Deformation Simulation and Control of Large Diameter Tube Sheet Weld Overlay

Literature Overview

The research by Du Jintao, Pan Xiujian, Wang Ziwei, and Zhang Jianxiao, published in Petrochemical Equipment (2016) from Lanzhou Lanchen Heavy Equipment Co., Ltd. and the Gansu Provincial Key Laboratory of Pressure Vessel Special Materials Welding, addresses the critical challenge of controlling deformation during weld overlay of large diameter tube sheets. Large tube sheets are integral components of heat exchangers and reactors in the petrochemical and power generation industries, and their dimensional accuracy directly affects assembly, sealing, and long-term structural integrity.

Core Technical Challenge

Large diameter tube sheets, typically ranging from 1000 mm to over 4000 mm in diameter, are subjected to weld overlay processes to provide corrosion resistance, erosion resistance, or to achieve specific dimensional tolerances at the tube sheet-to-shell joint. The weld overlay process introduces significant thermal stresses and plastic deformations due to the localized heating and cooling of a large, massive component. The resulting deformation can manifest as warping, barrel distortion, localized bulging, or changes in flatness, all of which compromise the functional performance of the tube sheet.

The complexity of deformation prediction for large tube sheets arises from several factors: the large scale of the component relative to the weld zone, the asymmetric weld sequence, the interaction between overlay welds and existing tube holes, and the constraints imposed by the tube sheet mounting configuration during welding.

Finite Element Simulation Approach

The research employs finite element analysis (FEA) to simulate the thermal-mechanical behavior of the tube sheet during weld overlay. The simulation approach typically involves:

• Thermal analysis: Modeling the transient temperature field using coupled Eulerian-Lagrangian or Lagrangian approaches, with appropriate heat source models (double-ellipsoidal, conical, or Gaussian) to represent the welding arc.
• Mechanical analysis: Using the thermal results as boundary conditions to compute residual stresses and deformations, incorporating plastic deformation models that account for material nonlinearity and temperature-dependent mechanical properties.
• Weld sequence modeling: Simulating the actual welding sequence, including multiple weld passes, to capture the progressive accumulation of deformation.

Deformation Patterns and Control Strategies

The simulation results typically reveal characteristic deformation patterns for large tube sheet weld overlay:

• Barrel distortion: Upward or downward curvature of the tube sheet surface due to asymmetric thermal expansion and contraction.
• Local bulging: Outward deformation near the weld zone caused by plastic yielding of the base metal.
• Edge warping: Rotation or bending of the tube sheet edges due to constraint asymmetry.
• Flatness deviation: Changes in the overall flatness of the tube sheet surface, particularly critical for gasket sealing surfaces.

Control strategies derived from the simulation include:

• Optimized weld sequence: Using symmetric welding patterns (spiral from center outward, or alternating opposite sides) to balance thermal input and minimize net deformation.
• Interpass temperature control: Maintaining interpass temperatures below 150 °C to limit plastic deformation accumulation.
• Backing plate application: Using rigid backing plates to constrain deformation during welding.
• Post-weld straightening: Predicting the magnitude and direction of deformation to plan corrective straightening operations.
• Multi-stage welding: Dividing the overlay into multiple stages with intermediate straightening to limit total deformation.

Engineering Practice and Quality Assurance

The practical implementation of deformation control for large tube sheet weld overlay requires careful integration of simulation results with shop-floor procedures. Key quality assurance measures include:

• Pre-weld dimensional survey: Establishing baseline flatness, diameter, and thickness measurements before welding begins.
• In-process monitoring: Using strain gauges or displacement sensors to monitor real-time deformation during welding.
• Post-weld inspection: Verifying final dimensions against acceptance criteria per applicable standards (ASME VIII Div.1, NB/T 47002, or API 660).
• Residual stress measurement: Employing ultrasonic or X-ray diffraction methods to quantify residual stresses and assess the need for post-weld stress relief.

Critical Reflections

The work by Du et al. exemplifies the increasing role of computational simulation in addressing complex manufacturing challenges in pressure vessel fabrication. The integration of thermal-mechanical FEA with practical welding procedure optimization provides a powerful framework for predicting and controlling deformation in large tube sheet overlay operations. However, the accuracy of simulation results depends critically on the fidelity of material property data, heat source models, and boundary condition assumptions. Engineers must validate simulation predictions against experimental measurements and refine models accordingly to ensure reliable deformation control in production environments.

Summary

The research on deformation simulation and control of large diameter tube sheet weld overlay provides a comprehensive methodology for predicting and managing the dimensional distortions that arise during overlay welding of large pressure vessel components. The combination of finite element analysis, optimized weld sequencing, and in-process monitoring offers a practical pathway to achieving the dimensional accuracy required for reliable heat exchanger and reactor assembly in the petrochemical and power generation industries.