Computer Simulation of Residual Stress Field in Clad Metals

Literature Overview and Core Content

This study note examines the computational simulation of residual stress fields in clad metal components, addressing one of the most persistent challenges in bimetal product manufacturing and pressure vessel fabrication. Residual stresses arising from the thermal mismatch between the base metal and the cladding layer during welding or bonding processes can significantly affect the long-term mechanical performance, fatigue life, and susceptibility to stress corrosion cracking of clad components.

The paper presents a finite element analysis (FEA) framework for predicting the residual stress distribution in weld-overlay clad plates and pressure vessels. The simulation accounts for the sequential deposition of overlay layers, the thermal expansion coefficient mismatch between dissimilar metals, and the plastic deformation that occurs during both heating and cooling phases of the welding process. The core contribution of the study is the development of a validated numerical model that can predict residual stress magnitudes and distributions with sufficient accuracy to support engineering design decisions.

Key Technical Points and Process Analysis

Residual Stress Mechanisms in Clad Metals

Residual stresses in clad metals originate from three primary mechanisms: thermal contraction upon cooling, phase transformation strains, and constraint effects due to the dissimilar thermal expansion coefficients of the base and overlay materials. In weld-overlay cladding, the rapid heating and cooling cycles produce localized plastic deformation, and the subsequent cooling of the overlay layer is constrained by the cooler base material, resulting in tensile residual stresses in the overlay and compressive stresses in the base metal near the interface.

Simulation Methodology and Validation

The study employs a thermo-mechanical coupled finite element model that incorporates the elastic-plastic constitutive behavior of both the base and overlay materials. The material properties, including yield strength, thermal expansion coefficient, and thermal conductivity, are defined as functions of temperature to capture the non-linear behavior during the welding thermal cycle. The model is validated against experimental measurements obtained using the hole-drilling method (per ASTM E837) and neutron diffraction techniques.

The simulation results demonstrate that the peak residual stresses occur in the region of the overlay layer adjacent to the weld interface, with values reaching up to 400 MPa for a typical 304 stainless steel overlay on a SA-516 Gr.70 carbon steel base plate. The stress distribution exhibits a characteristic pattern where longitudinal stresses are higher than transverse stresses, and the through-thickness stress component shows a gradient from compressive at the surface to tensile near the interface.

Impact of Process Parameters on Residual Stress

The study systematically investigates the influence of key process parameters on the residual stress field. Increasing the number of overlay passes generally increases the overall residual stress magnitude due to the cumulative thermal cycling effect, but each subsequent pass also introduces a degree of self-relaxation through plastic deformation. Higher welding speeds result in narrower heat-affected zones and more localized stress concentrations, while lower welding speeds produce broader stress fields with somewhat lower peak values.

Preheating the base material before overlay welding is identified as an effective strategy for reducing residual stresses. The simulation shows that preheating to 200°C reduces peak longitudinal residual stresses by approximately 25% compared to ambient temperature welding. Post-weld heat treatment (PWHT) at 620°C for carbon steel substrates can reduce residual stresses by 70–90%, but this approach is not always feasible for components already in service or for overlay materials that are sensitive to grain growth at elevated temperatures.

Engineering Practice Integration

In pressure vessel fabrication, the prediction of residual stresses is essential for evaluating the risk of stress corrosion cracking (SCC) in clad components exposed to aggressive environments. For example, a 316L stainless steel clad pressure vessel operating in a chloride-containing environment may be susceptible to SCC if the residual tensile stresses in the overlay exceed a critical threshold. The simulation results can be directly integrated with fracture mechanics analyses to predict crack growth rates and establish inspection intervals.

The PDCA (Plan-Do-Check-Act) framework is applicable to the iterative refinement of residual stress prediction models. The Plan phase involves defining the geometry, material properties, and welding sequence. The Do phase involves running the FEA simulation. The Check phase involves comparing simulation results with experimental measurements. The Act phase involves refining the model parameters and re-running the analysis until acceptable agreement is achieved.

Key Questions and Reflections

A significant question raised by the study is the scalability of the simulation approach from small coupon-sized specimens to full-scale pressure vessel components. The geometric complexity of real pressure vessels, with their varying curvatures, weld joints, and nozzles, introduces additional stress concentration effects that may not be captured in simplified simulation models. The study acknowledges this limitation and suggests that a hybrid approach, combining detailed local simulations with global structural analyses, may be the most practical solution for engineering applications.

Study Insights and Implications

The study provides valuable insight into the use of computational methods for predicting and managing residual stresses in clad metal components. The ability to simulate the residual stress field before fabrication enables engineers to optimize welding sequences, select appropriate preheating temperatures, and determine the necessity of post-weld stress relief treatments. This predictive capability reduces the reliance on trial-and-error approaches and supports the development of more efficient and reliable fabrication procedures for bimetal products and clad pressure vessels.