CLADDING TECHNOLOGY SHANXI CO., LTDFinite Element Analysis of Surface Residual Stress at Different Cladding Depths
Literature Overview
This paper, published in Metal Heat Treatment (2014) by Zhou Yuming and Shi Haifang from Liaoning Polytechnic College and Liaoning Technical University, presents a finite element analysis (FEA) of surface residual stresses at different depths in weld overlay cladding layers. Residual stress is a critical parameter that affects the fatigue life, stress corrosion cracking susceptibility, and dimensional stability of cladded components.
Core Technical Points
Thermal-Mechanical Coupled FEA Model
The finite element analysis of cladding residual stresses requires a coupled thermal-mechanical model that accounts for:
- Thermal analysis: Heat generation, conduction, convection, and radiation during the welding process.
- Phase transformation: The latent heat release/absorption during austenite-ferrite transformations.
- Plastic deformation: The development of plastic strain due to thermal expansion/contraction.
- Viscoelastic behavior: The time-dependent deformation at elevated temperatures.
The material properties must be temperature-dependent, including:
- Thermal conductivity: k(T)
- Specific heat: c(T)
- Coefficient of thermal expansion: α(T)
- Young's modulus: E(T)
- Yield strength: σy(T)
- Poisson's ratio: ν(T)
Residual Stress Distribution
The residual stress distribution in a multi-pass cladding layer exhibits a characteristic pattern that varies with depth from the surface:
| Depth from Surface (mm) | Longitudinal Stress (MPa) | Transverse Stress (MPa) |
|---|---|---|
| 0 (Surface) | −150 to −250 (Compressive) | −80 to −150 (Compressive) |
| 0.5 | −100 to −200 | −50 to −120 |
| 1.0 | −50 to −150 | −20 to −80 |
| 1.5 | 0 to −100 | 0 to −50 |
| 2.0 (Substrate) | 50 to 200 (Tensile) | 30 to 150 (Tensile) |
The compressive residual stresses at the surface are beneficial for fatigue life and stress corrosion cracking resistance, while the tensile stresses in the substrate can be detrimental. The transition from compressive to tensile stress typically occurs at a depth of 1–2 mm from the cladding surface.
Effect of Cladding Parameters on Residual Stress
| Parameter | Effect on Surface Compressive Stress |
|---|---|
| Increasing heat input | Decreases compressive stress |
| Increasing pass thickness | Increases compressive stress |
| Reverse welding sequence | Increases compressive stress |
| Shot peening after welding | Significantly increases compressive stress |
| Post-weld stress relief | Reduces both compressive and tensile stresses |
Numerical Model Validation
The FEA model must be validated against experimental measurements. Common experimental techniques include:
- X-ray diffraction (XRD): Measures surface residual stresses with a penetration depth of 10–50 μm.
- Neutron diffraction: Measures stresses at depths up to 50 mm.
- Hole drilling method: Measures stresses at various depths by removing material in increments.
| Method | Depth Resolution | Accuracy | Cost |
|---|---|---|---|
| XRD | 0–50 μm | ±10 MPa | Low |
| Neutron diffraction | 0–50 mm | ±20 MPa | High |
| Hole drilling | 0–5 mm | ±15 MPa | Medium |
| FEA prediction | 0–100 mm | ±30 MPa | Low |
Defect Analysis and Countermeasures
Residual Stress-Related Defects
- Distortion: Excessive residual stresses cause dimensional distortion that can exceed manufacturing tolerances.
- Cracking: High tensile residual stresses can initiate cracks, particularly in the HAZ or at the fusion boundary.
- Stress corrosion cracking: Tensile residual stresses in the presence of a corrosive environment can cause SCC, particularly in austenitic stainless steel cladding layers.
- Fatigue failure: Tensile residual stresses reduce the fatigue life by promoting crack initiation and propagation.
Engineering Countermeasures
- Shot peening: Applying shot peening to the cladding surface introduces additional compressive stresses that can reach −400 to −600 MPa, significantly improving fatigue life.
- Vibration stress relief (VSR): Applying low-amplitude, high-frequency vibrations to the cladded component can reduce residual stresses by 30–50%.
- Thermal stress relief: Post-weld heat treatment at 550–650 °C (for stainless steel) or 600–650 °C (for carbon steel) reduces residual stresses but may affect the microstructure and mechanical properties.
- Weld sequence optimization: Using a symmetric welding sequence or backing plate can reduce distortion and residual stresses.
Integration with Engineering Practice
In pressure vessel fabrication, the residual stress distribution in cladding layers must be considered in the design and qualification. The ASME Code allows for residual stress relaxation factors in the design calculations, but the actual stress distribution must be verified for critical applications.
For hydrogenation reactors and other high-pressure equipment, the residual stress state affects the susceptibility to hydrogen-induced cracking. Compressive residual stresses at the surface are beneficial, but the tensile stresses in the substrate can promote hydrogen embrittlement.
The FEA results must be incorporated into the inspection planning. Areas of high tensile residual stress are more susceptible to stress corrosion cracking and should be prioritized for non-destructive examination.
Key Reflections
The finite element analysis of cladding residual stresses provides valuable insights into the stress state of the component, but the accuracy of the predictions depends on the quality of the input data and the fidelity of the model. The temperature-dependent material properties, particularly the yield strength and thermal expansion coefficient, must be accurately characterized for the specific materials used.
The practical challenge is to balance the benefits of compressive residual stresses (improved fatigue life, reduced SCC susceptibility) with the risks of tensile residual stresses in the substrate (distortion, cracking). The welding procedure must be optimized to achieve the desired stress state while maintaining the required microstructure and mechanical properties.
From a quality assurance perspective, the residual stress measurements should be included in the inspection plan for critical cladding applications. The acceptance criteria for residual stresses should be defined based on the specific service conditions and the applicable code requirements.