Numerical Simulation and Residual Stress Analysis of Heat Exchanger Tube Sheet Cladding

Literature Overview

This study, published in Modern Manufacturing Engineering in 2016 by Wang Yuhao, He Qingzhong, Wang Jia, Guo Bin, Wang Yongbin, and Pu Hu from the School of Mechanical Engineering at Sichuan University of Science and Engineering, addresses a critical challenge in the fabrication of clad heat exchanger tube sheets. The research was supported by the Open Fund of the Sichuan Provincial Key Laboratory of Materials Corrosion and Protection (2013CL05), the Zigong City Key Science and Technology Program (2013C16), and the Sichuan University of Science and Engineering Graduate Innovation Fund (y2014037). The work focuses on finite element simulation of the cladding process on heat exchanger tube sheets, with particular attention to residual stress distribution and its implications for service performance.

Core Technical Content

Heat exchanger tube sheets are among the most complex components in pressure vessel fabrication, requiring both structural integrity and corrosion resistance at the tube-to-tubesheet junction. In aggressive service environments such as hydrogenation reactors, sour gas service, or high-temperature acid conditions, the tube sheet is typically clad with stainless steel or nickel-based alloys to provide the necessary corrosion resistance while maintaining the mechanical strength of a carbon steel or low-alloy steel base. The cladding process introduces significant thermal gradients that produce residual stresses, which can compromise the dimensional accuracy of the tube sheet, affect the quality of subsequent tube drilling and reaming, and potentially initiate cracking during service.

The researchers employed finite element analysis to model the thermal-mechanical coupling during the cladding process. The simulation incorporated the following key parameters:

The simulation revealed that the maximum residual stress occurs at the weld bead surface and decreases toward the base material. The peak longitudinal residual stress typically reaches 200–350 MPa for 304 stainless steel overlay on 16MnR base, which approaches the yield strength of the base material and warrants careful consideration in design and fabrication.

Key Technical Points and Analysis

Residual Stress Distribution Patterns

The numerical results demonstrate a characteristic stress distribution pattern that is consistent with established welding residual stress theory but with specific features unique to tube sheet geometry. The tube sheet's large flat area combined with the periodic presence of tube holes creates a complex stress field. The simulation shows that residual stresses are significantly affected by:

• The position of the tube holes relative to the cladding weld path
• The sequence of cladding passes and their interaction with previously deposited material
• The constraint imposed by the tube sheet's mounting flange and any attached shell sections
• The thermal mass effect of the thick tube sheet, which can either increase or decrease peak thermal gradients depending on the cladding direction

Effect of Process Parameters on Residual Stress

The study systematically varied several process parameters to identify their influence on residual stress magnitude and distribution. The following observations were made:

1. Heat input: Increasing heat input from 15 kJ/mm to 45 kJ/mm increased the peak residual stress by approximately 30–50%, as the larger thermal gradient and greater volume of material experiencing plastic deformation lead to higher拘束应力. However, higher heat input also produces a more gradual cooling rate, which can reduce the risk of cracking in the overlay layer.
2. Preheating temperature: Raising the preheat temperature from 100°C to 250°C reduced peak residual stress by approximately 15–25%. The elevated base temperature reduces the thermal gradient between the molten pool and the surrounding material, resulting in a more uniform temperature field and less severe thermal contraction stresses.
3. Cladding pass sequence: The order in which cladding passes are applied has a significant effect on the final residual stress state. A back-step welding sequence (welding from the center outward or in alternating directions) was found to produce a more uniform stress distribution compared to a sequential one-directional approach.
4. Clad layer thickness: Thicker clad layers (e.g., 6 mm versus 3 mm) produced higher residual stresses due to the greater volume of material undergoing thermal cycling and the increased constraint from the additional deposited layers.

Comparison with Experimental Measurements

The simulation results were validated against experimental measurements obtained through strain gauge technique and X-ray diffraction stress analysis. The agreement between simulated and measured values was generally within 10–15% for peak stress magnitudes, with slightly larger deviations in the stress gradient regions away from the weld centerline. This level of accuracy is considered acceptable for engineering prediction purposes, particularly when the goal is to identify critical stress zones and evaluate the effectiveness of stress-relief measures.

Engineering Practice Implications

Stress Relief Strategy Selection

Based on the simulation findings, several stress relief strategies were evaluated:

For heat exchanger tube sheets, post-weld heat treatment remains the most reliable method for achieving acceptable residual stress levels, particularly when the component will be subjected to fatigue loading or corrosive environments. However, the simulation results suggest that a combination of process optimization (controlled interpass temperature, optimized pass sequence) and partial stress relief can achieve acceptable results while reducing the thermal exposure of the component.

Impact on Tube Drilling and Reaming

One of the most practical concerns in tube sheet fabrication is the effect of cladding residual stress on the accuracy of tube hole drilling and reaming. The simulation indicates that residual stresses of 200–350 MPa can cause dimensional distortion of the tube sheet surface of 0.1–0.3 mm per 100 mm span, which is significant for tube hole alignment tolerances typically specified at ±0.05–0.10 mm. This finding underscores the importance of stress relief prior to tube drilling, or alternatively, the need to account for stress-induced distortion in the drilling and reaming process.

Design Considerations for Clad Tube Sheets

The residual stress analysis has important implications for the design of clad heat exchanger tube sheets:

• The allowable stress in the base material should be evaluated considering the superposition of operating stress and residual stress, particularly at the tube-to-tubesheet junction where stress concentration factors are already elevated.
• The clad layer thickness should be optimized to provide the required corrosion resistance without introducing excessive residual stress. For most applications, a clad thickness of 3–6 mm is sufficient, and thicker layers should be justified by specific corrosion requirements.
• The transition from clad to base material should be carefully managed, as the stress concentration at this interface can be a critical location for crack initiation under cyclic loading.

Key Questions and Reflections

Several questions arise from this research that merit further investigation:

1. Multi-pass interaction effects: The simulation primarily considered individual passes and their interaction with the base material. However, in practice, tube sheets may have 2–4 layers of cladding, and the interaction between multiple layers can produce complex stress states that are not easily predicted by single-pass simulations. A more comprehensive multi-layer simulation approach would provide more accurate predictions for thick clad layers.
2. Effect of tube hole presence: While the simulation accounts for the tube holes, the interaction between the cladding weld and the tube holes is complex. The tube holes act as stress concentrators and can redirect residual stress flow. Future work should investigate the specific effect of tube hole diameter, spacing, and edge condition on the residual stress field.
3. Long-term stress relaxation: The simulation captures the residual stress state immediately after welding. However, during service, residual stresses can relax through creep mechanisms, particularly at elevated temperatures. For applications involving temperatures above 200°C, the long-term stress state may be significantly different from the as-welded condition, and this should be considered in fatigue life assessment.
4. Material property accuracy: The accuracy of the simulation depends heavily on the material property data used, particularly the temperature-dependent stress-strain curves and thermal conductivity values. For clad materials such as 316L stainless steel, the material properties can vary significantly between suppliers and heats, and the simulation should ideally use material-specific data rather than generic values.

Study Insights and Implications

This research provides valuable quantitative insights into the residual stress behavior of clad heat exchanger tube sheets, bridging the gap between theoretical welding residual stress models and practical fabrication concerns. The finite element approach, when properly calibrated against experimental data, offers a powerful tool for optimizing cladding processes and predicting component performance.

The key takeaway for engineering practice is that residual stress management in clad tube sheets is not merely a matter of post-weld heat treatment but should be addressed through a holistic approach that includes process parameter optimization, welding sequence design, and appropriate stress relief measures. The simulation results confirm that the combination of controlled interpass temperature (200–250°C), optimized welding sequence (back-step or alternating direction), and post-weld heat treatment at 600°C can reduce residual stresses to acceptable levels (below 100 MPa) while maintaining the structural integrity of the clad layer.

Furthermore, this research highlights the importance of integrating numerical simulation into the fabrication planning process for critical components such as heat exchanger tube sheets. By predicting residual stress distributions before fabrication, engineers can identify potential problem areas, optimize the welding procedure, and ensure that the final component meets both structural and corrosion resistance requirements. This proactive approach to residual stress management represents a significant advancement over traditional trial-and-error methods and contributes to the reliability and longevity of pressure vessels in aggressive service environments.

Reference Value and Outlook

The methodology presented in this study is directly applicable to other clad components in pressure vessel fabrication, including shell plates, heads, and flanges. The finite element modeling approach can be adapted to different geometries, materials, and welding processes with appropriate modifications to the boundary conditions and material property data. Future research should focus on extending the simulation to include multi-scale modeling that captures both the macro-scale residual stress distribution and the micro-scale metallurgical effects, such as grain growth, phase transformation, and precipitation hardening, which can influence both the residual stress state and the corrosion resistance of the clad layer.