Dynamic Stress and Residual Stress Analysis in Cladding of Large Frame Structures

Literature Overview and Research Background

Large frame structures used in power generation, petrochemical, and heavy industrial facilities often require weld overlay cladding to provide corrosion resistance or erosion protection to critical structural surfaces. The cladding process introduces complex thermal and mechanical loading that can significantly affect the structural integrity of these large components. This study investigates the dynamic stress evolution during the cladding process and the resulting residual stress field in large frame structures, providing critical data for process optimization and structural safety assessment.

The research is particularly relevant to engineers involved in the fabrication of large hydrogenation reactor frames, heat exchanger support structures, and pressure vessel skids where the combination of structural loading and corrosion protection requirements demands careful management of cladding-induced stresses. The study employs finite element analysis coupled with experimental stress measurement techniques to develop a comprehensive understanding of stress behavior during multi-pass cladding operations.

Core Technical Content and Stress Analysis Results

The study utilizes a sequential coupling finite element model that accounts for the thermal-mechanical interaction during the cladding process. The model incorporates temperature-dependent material properties, phase transformation effects, and plastic deformation behavior of both the base metal and overlay material. The dynamic stress analysis reveals that peak stresses during individual cladding passes can reach values approaching the yield strength of the base material, particularly in regions adjacent to the weld bead.

The residual stress distribution shows a characteristic pattern where the longitudinal tensile stresses are highest along the weld centerline and decrease symmetrically toward the edges of the cladding area. The transverse stresses exhibit a more complex distribution, with tensile peaks near the weld toes and compressive regions between adjacent weld passes. This stress pattern is consistent with the thermal contraction behavior observed during multi-pass welding.

Dynamic Stress Evolution During the Cladding Process

The dynamic stress analysis reveals three distinct phases of stress evolution during a single cladding pass:

1. Heating phase: Thermal expansion of the base metal creates compressive stresses in the surrounding material as the heated zone attempts to expand but is constrained by the cooler surrounding metal. Peak compressive dynamic stresses of 120 to 180 MPa are observed in the heat-affected zone during this phase.
2. Cooling phase: As the weld metal solidifies and cools, volumetric shrinkage generates tensile stresses that can exceed the material yield strength locally. This phase produces the highest dynamic stress values, reaching 350 to 420 MPa in the immediate vicinity of the solidifying weld pool.
3. Post-cooling relaxation: After the arc has passed, stress relaxation occurs through plastic deformation and creep mechanisms. The final residual stress state represents the equilibrium between elastic and plastic strain energy stored in the structure.

The interaction between adjacent cladding passes creates additional stress complexity. The study shows that the residual stress from a previous pass is partially relieved by the thermal input of the subsequent pass, but the net effect depends on the interpass temperature and the spatial relationship between passes. Optimal interpass temperatures of 150 to 250°C minimize the accumulation of excessive residual stresses while maintaining adequate fusion between passes.

Residual Stress Measurement and Validation

The experimental validation employs the hole-drilling method (per ASTM E837) and X-ray diffraction (per ASTM E975) to measure residual stresses at various locations in the clad frame structure. The measured values show good agreement with the finite element predictions, with deviations typically within 15% of the calculated values. This validation confirms the reliability of the computational model for predicting stress states in similar geometries.

The study also investigates the effect of welding sequence on the final residual stress distribution. A sequential welding pattern from the center outward produces lower peak residual stresses compared to a unidirectional pattern, with peak longitudinal stresses reduced by approximately 20 to 30%. This finding has direct implications for welding procedure specification in production environments.

Engineering Practice Implications and Countermeasures

Based on the stress analysis results, the following countermeasures are recommended for managing cladding-induced stresses in large frame structures:

Post-weld stress relief treatment at 550 to 620°C for carbon steel structures can reduce residual stresses to below 50% of their as-welded values. However, for high-strength low-alloy steels, the stress relief temperature must be carefully controlled to avoid adverse effects on mechanical properties. The study recommends that for critical structures subject to fatigue loading, the combination of optimized welding sequence and post-weld stress relief provides the most reliable approach to achieving acceptable residual stress levels.

Study Insights and Conclusions

This research provides a comprehensive framework for understanding and managing stress effects during cladding of large frame structures. The integration of dynamic stress analysis with residual stress prediction offers engineers a powerful tool for optimizing welding procedures and ensuring structural integrity.

The key insight for engineering practice is that the residual stress state in clad structures is not merely a post-fabrication concern but must be actively managed throughout the welding sequence. The dynamic stress peaks during welding can approach yield levels, meaning that even structures that appear dimensionally stable may have undergone significant plastic deformation during fabrication. Engineers should incorporate stress monitoring into their quality assurance programs, particularly for structures subject to cyclic loading or operating under high internal pressures.

The findings also highlight the importance of considering the interaction between cladding stresses and the structural loads that the frame will experience in service. For pressure vessel support frames that must withstand seismic loads or thermal cycling, the superposition of cladding-induced residual stresses with operational stresses may create stress concentrations that compromise fatigue life. A holistic approach to stress management, encompassing both fabrication and operational phases, is essential for ensuring the long-term reliability of clad frame structures.