Finite Element Analysis of Residual Stress in Surfacing Layers at Different Depths

Literature Overview

The paper by Zhou Yuming and Shi Haifang, published in Heat Treatment of Metals (2014, Vol. 39, No. 7, pp. 153-156), presents a combined numerical and experimental investigation of welding residual stresses in surfacing layers at varying depths. Using the MARC finite element software, the authors developed a thermal-mechanical model that accounts for the welding heat source, welding time, and cooling time. The simulation results were validated against experimental measurements obtained using the hole-drilling method (blind hole method), demonstrating good agreement in stress distribution trends.

Core Technical Content

Residual stresses in surfacing deposits are a critical concern in surface engineering because they directly influence the service life of the component, particularly with respect to fatigue cracking, stress corrosion cracking, and delamination failure. The authors established a finite element model based on a constant plate thickness condition, incorporating the moving heat source characteristics of the welding process and the associated thermal cycles.

Modeling Approach

The finite element model employed a sequential coupled approach: first, a thermal analysis was performed to determine the temperature field evolution during welding and cooling, and then the thermal history was mapped onto a structural model to compute the residual stress distribution. This approach captures the essential physics of residual stress formation, including thermal expansion/contraction, plastic deformation, and stress relaxation during cooling.

Key Findings

The study revealed a consistent trend: welding residual stress increases with increasing surfacing layer depth. This finding has significant practical implications:

• Shallow surfacing layers (first pass or cover pass) experience lower residual stresses due to the stress-relieving effect of the adjacent air and the relatively free boundary condition.
• Deeper surfacing layers are constrained by the surrounding material on multiple sides, leading to greater restraint and higher residual stress levels.
• The agreement between FEM predictions and hole-drilling measurements confirms that the modeling assumptions (material properties, boundary conditions, heat source parameters) are reasonably accurate.

Engineering Practice Integration

In engineering practice, the residual stress distribution in surfacing deposits directly affects:

• Service life: High tensile residual stresses at the weld root or between layers promote fatigue crack initiation and propagation.
• Delamination risk: Excessive interlayer residual stresses can lead to spalling or delamination of the surfacing overlay under cyclic loading.
• Post-weld treatment requirements: Understanding the stress distribution helps determine whether stress-relief annealing is necessary and at what temperature and duration.

For critical applications such as:

• Boiler tube overlays
• Pressure vessel corrosion-resistant linings
• Pump impeller and turbine blade coatings
• Pipeline girth weld surfacing for sour service

The residual stress distribution must be carefully managed through process optimization, including:

1. Welding sequence optimization: Using a zigzag or back-step welding pattern to reduce peak residual stresses.
2. Interpass temperature control: Maintaining interpass temperatures between 150-250°C to promote stress relaxation without compromising hardness.
3. Post-weld stress relief: Applying heat treatment at 550-650°C for carbon steel substrates or appropriate temperatures for alloy systems.
4. Layer thickness management: Using thinner layers (2-3 mm) to reduce individual pass stress levels.

Key Questions and Reflections

The hole-drilling method used for validation is a well-established technique but has inherent limitations: it measures stress at a specific point and depth, requiring multiple measurements to reconstruct the full stress profile. The finite element model, while validated, still relies on simplifying assumptions regarding material behavior (elastic-plastic constitutive models) and heat source characterization. Future work should consider:

• Incorporating creep and stress relaxation effects at elevated temperatures
• Using more advanced experimental techniques such as neutron diffraction or X-ray synchrotron for non-destructive stress mapping
• Accounting for the effect of multi-layer multi-pass welding sequences on the cumulative stress state
• Considering the influence of base metal properties (yield strength, thermal conductivity) on the residual stress distribution

Study Implications and Outlook

This research provides a robust methodology for predicting and managing residual stresses in surfacing applications. The combination of finite element analysis with experimental validation offers a reliable framework for process optimization. For engineers, the key takeaway is that residual stress increases with surfacing depth, and this trend can be predicted and mitigated through appropriate process design. The MARC-based approach can be adapted to other finite element platforms (ANSYS, ABAQUS) with appropriate modifications to the material models and boundary conditions. The validated model serves as a valuable tool for pre-production process planning, enabling engineers to optimize welding parameters before physical trials, thereby reducing development costs and time-to-market for surfacing applications.