Study Note on Dynamic Simulation of Cladding Welding Thermal Stresses Using ANSYS

Literature Overview

This study employs finite element analysis (FEA) on the ANSYS platform to model the dynamic thermal-mechanical behaviour of weld overlay/cladding processes. The research addresses a fundamental challenge in cladding engineering: predicting residual stresses, distortion, and potential cracking in overlay layers and the base metal during and after welding. Thermal stress analysis is essential for understanding overlay failure mechanisms, optimising welding sequences, and preventing costly fabrication defects.

Simulation Methodology

The study uses a coupled thermo-mechanical analysis approach, solving the heat transfer problem first to obtain temperature field distributions, then using these temperature histories as input for the mechanical stress analysis. The key modelling parameters include:

The material properties are defined as functions of temperature, incorporating the phase transformation behaviour of the base metal and overlay metal. For carbon steel substrates, the yield strength drops significantly above 400°C, while the austenitic stainless steel overlay maintains its ductility at higher temperatures due to its stable austenitic structure.

Key Simulation Results

The simulation reveals several important findings regarding thermal stress behaviour in cladding welds:

1. Peak residual stress distribution: The maximum tensile residual stress in the overlay layer typically ranges from 200-350 MPa, concentrated in the weld centreline region. The stress magnitude is directly proportional to the welding heat input and inversely proportional to the number of overlay passes.
2. Stress gradient across the overlay thickness: The top layer of the overlay experiences lower residual stresses compared to the root layer adjacent to the base metal interface. This is attributed to the self-relaxation effect of the top layer, which cools last and experiences less constraint.
3. Effect of interpass temperature: Increasing the interpass temperature from 50°C to 250°C reduces peak residual stresses by approximately 20-30%, but may increase the risk of sensitisation in austenitic stainless steel overlays.
4. Effect of welding sequence: Sequential welding from one end to the other produces asymmetric residual stress distributions and significant angular distortion, while symmetric welding from the centre outward produces more uniform stress distributions and reduced distortion.

Comparison with Experimental Data

The simulation results are validated against experimental measurements obtained through strain gauge monitoring and X-ray diffraction stress measurement. The agreement between simulated and measured residual stresses is generally within ±15%, which is considered acceptable for engineering prediction purposes. The largest discrepancies occur in the near-surface region where the Gaussian heat source model may not accurately represent the actual heat input distribution.

Engineering Practice Applications

The thermal stress simulation results have direct applications in cladding engineering practice:

• Welding sequence optimisation: For large vessel heads with extensive overlay areas, the simulation can predict the optimal welding sequence to minimise distortion and residual stresses. Symmetric welding from the centre outward is recommended for circular geometries, while alternating welding from opposite sides is preferred for rectangular geometries.
• Cracking risk assessment: By comparing predicted residual stresses with the fracture toughness of the overlay material, the simulation can identify regions with high cracking risk. For nickel-based alloy overlays such as Inconel 625, which have lower fracture toughness than austenitic stainless steels, the cracking risk is higher and requires more conservative welding parameters.
• Post-weld heat treatment (PWHT) design: The simulation can predict the effectiveness of PWHT in relieving residual stresses. Typical PWHT at 600-650°C for 2-4 hours reduces residual stresses to below 50% of the peak values, but the effectiveness depends on the constraint conditions imposed by the vessel geometry.

Key Reflections

The study demonstrates that finite element simulation is a powerful tool for understanding and controlling thermal stress behaviour in cladding processes. However, the accuracy of the simulation depends critically on the quality of the input data, including the heat input model, material property definitions, and boundary conditions. Engineers should validate simulation models against experimental data before using them for production decisions. The simulation should be viewed as a decision-support tool rather than a definitive prediction tool.

Study Insights and Implications

The dynamic thermal stress simulation of cladding welding processes bridges the gap between theoretical understanding and practical engineering decisions. The ability to predict residual stress distributions enables proactive measures such as welding sequence optimisation, pre-stressing techniques, and post-weld treatment design, all of which contribute to improved overlay quality and service reliability. For pressure vessel fabrication, where overlay layers must withstand both corrosion and mechanical loading, the integration of thermal stress simulation into the design and fabrication workflow represents a significant advancement in engineering practice. Future work should focus on incorporating microstructural evolution models and cracking prediction models into the simulation framework to provide more comprehensive predictions of overlay performance.