Finite Element Analysis of Residual Stress Distribution at Different Depths in Overlay Layers

Literature Overview

The study by Zhou Yuming and Shi Haifang (2014), conducted at the Liaoning Polytechnic College and Liaoning Technical University, presents a finite element analysis (FEA) approach to characterize residual stress distributions at various depths within weld overlay layers. This computational work addresses a fundamental challenge in cladding technology—the prediction and management of residual stresses that develop during the multi-pass deposition process. The research contributes to the understanding of how stress states evolve through the thickness of the overlay and how they influence service performance.

Core Technical Points

Residual Stress Formation Mechanisms in Overlay Welding

Residual stresses in overlay layers arise from the complex interplay of thermal and mechanical effects during sequential deposition:

1. Thermal stresses: Generated by non-uniform temperature distribution during heating and cooling of each pass
2. Phase transformation stresses: Resulting from volume changes associated with austenite-to-ferrite or austenite-to-martensite transformations
3. Plastic deformation stresses: Caused by localized plastic flow in the weld metal and HAZ during each deposition cycle
4. Constraining stresses: Resulting from the geometric constraint imposed by previously deposited layers and the base metal

FEA Modeling Approach

The finite element model employed in this study typically incorporates:

Residual Stress Distribution Characteristics

The analysis reveals characteristic residual stress profiles through the overlay thickness:

At the overlay surface (0-2 mm depth):

• Predominantly compressive longitudinal stresses (-100 to -300 MPa)
• Transverse stresses may be tensile or compressive depending on weld geometry
• Compressive surface stresses are beneficial for fatigue resistance

In the mid-overlay region (2-5 mm depth):

• Stress magnitudes decrease in absolute value
• Transition from compressive to mixed stress state
• Maximum principal stress typically in the longitudinal direction

At the bond line (overlay-base interface):

• Highest tensile residual stresses (150-400 MPa in the longitudinal direction)
• Multi-axial stress state with potential for delamination
• Stress concentration at interface defects or lack of fusion

In the base metal HAZ:

• Compressive stresses immediately adjacent to the bond line
• Tensile stresses at greater distances from the weld
• Stress relaxation due to plastic deformation at elevated temperatures

Comparison with Experimental Data

The agreement between FEA predictions and experimental measurements (typically obtained by X-ray diffraction or neutron diffraction) validates the modeling approach while highlighting the inherent uncertainties in residual stress prediction.

Practical Implications for Cladding Engineering

Stress Management Strategies

Based on the FEA results, several practical strategies can be employed to manage residual stresses in overlay layers:

1. Weld sequence optimization: Back-step welding, skip welding, and symmetric deposition patterns can redistribute stress more uniformly
2. Post-weld heat treatment (PWHT): Stress relief at 550-650 °C for low-alloy overlays or 600-700 °C for stainless steel overlays significantly reduces peak residual stresses
3. Shot peening or laser peening: Introducing beneficial compressive stresses at the overlay surface to counteract tensile stresses at the bond line
4. Mechanical vibration stress relief (VSR): Effective for reducing residual stresses by 30-50% in thick overlay sections
5. Thermal spray pre-treatment: Pre-heating the base metal to reduce thermal gradients during overlay deposition

Implications for Pressure Vessel Applications

For clad pressure vessels governed by GB/T 150 or ASME Section VIII, residual stress management is critical because:

• Tensile residual stresses at the bond line can initiate delamination under cyclic pressure loading
• Combined residual and operational stresses may exceed the allowable stress limits specified in design codes
• Stress corrosion cracking (SCC) susceptibility increases significantly in the presence of tensile residual stresses
• Hydrogen-induced cracking (HIC) risk is elevated in high-stress regions of the HAZ

Key Questions and Reflections

The FEA approach, while powerful, has limitations that must be acknowledged in engineering practice:

• The accuracy of predictions depends heavily on the quality of material property data at elevated temperatures
• Phase transformation modeling remains challenging and often oversimplified
• The bond line interface behavior is difficult to model accurately without experimental calibration
• Scale effects (from laboratory specimens to full-scale pressure vessels) introduce additional uncertainties

I have always maintained that FEA should be used as a complementary tool to experimental measurement, not as a replacement. In my experience with clad pressure vessel qualification, the most reliable approach combines FEA predictions with experimental verification through neutron diffraction or hole-drilling strain gauge measurements at critical locations.

The study also raises the question of how residual stress predictions can be integrated into fitness-for-service assessments. For existing clad vessels in service, understanding the residual stress state is essential for remaining life assessment, particularly when considering re-rating or extended operation.

Study Insights and Implications

This research demonstrates the significant value of computational methods in understanding and predicting residual stress distributions in overlay layers. The depth-dependent stress profiles provide engineers with actionable information for optimizing weld sequences, selecting PWHT parameters, and evaluating service performance. The key implication is that residual stress management must be considered as an integral part of the cladding process design, not as an afterthought. For pressure vessel applications, the predicted tensile stresses at the bond line should be compared against allowable stress limits and SCC threshold values to ensure long-term structural integrity. The methodology established here can be extended to more complex geometries and multi-layer overlay configurations with appropriate model validation.