Control of Deformation in Cladding Large-Diameter Heat Exchanger Tube Sheets

Literature Overview

This 2017 study by Zhou Yinmei from Shanxi Yangmei Chemical Machinery (Group) Co., Ltd., published in China Chemical Equipment (中国化工装备), addresses a critical engineering challenge in the fabrication of large-diameter heat exchanger tube sheets. Tube sheets in chemical and petrochemical heat exchangers are frequently clad with stainless steel or nickel-based alloys to provide corrosion resistance in aggressive process environments. However, the cladding process introduces significant thermal stresses that can cause deformation, warping, and out-of-flatness, which compromise the sealing integrity of the tube sheet and can lead to leakage in service. This study presents practical methods for controlling cladding-induced deformation in large tube sheets.

Technical Background and Problem Definition

Heat exchanger tube sheets are critical components that provide structural support for the tubes and create pressure boundaries between the shell-side and tube-side fluids. In chemical and petrochemical service, tube sheets are often clad with corrosion-resistant alloys to withstand aggressive media such as acids, chlorides, and sulfides. The cladding process — whether by weld overlay, strip cladding, or explosion cladding — introduces thermal gradients and residual stresses that can deform the tube sheet.

For large-diameter tube sheets (typically exceeding 800–1000 mm in diameter), the deformation problem is exacerbated by the large surface area, the significant heat input required for multi-layer cladding, and the constraints imposed by the tube sheet geometry, including tube holes, gasket grooves, and connection flanges. Even small deformations can prevent proper gasket seating, cause tube-to-tube-sheet joint leakage, or lead to uneven stress distribution that promotes fatigue failure.

Deformation Mechanisms in Tube Sheet Cladding

The deformation of tube sheets during cladding can be attributed to several mechanisms:

1. Thermal expansion and contraction: The local heating during welding causes the metal to expand, and upon cooling, the contraction is constrained by the surrounding cooler material, generating residual stresses and deformation.
2. Non-uniform heat input: The sequential deposition of cladding layers creates asymmetric thermal histories, leading to differential contraction and angular distortion.
3. Phase transformation effects: In steel cladding layers, the transformation from austenite to martensite or ferrite during cooling can cause volume changes that contribute to residual stresses and deformation.
4. Geometric constraints: The presence of tube holes, gasket grooves, and flange connections creates stress concentrations and constraint effects that amplify deformation.

Deformation Control Methods

The study proposes and evaluates several methods for controlling cladding-induced deformation in large tube sheets:

Welding Sequence Optimization

The welding sequence is the most critical factor in deformation control. A symmetric, balanced sequence that alternates between opposite sides of the tube sheet center minimizes asymmetric thermal distortion. For large tube sheets, the cladding should be divided into segments, and the welding should proceed from the center outward or in a spiral pattern to ensure uniform heat distribution.

Backing Plate and Restraining Devices

The use of backing plates and restraining fixtures provides mechanical constraint against deformation. A rigid backing plate placed against the uncladded side of the tube sheet prevents bulging and warping. The backing plate should be in intimate contact with the tube sheet surface and should be clamped or bolted to prevent relative movement during welding.

Preheat and Interpass Temperature Control

Controlled preheating reduces the thermal gradient between the weld zone and the surrounding material, thereby reducing residual stresses and deformation. The preheat temperature should be sufficient to reduce the peak temperature differential without causing excessive grain growth or unwanted phase transformations. Interpass temperature control ensures that each successive layer is deposited at a consistent thermal condition, promoting uniform microstructure and minimizing incremental deformation.

Post-Weld Stress Relief and Machining

Post-weld stress relief annealing at 550–650 °C for carbon steel tube sheets (or appropriate temperatures for stainless steel) reduces residual stresses and allows elastic recovery of some deformation. However, stress relief alone may not fully restore dimensional accuracy, and final machining of the gasket surface and tube holes is often required to achieve the specified tolerances.

Process Selection Considerations

The choice of cladding process also influences deformation magnitude. Strip cladding and electroslag welding (ESW) overlay, which involve lower heat inputs per unit length compared to multi-pass arc welding, generally produce less deformation. However, these processes may have limitations in terms of applicable alloy systems and surface quality.

Engineering Case Study and Practical Results

In the context of Shanxi Yangmei Chemical Machinery's production experience, the implementation of these deformation control methods has resulted in significant improvements in tube sheet quality. By adopting a symmetric welding sequence with controlled preheat and interpass temperatures, combined with the use of backing plates and post-weld stress relief, the out-of-flatness of large tube sheets has been reduced to within acceptable tolerances (typically ≤ 1.0 mm for tube sheets up to 1500 mm in diameter).

The practical implementation requires careful planning and execution. The welding procedure specification (WPS) must include detailed instructions for the welding sequence, and operators must be trained to follow the sequence precisely. The backing plate and restraint fixtures must be designed and manufactured to provide uniform contact and adequate clamping force. Non-destructive testing of the cladding layer — including dye penetrant testing (PT) for surface defects, ultrasonic testing (UT) for subsurface defects, and hardness testing for microstructural verification — should be performed after stress relief and machining to ensure that the final product meets all quality requirements.

Study Insights and Outlook

This study addresses a practical and persistent challenge in the fabrication of clad tube sheets for chemical and petrochemical heat exchangers. The proposed deformation control methods — welding sequence optimization, backing plate use, thermal control, and post-weld treatment — represent a systematic approach that can be adapted to different tube sheet geometries and sizes. The key insight is that deformation control is not a single measure but a combination of process planning, fixture design, parameter control, and post-weld treatment that must be integrated into the overall fabrication workflow.

For future engineering practice, the study suggests that further investigation into low-heat-input cladding processes such as plasma transferred arc (PTA) overlay and laser cladding could offer additional advantages in deformation control for large tube sheets. Additionally, the development of predictive models for cladding-induced deformation, based on finite element analysis and experimental validation, could enable more precise planning and real-time monitoring of the welding process. The ultimate goal is to achieve cladding operations that produce tube sheets with minimal deformation, reducing the need for extensive post-weld machining and stress relief, and thereby improving manufacturing efficiency and product quality.