CLADDING TECHNOLOGY SHANXI CO., LTDNumerical Simulation and Residual Stress Analysis of Heat Exchanger Tube Sheet Cladding
Literature Overview
This study presents a numerical simulation of the cladding process for heat exchanger tube sheets, with a focus on residual stress distribution and its implications for component performance. Heat exchanger tube sheets are critical pressure boundary components that must withstand high temperatures, pressures, and corrosive media. The cladding layer, typically an austenitic stainless steel (e.g., 316L, 321, or 347) or nickel-based alloy, provides corrosion resistance while the base material (e.g., carbon steel or low-alloy steel) provides structural strength. The residual stress state induced by the cladding process directly affects the tube sheet's resistance to stress corrosion cracking, fatigue life, and dimensional stability during subsequent tube expansion and assembly operations.
Core Technical Points
Cladding Process Configuration
Heat exchanger tube sheets are typically clad using one of the following methods:
- Electroslag welding (ESW) overlay: Common for thick tube sheets (20–50 mm) with large surface areas, providing high deposition rates and good dilution control.
- Submerged arc welding (SAW) overlay: Used for medium-thickness tube sheets (10–30 mm) where moderate deposition rates are acceptable.
- Plasma transferred arc (PTA) powder cladding: Employed for thin tube sheets (5–15 mm) or where low dilution and high-quality surface finish are required.
- Strip cladding (roll-bonded): Used as a pre-clad tube sheet blank, with the cladding layer bonded during the rolling process rather than by welding.
The study focuses on ESW and SAW overlay processes, which are the most commonly used methods for tube sheet cladding in pressure vessel fabrication. The numerical model employs a finite element approach with a sequential thermomechanical coupling strategy, where the thermal analysis is performed first to determine the temperature field and cooling rates, followed by the mechanical analysis to compute the residual stress distribution.
Residual Stress Distribution
The residual stress distribution in a clad heat exchanger tube sheet is characterized by a complex multi-axial stress state that varies with position relative to the weld beads and the tube sheet geometry. The simulation reveals that the maximum longitudinal residual stress (parallel to the weld bead direction) reaches 250–350 MPa in the cladding layer, with values decreasing to 100–200 MPa in the base metal. The transverse residual stress (perpendicular to the weld bead direction) is typically lower, ranging from 100–200 MPa in the cladding layer. The radial stress (through the thickness) is compressive in the cladding layer near the surface and tensile near the clad interface.
| Position | Longitudinal Stress | Transverse Stress | Radial Stress |
|---|---|---|---|
| Cladding surface | 250–350 MPa (tensile) | 100–200 MPa (tensile) | 0–50 MPa (compressive) |
| Clad interface | 150–250 MPa (tensile) | 80–150 MPa (tensile) | 50–100 MPa (tensile) |
| Base metal (10 mm from interface) | 80–150 MPa (tensile) | 40–80 MPa (tensile) | 20–50 MPa (compressive) |
| Base metal core | 30–80 MPa (tensile) | 20–50 MPa (tensile) | 0–20 MPa (compressive) |
Influence of Weld Sequence on Residual Stress
The weld sequence is a critical process variable that significantly influences the residual stress distribution in clad tube sheets. The study compares several weld sequence strategies:
- Single-direction sequential welding: Welds are deposited in a single direction from one end of the tube sheet to the other. This produces the highest longitudinal residual stresses due to the progressive accumulation of thermal strain.
- Alternating direction welding: Welds are deposited in alternating directions, with each pass starting from the opposite end. This partially cancels the longitudinal residual stresses but may introduce additional transverse stresses.
- Symmetric welding: Welds are deposited from the center of the tube sheet outward in both directions simultaneously. This produces the most symmetric residual stress distribution and minimizes angular distortion.
- Multi-pass with interpass temperature control: Multiple passes are deposited with controlled interpass temperatures (typically 150–250 °C for austenitic stainless steel cladding), which reduces the peak temperature and moderates the residual stress magnitudes.
| Weld Sequence | Max Longitudinal Stress | Max Angular Distortion | Processing Time |
|---|---|---|---|
| Single-direction | 320 MPa | 2.5 mm | 1.0× |
| Alternating direction | 240 MPa | 1.2 mm | 1.3× |
| Symmetric | 180 MPa | 0.5 mm | 1.5× |
| Multi-pass with interpass control | 200 MPa | 0.8 mm | 1.8× |
Residual Stress and Component Performance
Stress Corrosion Cracking (SCC) Resistance
The residual stress state in a clad heat exchanger tube sheet is a critical factor in determining the susceptibility to stress corrosion cracking, particularly in chloride-containing environments. The threshold stress for SCC in austenitic stainless steels (e.g., 316L) is typically in the range of 100–200 MPa, depending on the chloride concentration, temperature, and material condition. The simulation results indicate that the longitudinal residual stresses in the cladding layer can exceed this threshold in the weld bead region, creating a risk for SCC initiation in the heat-affected zone (HAZ) and the weld metal itself.
Post-weld heat treatment (PWHT) is the primary method for reducing residual stresses in clad tube sheets. However, PWHT of austenitic stainless steel cladding layers is complicated by the risk of sensitization (chromium carbide precipitation at grain boundaries) in the temperature range of 450–850 °C. The study recommends a PWHT temperature of 420–450 °C for 316L cladding to achieve a balance between residual stress relief and sensitization avoidance, with a soaking time of 1 hour per 25 mm of cladding thickness.
Fatigue Life and Tube Expansion
The residual stress distribution also affects the fatigue life of the clad tube sheet, particularly in the tube hole regions where the tube expansion operation introduces additional localized stresses. The simulation shows that the residual stresses in the tube hole region are influenced by the proximity of the cladding weld beads. Weld beads located within 2–3 times the cladding thickness from the tube hole centerline can increase the local residual stress by 50–100 MPa, reducing the fatigue life by 20–40% in cyclic loading conditions.
The tube expansion operation, which is performed after cladding to secure the tubes in the tube sheet, introduces additional plastic deformation and residual stresses in the tube sheet. The interaction between the cladding-induced residual stresses and the tube expansion-induced residual stresses can be either beneficial or detrimental, depending on the stress state superposition. Compressive residual stresses in the tube hole region are beneficial for fatigue life, while tensile residual stresses are detrimental. The study recommends that the cladding weld sequence be designed to produce compressive residual stresses in the tube hole regions, which can be achieved by directing the final weld passes toward the tube hole area.
Standards and Acceptance Criteria
Comparison with Standards
The residual stress levels predicted by the simulation are compared with the acceptance criteria specified in relevant standards for clad pressure vessel components.
| Standard | Residual Stress Limit | Testing Method | Applicable Component |
|---|---|---|---|
| GB/T 150 | Not explicitly specified | N/A | Pressure vessels |
| NB/T 47002 | Not explicitly specified | N/A | Pressure vessel components |
| ASME VIII Div.1 | Not explicitly specified | N/A | Pressure vessels |
| ASME VIII Div.2 | Not explicitly specified | N/A | Pressure vessels |
| API 934 | Not explicitly specified | N/A | Strip cladding |
| EN 10028-7 | Not explicitly specified | N/A | Clad plates |
| NACE MR0175 | Implied through SCC resistance | Hydrogen blistering test | Clad components in sour service |
The absence of explicit residual stress limits in most standards reflects the difficulty of measuring residual stresses in complex geometries and the challenge of establishing universal acceptance criteria. However, the study's findings provide a technical basis for establishing project-specific residual stress limits based on the service conditions and performance requirements of the specific component.
Integration with Engineering Practice
Application to Hydrogenation Reactor Tube Sheets
A practical case involved the cladding of a hydrogenation reactor tube sheet made of 16MnR base steel with a 316L stainless steel cladding layer. The tube sheet had a thickness of 40 mm and a diameter of 1200 mm, with 400 tube holes of 25 mm diameter. The cladding was performed using ESW overlay with a symmetric weld sequence and an interpass temperature of 200 °C. The simulated residual stress distribution showed maximum longitudinal stresses of 220 MPa in the cladding layer, which were reduced to 80 MPa after PWHT at 430 °C for 2 hours. Post-PWHT residual stress measurements using the hole-drilling method confirmed the simulated values within a 15% margin of error, validating the simulation approach.
Case Example: Heat Exchanger Tube Sheet Residual Stress Management
Another case involved a 316L clad 20# steel heat exchanger tube sheet with a thickness of 25 mm and a diameter of 800 mm. The initial cladding process used a single-direction weld sequence, resulting in residual stresses of 340 MPa in the cladding layer. After tube expansion, the tube sheet exhibited cracking at three tube holes located near the weld beads. The root cause analysis identified the combination of high residual stresses and the tube expansion-induced tensile stresses as the primary cause of cracking. The process was revised to use a symmetric weld sequence with a 200 °C interpass temperature, and PWHT was applied at 430 °C. The revised process produced residual stresses of 160 MPa in the cladding layer, and no cracking was observed after tube expansion.
Key Questions and Reflections
The study raises an important question about the accuracy of residual stress predictions in complex geometries such as tube sheets with hundreds of tube holes. The numerical model simplifies the tube hole geometry and does not fully capture the interaction between the cladding weld beads and the tube hole stress concentrations. Future work should develop more refined models that incorporate the tube hole geometry and the tube expansion process to provide more accurate predictions of the final residual stress state.
Another reflection concerns the validation of simulation results against experimental measurements. While the hole-drilling method is commonly used for residual stress measurement, it is limited to surface and near-surface measurements and cannot capture the through-thickness stress distribution. The study recommends the use of neutron diffraction or X-ray diffraction for through-thickness residual stress measurements in critical components, although these methods are more expensive and less accessible.
Study Insights and Implications
The most significant insight from this study is the demonstration that the weld sequence is a powerful tool for controlling the residual stress distribution in clad tube sheets. By carefully designing the weld sequence to produce compressive residual stresses in critical regions (such as tube holes), the fatigue life and SCC resistance of the component can be significantly improved. The study's simulation approach provides a practical tool for predicting residual stress distributions and optimizing the weld sequence before fabrication, reducing the need for costly trial-and-error approaches. Engineers working on clad tube sheet fabrication should adopt simulation-based weld sequence design as a standard practice, particularly for critical components where residual stress control is essential for long-term reliability.