CLADDING TECHNOLOGY SHANXI CO., LTDNumerical Simulation of Heat Exchanger Tube Sheet Cladding and Residual Stress Analysis
Literature Overview
This study presents a numerical simulation approach for analyzing the weld overlay (cladding) process on heat exchanger tubesheets, with particular emphasis on residual stress development during the overlay welding sequence. Heat exchanger tubesheets are critical pressure-containing components that frequently require cladding to achieve corrosion resistance against process fluids while maintaining the mechanical strength of a carbon steel or low-alloy steel base material. The literature addresses a fundamental challenge in bimetallic pressure vessel fabrication: predicting and controlling residual stresses that arise during the multi-pass overlay welding process.
The study employs finite element analysis (FEA) to model the thermal and mechanical behavior of a tubesheet during sequential overlay welding. The simulation captures the nonlinear material behavior, including elastic-plastic deformation, thermal expansion, and creep relaxation, across the entire welding sequence. The results provide insight into stress distribution patterns, distortion predictions, and the effectiveness of process control strategies for minimizing detrimental residual stresses.
Core Technical Analysis
The numerical model incorporates several key physical phenomena. The thermal analysis solves the transient heat conduction equation with moving heat sources representing the welding arc. The mechanical analysis employs an elastic-plastic constitutive model with kinematic hardening to capture cyclic loading effects during multi-pass welding. The interaction between thermal and mechanical fields is achieved through a sequentially coupled approach, where temperature fields from the thermal analysis serve as boundary conditions for the mechanical analysis.
The material properties used in the simulation include temperature-dependent Young's modulus, Poisson's ratio, yield stress, thermal conductivity, specific heat, and thermal expansion coefficient. For the overlay material, typical nickel-based alloys or austenitic stainless steels are modeled, while the base material represents a common pressure vessel steel such as SA-516 Gr.70 or SA-387 Gr.11.
| Parameter | Base Material (SA-516 Gr.70) | Overlay Material (304L / Inconel 625) |
|---|---|---|
| Yield strength at 20°C | 255 MPa | 205 MPa (304L) / 310 MPa (625) |
| Thermal conductivity at 20°C | 45 W/(m·K) | 16.3 W/(m·K) (304L) / 11.4 W/(m·K) (625) |
| Coefficient of thermal expansion | 12 × 10⁻⁶ /K | 17.3 × 10⁻⁶ /K (304L) |
| Elastic modulus at 20°C | 200 GPa | 193 GPa (304L) / 210 GPa (625) |
| Melting point | 1480°C | 1400°C (304L) / 1350°C (625) |
The residual stress analysis reveals characteristic patterns: compressive stresses develop in the overlay layer due to the sequence of solidification and cooling, while tensile stresses concentrate at the overlay-base interface and in the base material near the weld zone. The peak tensile residual stresses can reach 200–350 MPa depending on the overlay thickness, welding parameters, and sequence strategy. These stresses are particularly concerning because they can reduce the fatigue life of the tubesheet and potentially initiate cracking in the heat-affected zone.
Process Optimization Strategies
The simulation results inform several process optimization strategies that are directly applicable to engineering practice. The welding sequence plays a critical role in residual stress distribution. A symmetric welding pattern, where passes are laid down in a sequence that maintains thermal balance across the tubesheet, produces lower peak stresses than a sequential pattern. The literature demonstrates that a spiral or zigzag pattern starting from the center and progressing outward reduces distortion by 30–50% compared to a straight-line sequence.
Preheating and interpass temperature control emerge as effective tools for managing residual stresses. A preheat temperature of 100–200°C reduces the cooling rate and allows greater plastic relaxation during welding, resulting in lower residual stress magnitudes. However, excessive preheat can lead to grain growth in the base material and reduced hardness in the heat-affected zone. The optimal preheat temperature represents a balance between stress reduction and microstructural control.
Post-weld stress relief is another critical consideration. The simulation quantifies the effectiveness of various stress relief cycles. A conventional PWHT at 580–620°C for 1 hour per 25 mm of thickness reduces residual stresses by 60–80%, but the high temperature may cause sensitization in the stainless steel overlay layer. Alternative approaches, such as thermal stress relief at lower temperatures (350–450°C) or mechanical stress relief through controlled vibration, are evaluated as potential compromises between stress reduction and microstructural integrity.
| Process Parameter | Recommended Range | Effect on Residual Stress |
|---|---|---|
| Preheat temperature | 100–200°C | Reduces peak stress by 15–25% |
| Interpass temperature | ≤ 250°C (stainless overlay) | Prevents sensitization, moderate stress reduction |
| Travel speed | 150–300 mm/min | Higher speed reduces heat input, increases stress gradient |
| Heat input | 0.5–2.0 kJ/mm | Lower heat input increases stress magnitude |
| Pass thickness | 3–6 mm | Thinner passes reduce per-pass stress but increase cycle count |
| PWHT temperature | 580–620°C | Reduces stress by 60–80% but risks sensitization |
Defect Analysis and Quality Control
The residual stress analysis has direct implications for defect prediction and quality control. High tensile residual stresses at the overlay-base interface increase susceptibility to hydrogen-induced cracking (HIC) and sulfide stress corrosion cracking (SSC), particularly in sour service environments. The simulation can identify regions of maximum stress concentration, guiding the placement of non-destructive testing (NDT) efforts.
For tubesheets in hydrogenation reactors or sour gas service, the combined effect of residual stress and operating stress must be evaluated. The simulation provides the basis for a stress evaluation that combines the as-welded residual stress field with the operating stress field from internal pressure and thermal gradients. This combined stress analysis is essential for compliance with design codes such as ASME VIII Div.1 and Div.2, which require demonstration that the maximum principal stress does not exceed allowable limits.
The literature also addresses the effect of overlay thickness on residual stress magnitude. Thicker overlays introduce greater restraint on the base material, leading to higher tensile stresses in the base. However, thinner overlays may not provide adequate corrosion protection and may be more susceptible to mechanical damage during service. The optimal overlay thickness represents a trade-off between corrosion protection, residual stress management, and mechanical integrity.
Engineering Practice Integration
In practical fabrication, the numerical simulation results must be validated through experimental measurement. Strain gauge measurements during welding, X-ray diffraction (XRD) residual stress measurements after welding, and neutron diffraction measurements for deep stress profiling provide the experimental data needed to calibrate and validate the numerical model. The literature demonstrates good agreement between simulated and measured residual stress profiles, with deviations typically within 10–20% of the peak values.
For quality assurance purposes, the simulation can be used to establish acceptance criteria for residual stress levels. A maximum tensile residual stress of 200 MPa at the overlay-base interface, for example, may be specified as an acceptance criterion for tubesheets in high-integrity applications. This criterion can be verified through XRD measurements at critical locations, with the measurement plan informed by the simulation results.
Study Insights and Conclusions
The most significant contribution of this literature is the demonstration that numerical simulation provides a powerful tool for predicting and controlling residual stresses in overlay welding operations. For the cladding engineer, this means that process development can be accelerated through simulation-based optimization, reducing the need for extensive trial-and-error testing. The ability to predict residual stress distributions before fabrication begins enables proactive design of welding sequences, preheat protocols, and stress relief procedures.
However, the literature also highlights limitations that must be acknowledged. The accuracy of the simulation depends on the quality of material property data, the fidelity of the heat source model, and the appropriateness of the constitutive model for cyclic thermo-mechanical loading. In practice, material properties at elevated temperatures are often poorly characterized, leading to uncertainties in the predicted stress fields. Future work should focus on improving material databases and developing more sophisticated constitutive models that capture the complex behavior of overlay materials during welding.
The practical implication for bimetal pressure vessel fabrication is clear: numerical simulation should be an integral part of the process qualification and design verification workflow, complementing rather than replacing experimental testing and code compliance procedures.