Dynamic Simulation and Stress Characteristics of Tube Sheet Weld Overlay Based on ABAQUS

Literature Overview

This study by Wang Puquan, He Qingzhong, Yan Yunqi, Guo Shuai, and Duan Peng from the School of Mechanical Engineering at Sichuan University of Science and Engineering investigates the thermal-mechanical coupling behavior during weld overlay of heat exchanger tube sheets using ABAQUS finite element analysis. Funded by the Key Application Research Program of the Sichuan Provincial Key Laboratory for Materials Corrosion and Protection (2013CL05) and the Zigong City Key Science and Technology Program (2013C16), the work addresses a critical practical challenge: predicting residual stress distribution and distortion in tube sheets subjected to weld overlay cladding.

The tube sheet is one of the most mechanically and thermally critical components in heat exchangers, particularly in hydrogenation reactors, high-pressure separators, and cryogenic service equipment. When corrosion-resistant overlay is applied to the tube sheet surface, the localized thermal input inevitably induces complex residual stress fields that can compromise both the bond integrity of the overlay and the structural integrity of the tube-to-tube-sheet joints.

Core Technical Approach

The researchers employed a sequential coupling approach in ABAQUS, first solving the transient thermal problem and then mapping the temperature history onto the mechanical analysis. The thermal model utilized the DFLUX subroutine to simulate moving heat sources representing the welding arc, while the mechanical model incorporated temperature-dependent material properties for both the base material and the overlay alloy.

The moving heat source was modeled as a double-ellipsoidal Goldak distribution, which better captures the asymmetric temperature profile of a real welding arc compared to a simple Gaussian source. The simulation considered multiple passes with proper interpass cooling intervals.

Key Findings on Stress Distribution

The analysis revealed that peak tensile residual stresses concentrate in the weld cap region, reaching values of 280–350 MPa for a single-pass simulation. However, with multi-pass welding sequences incorporating back-step welding and proper restraint, the peak stresses can be reduced to approximately 180–220 MPa. The stress distribution follows a characteristic pattern: maximum longitudinal tensile stress in the weld centerline, transitioning to compressive stress in the heat-affected zone and beyond.

A particularly important finding is the interaction between the overlay residual stress and the pre-existing tube-hole stress concentration. The tube holes in the tube sheet act as stress concentrators, and when the overlay residual stress field superimposes on these geometric discontinuities, localized stress peaks can exceed 400 MPa near the tube-hole boundaries in the overlay layer. This has direct implications for overlay cracking susceptibility, particularly in nickel-based alloys with limited ductility.

Engineering Practice Implications

From an engineering practice perspective, this study validates several important fabrication guidelines that experienced cladding engineers already follow intuitively:

1. Back-step welding or skip welding sequences effectively reduce peak residual stresses by distributing the thermal input more evenly across the tube sheet surface.
2. Preheating to 100–150 °C reduces the peak cooling rate and consequently the magnitude of thermal gradients, which directly lowers residual stress levels.
3. Post-weld heat treatment (PWHT) at 590–620 °C for carbon steel base materials can reduce residual stresses by 60–80%, but the hold time and ramp rates must be carefully controlled to avoid sensitization of the stainless steel overlay.

The study also highlights a practical challenge: the tube sheet overlay must be compatible with subsequent tube-to-tube-sheet welding. If the overlay extends over the tube holes, the tube welding process must be designed to avoid excessive dilution from the overlay material, which could compromise the mechanical properties of the tube weld.

Key Questions and Reflections

One question that this study raises but does not fully answer is the long-term fatigue behavior of the tube sheet under combined cyclic pressure loading and the frozen residual stress field from the overlay. In hydrogenation service, tube sheets experience thousands of pressure cycles, and the superposition of residual tensile stress with cyclic stress could accelerate fatigue crack initiation, particularly at the tube-hole edges.

Another reflection concerns the applicability of the simulation to different welding processes. The study focuses on SAW overlay, which is common for thick tube sheets, but in practice, GTAW and GMAW are also frequently used for tube sheet overlay, especially where access is limited or where thinner overlay layers are required. The thermal input profiles differ significantly between these processes, and the residual stress predictions would need to be recalibrated accordingly.

Study Insights and Outlook

This work demonstrates that finite element simulation has matured to the point where it can provide actionable guidance for tube sheet overlay fabrication. The key value lies not merely in predicting stress magnitudes but in optimizing welding sequences and thermal management strategies before committing to physical fabrication on expensive tube sheets. For engineers involved in hydrogenation reactor fabrication, this type of analysis should become standard practice, integrated with welding procedure qualification (WPQ) and welder performance qualification (WPQ) to ensure that the simulated conditions match the actual fabrication parameters. Future work should extend to multi-physics simulations that incorporate hydrogen embrittlement effects in the overlay layer, which is particularly relevant for high-pressure hydrogen service where the overlay material must resist both corrosion and hydrogen damage simultaneously.