CLADDING TECHNOLOGY SHANXI CO., LTDInterpass Stress Analysis and Process Optimization in Metal Overlay Forming
Literature Overview and Research Background
This study addresses the critical issue of interpass stress development during multi-pass metal overlay forming processes and proposes process optimization strategies to mitigate adverse stress effects. In weld overlay applications, particularly for thick overlay layers deposited through multiple passes, the cumulative thermal cycling creates complex residual stress fields that can significantly affect the mechanical performance, dimensional stability, and service life of the overlay component. Understanding the evolution of interpass stresses and implementing targeted process controls is essential for producing high-quality overlay components that meet the demanding requirements of pressure vessel fabrication, corrosion-resistant linings, and wear-resistant surfaces.
Core Technical Points
Thermal Stress Mechanisms and Evolution
The interpass stress field in multi-pass overlays develops through a combination of thermal stresses (arising from differential thermal expansion and contraction) and mechanical stresses (resulting from plastic deformation during welding). During each welding pass, the deposited material experiences rapid heating followed by cooling, creating a transient thermal gradient that drives plastic deformation. As subsequent passes are deposited, the previously solidified material is reheated, leading to stress redistribution and partial relaxation of previously accumulated stresses. However, this stress relief is incomplete, and the net residual stress field continues to evolve with each additional pass.
The magnitude and distribution of interpass stresses depend on several factors, including the heat input per pass, the interpass temperature, the number of passes, the weld geometry, and the constraint imposed by the base material and previously deposited layers. In constrained overlay configurations (such as those applied to thick-walled pressure vessels), the base material acts as a rigid constraint, leading to higher tensile residual stresses in the overlay layer. In unconstrained configurations (such as overlays applied to thin sheets), the stresses are lower but may cause significant distortion.
| Process Parameter | Low Value | High Value | Effect on Interpass Stress |
|---|---|---|---|
| Heat input per pass | 0.5 kJ/mm | 8.0 kJ/mm | Higher heat input increases tensile stress magnitude |
| Interpass temperature | 50°C | 300°C | Higher interpass temperature reduces stress accumulation |
| Number of passes | 1 | 10 | More passes increase cumulative stress but allow redistribution |
| Weld bead width | 5 mm | 20 mm | Wider beads increase constraint and tensile stress |
| Weld bead spacing | 2 mm | 10 mm | Wider spacing reduces interaction between adjacent beads |
| Travel speed | 50 mm/min | 400 mm/min | Faster travel reduces heat input and lowers stress |
Stress Measurement and Analysis Methods
The accurate measurement of residual stresses in overlay layers is challenging due to the thin cross-section, complex geometry, and the presence of multiple material interfaces. The study employs a combination of X-ray diffraction (XRD) for surface stresses, neutron diffraction for through-thickness stresses, and the hole-drilling method for stress relaxation measurements. Each method has its limitations: XRD is limited to surface stresses within the top 50–200 μm, neutron diffraction requires specialized facilities, and the hole-drilling method is destructive and limited to stresses within a depth of approximately 1.5 times the hole diameter.
The finite element analysis (FEA) of the welding process provides a complementary approach to experimental measurement, allowing for the prediction of stress fields throughout the entire overlay component. The FEA model should incorporate the temperature-dependent material properties of both the base material and the overlay material, the thermal and mechanical boundary conditions, and the welding heat source model (typically a double-ellipsoidal or Gaussian heat source). The accuracy of the FEA predictions is validated by comparing the calculated stress distributions with experimental measurements at multiple locations and depths.
Stress Relief and Process Optimization Strategies
| Optimization Strategy | Mechanism | Effectiveness | Implementation Difficulty |
|---|---|---|---|
| Controlled interpass temperature | Thermal stress relaxation | High | Moderate (requires temperature monitoring) |
| Multi-directional welding sequence | Stress cancellation | High | Low (requires programming) |
| Interpass hammering | Plastic deformation, stress relief | Moderate | Moderate (requires skill) |
| Post-weld heat treatment (PWHT) | Stress relaxation, microstructure refinement | Very High | High (requires furnace) |
| Reduced heat input | Lower thermal gradients | Moderate | Low (requires process adjustment) |
| Backing layer / stress-relief layer | Stress buffering | Moderate | Low |
| Vibration-assisted welding | Stress relaxation through vibration | Moderate | High (requires specialized equipment) |
The most effective strategy for managing interpass stresses is the combination of a carefully designed welding sequence (such as a symmetric or step-back welding pattern) with controlled interpass temperatures maintained between 150–250°C. This combination allows for partial stress relief during the welding process itself, reducing the need for extensive post-weld heat treatment. For critical applications where residual stresses must be minimized (such as hydrogen service pressure vessels), a post-weld heat treatment at 600–700°C for a duration of 1 hour per 25 mm of wall thickness is recommended to achieve stress levels below 30 MPa.
Engineering Practice Integration
Application to Pressure Vessel Cladding
In the fabrication of clad-plate pressure vessels, the interpass stress management is particularly critical due to the high constraint imposed by the thick base plate and the potential for hydrogen-induced cracking (HIC) in low-stress-corrosion cracking (LSCC) susceptible materials. The overlay layer, typically deposited using electroslag welding (ESW) or submerged arc welding (SAW), is subject to significant tensile residual stresses that can promote crack initiation and propagation, particularly in the presence of sulfide inclusions or high hydrogen content.
The recommended welding sequence for ESW cladding involves a multi-pass approach with a controlled interpass temperature of 200–300°C, a welding current of 4000–6000 A, and a travel speed of 150–250 mm/min. The welding sequence should follow a symmetric pattern (welding from the center outward) to minimize distortion and stress concentration. For thick overlays (greater than 15 mm), the use of a backing layer or stress-relief layer is recommended to buffer the stress transfer from the overlay to the base material.
Quality Control and Acceptance Criteria
| Inspection Method | Stress Measurement Capability | Typical Acceptance Criteria |
|---|---|---|
| X-ray diffraction (XRD) | Surface stresses (0–200 μm depth) | Tensile stress < 150 MPa |
| Neutron diffraction | Through-thickness stresses | Tensile stress < 100 MPa |
| Hole drilling | Local stresses (0–1.5 × hole diameter) | Stress gradient < 50 MPa/mm |
| Magnetoelastic method | Surface and near-surface stresses | Stress < 200 MPa |
| Strain gauge method | Local stress relaxation | Relaxation < 50 MPa |
The acceptance criteria for residual stresses should be established based on the service conditions and the susceptibility of the material to stress-corrosion cracking. For hydrogen service applications (such as hydrogenation reactors and ammonia synthesis loops), the tensile residual stress should be limited to below 100 MPa to minimize the risk of HIC and LSCC. For general corrosion-resistant overlay applications, a tensile stress limit of 150–200 MPa is typically acceptable.
Key Questions and Reflections
The study raises important questions about the long-term stability of residual stresses under service conditions. While post-weld heat treatment can effectively reduce residual stresses, subsequent thermal cycling during service (such as the repeated heating and cooling of a pressure vessel during operation) can re-introduce stresses through differential thermal expansion. The interaction between service-induced stresses and the residual stress field from welding is a complex phenomenon that is not fully understood and requires further investigation.
Additionally, the study highlights the importance of considering the interaction between residual stresses and other microstructural features, such as grain orientation, phase distribution, and inclusion morphology. The combined effect of these factors on the mechanical performance and fracture resistance of overlay layers is a critical area for future research. The development of predictive models that can account for these interactions would greatly enhance the ability of engineers to design reliable overlay solutions for demanding applications.
Study Insights and Implications
The research on interpass stress analysis and process optimization in metal overlay forming provides a comprehensive framework for managing residual stresses in multi-pass overlay applications. The identification of key process parameters and their influence on the stress field, combined with the development of effective mitigation strategies, offers practical guidance for engineers involved in overlay fabrication. The integration of thermal stress management with microstructural control represents a holistic approach to overlay engineering that goes beyond simple process optimization to address the fundamental metallurgical mechanisms governing overlay performance. For the pressure vessel industry, the findings of this study have direct implications for the design and fabrication of clad components in hydrogen service, where residual stress management is a critical safety consideration. The systematic approach to stress analysis, measurement, and mitigation presented in this study serves as a valuable reference for developing quality assurance protocols and ensuring the long-term reliability of overlay components in demanding industrial applications.