction, heat losses due to convection and radiation and heat input from the welding
torch along with the effects of filler metal deposition, is of paramount importance for the
determination of realistic temperature profiles. The governing equation for transient heat transfer
analysis during welding process is given by Eq.
EXPERIMENTAL VERIFICATION
The appropriate way to ensure the reliability of the numerical simulations and to extend the
utilization of the research work for shop floor applications is by conducting full-scale experiments
with proper instrumentation for data measurement. For arc welding experiments, automatic welding
setup with minimum human intervention and skill is considered as mandatory for the proper
validation of numerical results due to the variations associated with the skill of the operators and
rotary synchronization problems. Similarly, the careful data acquisition during the experiments is of
critical importance and demands a proper data measurement and analysis system. In the present
research, to ensure the reliability of the FE models, GTAW experiments on two thin-walled pipe
with similar geometric and welding process parameters from the finite element models are
conducted. Low stainless steel equivalent to AH36 with chemical composition as shown in Table 1
having slight variations in chemical composition from the material model used in the simulation is
utilized. Similar approximations were made in the past by [22] with comparable measured and
predicted results. In addition to the FE parameters, argon with 99.999% purity was used as shielding
gas with flow rate of 15 liters/min. Commercially available high-tech, fully automatic SAF GTAW
welding equipment along with rotary positioners and welding fixtures was used to reflect the desired
structural boundary conditions. Single pass welding equipment along with rotary positioners and
welding fixtures was used to reflect the desired structural boundary conditions. Single pass butt-weld
geometry is used with single "V" groove having included angle of 90o and 2 mm root opening as
shown in Fig. 4. The welding specimen consists of two 150 mm outer diameter and 3 mm wall
thickness cylinders. Two tack welds starting from 0° (weld start position) equally spaced at 180°,
each with length of ~ 8 mm were placed. These tack welds were also used to create a root opening
prior to welding by insertion of additional spacers of 2 mm at some appropriate locations during tack
welding
Stress contour plots
From previous discussion it is evident that due to weld start/end and tack welds, the residual
stress varies significantly along the entire periphery. Fig. 10 and Fig. 11 for cylinder outer and inner
surfaces, respectively, presents axial residual stresses at four different cross sections from weld start
position at 0o. No presentable variation is observed because the data shown is away from the weld
start/end and tack weld orientations. Similarly, slight variation in hoop residual stress patterns is
observed from Fig. 12 and Fig. 13 for outer and inner surfaces, respectively. Again, the data at weld
start/end and tack weld location(s) is missing in this case. In order to get a better insight of the stress
variation along the hoop co-ordinates, hoop residual stress fields on outer and inner surfaces are
shown in Fig. 15 and Fig. 16, respectively. From Fig. 15, on outer surfaces the stress pattern on the
whole periphery is strongly affected by the weld start/end and tack weld at 180°. Highly fluctuating
stress patterns along the entire periphery, transverse to weld direction (axial direction) are obtained.
Pronounced localized stress reduction in and around the weld start/end and tack weld locations is
shown. However, these effects are slightly less significant at the weld end location. Transverse to
weld direction and away from the weld line, stress reversal is shown along the entire periphery with
some exceptions at weld start/end and tack weld positions. Hoop residual stress fields from Fig. 16
on inner surface reveals that the effect
CONCLUSIONS
Computational methodology and techniques based on finite element analysis for the
prediction of temperature profiles and subsequent weld-induced residual stress fields and distortion
patterns in GTA welded thin-walled pipe of low stainless steel are developed and implemented
successfully with close correlation to the experimental investigations. Detailed results and discussion
pertaining to residual stress fields are presented. Further, the author's also present some data
pertaining to residual deformations. The following are the significant conclusions from the results
presented. (1) In the tig-welded pipes, axial residual stresses in the weld vicinity are compressive in
the outer surface and tensile in the inner surface. Hoop stresses are tensile in both inner and outer