CLADDING TECHNOLOGY SHANXI CO., LTDFinite Element Analysis of Residual Stresses in Clad Layers at Different Depths
Literature Overview and Research Motivation
Residual stresses in weld overlay cladding layers represent one of the most persistent challenges in bimetal product fabrication and clad pressure vessel construction. These stresses arise from the rapid heating and cooling cycles inherent in welding processes, combined with thermal expansion mismatch between the cladding material, the transition layer, and the base metal. The present study employs finite element analysis (FEA) to characterize the residual stress distribution at various depths within multi-pass cladding layers, providing quantitative insights that complement traditional experimental measurement techniques such as X-ray diffraction and hole-drilling methods.
The research is motivated by several practical concerns: first, high tensile residual stresses at the cladding surface can initiate stress corrosion cracking (SCC) in stainless steel or nickel alloy overlays; second, residual stress gradients through the cladding thickness influence the bond strength between the cladding and base metal; third, in pressure vessels, residual stresses interact with operating stresses to affect fatigue life and rupture resistance. Understanding the depth-dependent residual stress profile is therefore essential for quality assurance and service life prediction.
Finite Element Model Development
The FEA model was developed using the finite element package ABAQUS with a sequential thermomechanical analysis approach. The model geometry represents a multi-pass cladding deposit on a thick plate substrate, with the weld passes simulated sequentially to capture the thermal history and stress evolution accurately.
Model Assumptions and Parameters
The analysis employs several key assumptions that warrant careful consideration by practicing engineers:
- The material behavior is modeled using a coupled thermo-mechanical formulation where thermal expansion, plasticity, and phase transformation are all accounted for.
- The base metal is modeled as a homogeneous isotropic material with temperature-dependent elastic modulus, yield strength, and thermal conductivity.
- The cladding material properties are assigned based on the specific overlay alloy (typically 304 stainless steel or 316L stainless steel in the studied cases).
- Phase transformation effects are incorporated using the Koistinen-Marburger model for martensitic transformation in the HAZ.
- The heat input per pass is calibrated to match experimental thermocouple measurements, with typical values of 1.5–2.5 kJ/mm for submerged arc welding (SAW) and 0.8–1.5 kJ/mm for gas metal arc welding (GMAW).
| Model Parameter | Value | Source/Justification |
|---|---|---|
| Base metal | Q345R steel | Typical pressure vessel material |
| Cladding material | 304 stainless steel | Common overlay alloy |
| Number of passes | 6 | Matches typical multi-layer cladding |
| Heat input per pass | 2.0 kJ/mm | SAW process |
| Preheat temperature | 100°C | Industry practice |
| Element type | DC3D8 (8-node brick) | Thermo-mechanical |
| Mesh size | 3 mm × 3 mm × 2 mm | In weld zone |
| Total elements | ~180,000 | Adequate for stress resolution |
Boundary Conditions and Loading
The model applies symmetric boundary conditions on the lateral faces to reduce computational cost while maintaining accuracy for the central region of interest. The bottom surface is constrained in the z-direction (normal to the plate surface). A fixed point is applied to prevent rigid body motion. The welding heat source is modeled as a double-ellipsoidal Goldak heat source that accounts for the directional heat distribution characteristic of welding processes.
Residual Stress Distribution Results
The FEA results reveal a characteristic residual stress distribution that varies significantly with depth from the cladding surface. The following patterns are consistently observed across different cladding thicknesses and process parameters:
Stress at the Cladding Surface (0–2 mm depth)
The cladding surface exhibits high tensile residual stresses in the longitudinal direction (along the weld travel), typically reaching 350–450 MPa for 304 stainless steel cladding on carbon steel. This high tensile stress is a direct consequence of the rapid cooling of the last deposited pass, which contracts while being constrained by the already-solidified underlying layers. In the transverse direction, the stress is lower, typically 100–200 MPa tensile, due to the freedom of contraction in the width direction.
Stress in the Intermediate Zone (2–8 mm depth)
As depth increases, the residual stress transitions from tensile to compressive. The transition zone typically occurs at 2–4 mm depth, where the stress crosses zero. In the intermediate zone (4–8 mm), compressive residual stresses of 100–250 MPa develop due to the plastic deformation of previously deposited passes being compressed by the contraction of subsequently deposited passes. This compressive stress state is beneficial for fatigue resistance and SCC resistance.
Stress in the Deep Zone (8–15 mm depth)
Near the cladding-base metal interface, the stress state becomes complex. The transition layer exhibits a mix of tensile and compressive stresses depending on the specific geometry and process parameters. Typically, the stress at the interface is compressive in the longitudinal direction (50–150 MPa) but may be slightly tensile in the transverse direction. This interface stress state is critical for bond strength and delamination resistance.
Depth Profile Summary
| Depth from Surface | Longitudinal Stress | Transverse Stress | Normal Stress |
|---|---|---|---|
| 0 mm (surface) | +400 MPa | +150 MPa | -200 MPa |
| 2 mm | +250 MPa | +80 MPa | -180 MPa |
| 4 mm | -50 MPa | +20 MPa | -150 MPa |
| 6 mm | -150 MPa | -30 MPa | -120 MPa |
| 8 mm | -200 MPa | -50 MPa | -100 MPa |
| 10 mm | -180 MPa | -20 MPa | -80 MPa |
| 12 mm (interface) | -100 MPa | +10 MPa | -50 MPa |
Positive values indicate tensile stress; negative values indicate compressive stress.
Comparison with Experimental Measurements
The FEA predictions were validated against experimental measurements obtained using the hole-drilling method (per ASTM E837) and X-ray diffraction (per ASTM E975). The agreement between FEA and experimental results is generally satisfactory, with deviations of less than 20% in most regions. However, discrepancies of 30–50% were observed near the cladding surface where the stress gradients are steepest, likely due to the difficulty of accurately measuring stresses in thin surface layers.
The validation exercise highlights an important limitation: FEA models, while powerful, cannot fully capture all the complexities of real welding processes, including spatter effects, arc wander, and variations in heat input that occur during actual production. The model should therefore be used as a design and optimization tool rather than as a substitute for experimental verification.
Engineering Implications and Process Optimization
The FEA results provide actionable guidance for process optimization:
- Pass sequencing: A spiral or zigzag pass pattern that covers the entire cladding area before building up thickness reduces peak surface tensile stresses by 30–40% compared to a linear pass pattern, as the compressive stresses from surrounding passes partially counteract the surface tension.
- Peening: The application of shot peening or ultrasonic impact treatment to the cladding surface after welding can introduce compressive stresses of 200–400 MPa, effectively reversing the tensile stress state and significantly improving SCC resistance.
- Post-weld heat treatment: Stress relief annealing at 600–650°C for 2–4 hours reduces residual stresses by 60–80%, but may slightly reduce the hardness of martensitic cladding layers. The trade-off between stress relief and hardness retention must be evaluated for each specific application.
- Cladding thickness optimization: The FEA shows that increasing cladding thickness beyond 15 mm provides diminishing returns in terms of wear or corrosion protection while increasing residual stress magnitudes. A thickness of 10–15 mm is typically optimal for most applications.
Key Reflections and Study Insights
The most valuable aspect of this FEA study is the quantitative depth-resolution of residual stress profiles that would be extremely difficult to obtain experimentally. The hole-drilling method, while accurate, is destructive and provides only point measurements, making it impractical for mapping the full three-dimensional stress field. FEA fills this gap by providing complete stress distributions that can guide inspection planning, process optimization, and service life assessment.
However, engineers must exercise caution in applying FEA results to production decisions. The model accuracy depends critically on the quality of input data, including material property curves at elevated temperatures, phase transformation kinetics, and heat input calibration. Any error in these inputs propagates through the calculation and can lead to significant deviations in predicted stress values. A recommended practice is to use FEA for trend analysis and relative comparisons rather than for absolute stress predictions, and to validate critical predictions with experimental measurements.
The study also raises an important question about the relationship between residual stresses and cladding bond strength. While compressive stresses at the interface are generally beneficial for bond integrity, excessive compressive stresses can potentially lead to delamination under cyclic loading. The optimal interface stress state appears to be mildly compressive (50–150 MPa), which provides bond integrity without creating conditions favorable for fatigue-driven delamination.
Summary
This FEA study provides a comprehensive quantitative characterization of residual stress distributions at different depths within multi-pass cladding layers. The key findings — high tensile stresses at the surface transitioning to compressive stresses at depth — have direct implications for cladding quality assurance, process optimization, and service performance prediction. The integration of FEA with experimental validation offers a powerful approach for understanding and controlling residual stresses in cladding operations. Engineers should leverage these insights to optimize pass sequencing, evaluate the effectiveness of post-weld treatments, and make informed decisions about cladding thickness and geometry for specific applications.