CLADDING TECHNOLOGY SHANXI CO., LTDStudy Note on Numerical Simulation and Residual Stress Analysis of Heat Exchanger Tube Sheet Cladding
Literature Overview
This 2016 paper by researchers from Sichuan University of Science and Engineering, published in Modern Manufacturing Engineering, presents a numerical simulation study of the cladding process on heat exchanger tube sheets, with a focus on residual stress distribution and its implications for structural integrity. Heat exchanger tube sheets are critical components in pressure vessels and heat exchangers, and their cladding—typically with stainless steel or nickel-based alloys—is essential for corrosion resistance in aggressive process environments. The study leverages finite element analysis (FEA) to predict residual stress patterns that arise during multi-pass overlay welding, providing valuable insights for process optimization and defect prevention.
Core Technical Approach
The research employs a thermal-mechanical coupled finite element model to simulate the residual stress state in a tube sheet during cladding. The simulation accounts for:
- The sequential deposition of multiple weld passes, each introducing localized thermal gradients.
- The elastic-plastic material behavior of both the base metal (typically carbon steel or low-alloy steel) and the overlay layer (typically austenitic stainless steel such as 304 or 316).
- Thermal expansion mismatch between the ferritic base metal and the austenitic overlay, which is a primary driver of residual stress development.
The tube sheet geometry is modeled as a thick cylindrical plate with a central hole for tube insertion, and the cladding layer is deposited on the tube-side face, which is exposed to the process fluid.
Residual Stress Distribution Patterns
The simulation results reveal several characteristic residual stress patterns:
| Stress Component | Location | Typical Magnitude | Implications |
|---|---|---|---|
| Longitudinal Tensile | Overlay center, far from weld start/end | 200–400 MPa | Risk of stress corrosion cracking in chloride environments |
| Circumferential Tensile | Near the outer edge of the cladding | 150–350 MPa | Potential for edge cracking |
| Radial Compressive | At the overlay-base metal interface | -50 to -150 MPa | Generally beneficial for fatigue life |
| Transverse Tensile | Weld toe region | 100–250 MPa | Susceptible to fatigue crack initiation |
The thermal expansion mismatch between austenitic stainless steel (approximately 17.3 × 10⁻⁶ /K) and carbon steel (approximately 12.0 × 10⁻⁶ /K) is a dominant factor. As the overlay cools, the austenitic layer contracts more than the ferritic substrate, generating tensile residual stresses in the overlay and compressive stresses in the base metal.
Process Optimization Strategies
Based on the simulation results, several process optimization strategies are proposed:
- Weld sequence design: Using a symmetric welding sequence from the center outward, or employing a spiral pattern, can reduce peak residual stresses compared to a single-direction linear sequence.
- Preheating: Applying a uniform preheat temperature of 100–150 °C reduces thermal gradients and lowers residual stress magnitudes.
- Interpass temperature control: Maintaining interpass temperatures below 150 °C prevents excessive softening of the base metal while still allowing some stress relaxation.
- Post-weld heat treatment (PWHT): Stress relief annealing at 620–650 °C for carbon steel tube sheets can reduce residual stresses by 50–80%, though this must be balanced against potential sensitization of the austenitic overlay.
Comparison of Cladding Processes for Tube Sheets
The study implicitly compares different cladding approaches:
| Cladding Method | Typical Residual Stress Level | Dilution Control | Suitability for Tube Sheets |
|---|---|---|---|
| SAW overlay | High (300–500 MPa) | Moderate | Thick overlays (>3 mm) |
| GTAW overlay | Moderate (200–350 MPa) | Low | Thin overlays, precise control |
| ESW overlay | Moderate (250–400 MPa) | Moderate | Very thick overlays (>6 mm) |
| PTA cladding | Low-Moderate (150–300 MPa) | Very low | Thin, high-quality overlays |
| Explosive cladding | Low (<150 MPa) | None | Bonded layers, no weld dilution |
Engineering Practice and Quality Control
In practical tube sheet cladding operations, the following quality control measures are essential:
- Pre-weld inspection: Verification of base metal composition, hardness, and absence of defects such as laminations or inclusions.
- Weld procedure qualification: Following NB/T 47014 or ASME IX procedures to ensure that the selected welding process, filler metal, and parameters produce acceptable results.
- Post-weld NDE: Ultrasonic testing (UT) per JB/T 4730 for detection of lack of fusion and cracks at the overlay-base metal interface; magnetic particle testing (MT) or penetrant testing (PT) for surface defects.
- Residual stress measurement: X-ray diffraction or hole-drilling methods to verify that residual stresses are within acceptable limits, particularly for components subject to stress corrosion cracking risk.
Key Reflections and Study Insights
The numerical simulation approach presented in this paper is a powerful tool for predicting residual stress distributions without the need for extensive physical testing. However, several limitations should be acknowledged:
- The accuracy of FEA predictions depends heavily on the quality of material property data, particularly the temperature-dependent stress-strain curves and thermal properties.
- The model assumes idealized boundary conditions that may not fully capture the constraints imposed by the actual tube sheet geometry and its attachment to the pressure vessel shell.
- The effect of welding-induced distortion on the final dimensional accuracy of the tube sheet is not fully addressed, which is critical for tube installation.
The study's most valuable contribution is the demonstration that weld sequence optimization can significantly reduce peak residual stresses, which directly impacts the risk of stress corrosion cracking in service. For engineers designing cladding procedures for heat exchanger tube sheets, this finding underscores the importance of not only selecting the appropriate welding process but also carefully planning the weld sequence to minimize residual stress accumulation.