COMPUTATIONAL WELDING MECHANICS
Coupling Thermal, Microstructure and Stress Analysis
The first time step computed the displacement, strain and stress due to applying the internal pressure, [10]. Then the weld pool was positioned in the deformed geometry for each subsequent time step. If a groove formed under the weld, the thermal analysis took into account this thinning of the pipe wall. When the arc was extinguished, the weld was allowed to cool to room temperature. The final time step returned the internal pressure to I atm.
Welding on a pressurized natural gas pipeline is simulated using computational weld mechanics to determine if CWM can provide useful estimates of the risk of bum through, [10]. The critical input data in addition to the internal pressure in the pipe, the geometry of the pipe, the size and shape of the weld pool including weld reinforcement, are the convection coefficient on the internal pipe surface and the temperature dependence of the viscosity of the pipe metal near the melting point.
The CWM analysis shows that above a critical state creep under the weld pool thins the pipe wall and forms a groove. When the pipe wall is thinned by the groove during welding, the internal pipe wall temperature increases under the weld pool during welding. This nonlinear interaction further increases the temperature on the internal pipe wall under the weld pool and further accelerates creep and the actual bum-through. The analyses shows significant thinning exactly in those welds that burned-through or were at high risk of bum-through. No significant thinning is predicted in those welds for which no significant thinning was reported experiments. We conclude that CWM can compute useful estimates of the risk of bum-through when welding on pressurized pipelines.
The thickness of the liquid layer and the concentration of carbon as a function of distance from the inner wall of the pipe have been computed. A 3D transient temperature, displacement, stress and strain have been computed and coupled to the model for growth of the carburized layer.
The 3D transient nonlinear thermal stress analysis used a viscoplastic material model and 5-node bricks. Temperature dependent Young's modulus, Poisson's ratio, Yield strength, hardening modulus and viscosity are used. Rigid body modes in the pipe were constrained. The internal surface of the pipe was pressurized to 900 psig (6.2 MPa). The transient temperature field of the thermal analysis was applied to each time step. The displacement, strain and stress were computed for 40 time steps for each weld, i. e., the weld moved approximately 0.1 in (2.5 mm) in each time step. The stress analysis was done on the full vessel.
- Build an FEM mesh of the pressure vessel including the slots machined for welds and filler metal to simulate the welds described in the [8],
- For each repair weld, specify data of position, current, voltage, arc efficiency, welding speed, weld metal cross-section geometry and weld pool semi-axes lengths,
- For each weld compute the 3D transient temperature, microstructure, displacement, strain and stress,
- Post-process results of each weld analysis to visualize the temperature, displacement, strain and stress. The deformation of the internal pipe wall under the weld was of particular interest,
- Compare the temperature and deformation computed with CWM with experimental temperature and deformation as estimated from macrographs in [8],
- Interpret the results of the analyses.
