Large Diameter Tube Sheet Cladding Manufacturing Technology

Technical Background and Challenges

The literature from Lanzhou Lanchi Heavy Equipment Co., Ltd., published in 2016, addresses the manufacturing technology for cladding large diameter tube sheets, which are critical components in heat exchangers, reactors, and other pressure vessels used in the chemical and petrochemical industries. Large tube sheets, typically with diameters exceeding 1500 mm and often reaching 3000 mm or more, present unique manufacturing challenges related to distortion control, weld quality, and dimensional accuracy. The cladding of these components is typically performed to provide a corrosion-resistant surface on one or both faces while maintaining the structural integrity provided by a carbon steel or low-alloy steel base.

The primary challenge in large tube sheet cladding is the control of welding distortion. As the cladding welds are deposited, differential thermal expansion between the cladding layer and the base material creates residual stresses that can cause warping, bowing, and out-of-plane distortion. For large diameters, even small angular distortions can result in significant deviations from flatness, which can compromise the sealability of the tube sheet in a heat exchanger shell and make tube installation difficult.

Manufacturing Process and Technology

The manufacturing process for cladded large tube sheets typically involves several stages: base tube sheet fabrication, surface preparation, cladding welding, stress relief, and final machining. The cladding welding process itself can be performed using various methods depending on the required cladding thickness and material combination.

For thick cladding layers (greater than 3 mm), submerged arc welding (SAW) is commonly used due to its high deposition rate and good weld quality. For thinner cladding layers (1–3 mm), plasma transferred arc (PTA) welding or gas metal arc welding (GMAW) may be preferred for their precision and lower heat input. The choice of process is also influenced by the material combination; for example, welding nickel-based alloys on carbon steel typically requires lower heat input processes to minimize dilution and avoid cracking.

The welding sequence for large tube sheets is critical to distortion control. A common approach is to weld in a spiral pattern from the center outward, or to use a segmented pattern that alternates between different quadrants of the tube sheet. The goal is to ensure that the thermal expansion forces are balanced and do not create net moments that would cause warping. The welding sequence is typically determined through finite element analysis (FEA) of the thermal and residual stress fields, with experimental validation on test plates.

Preheating is an important aspect of the process, particularly for thick tube sheets and when welding high-carbon-equivalent base materials. Preheating reduces the cooling rate and minimizes the risk of hydrogen-induced cracking in the heat-affected zone (HAZ). Typical preheat temperatures range from 100°C to 200°C depending on the base material carbon equivalent and the cladding material. The preheat must be maintained throughout the welding sequence, which for large tube sheets requires significant heating capacity and temperature monitoring.

Distortion Control Strategies

Distortion control in large tube sheet cladding is achieved through a combination of design, process, and mechanical methods. The following table summarizes the primary strategies.

The use of a backing plate is one of the most effective methods for controlling distortion. The tube sheet is tack-welded to a flat backing plate before cladding, and the cladding is applied to the opposite face. The backing plate provides a rigid constraint that prevents the tube sheet from warping during welding. After cladding is complete, the tube sheet is separated from the backing plate, and any residual distortion is corrected through machining or thermal straightening.

Post-weld stress relief is another critical step. For carbon steel tube sheets with stainless steel cladding, a stress relief treatment at 550–650°C is typically performed to reduce residual stresses and minimize the risk of stress corrosion cracking in the cladding layer. However, the stress relief temperature must be carefully controlled to avoid sensitization of the stainless steel cladding, which can lead to intergranular corrosion. For this reason, the stress relief temperature is often limited to 620°C or below, and the hold time is minimized.

Material Selection and Interface Considerations

The material combination for tube sheet cladding is determined by the service conditions, including the corrosive medium, operating temperature, and pressure. Common combinations include carbon steel with 304 or 316L stainless steel cladding, low-alloy steel with Inconel 625 cladding, and carbon steel with nickel-based alloy cladding. The dilution at the interface between the cladding and base metal is a critical factor that affects the corrosion resistance and mechanical properties of the overlay.

For stainless steel cladding on carbon steel, the dilution can introduce carbon and manganese into the overlay, which can promote the formation of chromium carbides and reduce the pitting resistance of the cladding layer. To mitigate this, a transition layer of high-nickel material such as 309L stainless steel may be applied as the first pass, followed by the desired cladding material in subsequent passes. This approach reduces the dilution effect and provides a more corrosion-resistant interface.

The hardness profile across the cladding layer is another important quality indicator. A uniform hardness distribution indicates good metallurgical homogeneity, while hardness gradients or hard spots can indicate segregation, phase transformation, or excessive dilution. Hardness measurements are typically performed on a cross-section of the cladding layer, from the base metal through the interface to the surface of the overlay.

Quality Assurance and Inspection Requirements

The quality assurance program for cladded large tube sheets is comprehensive and includes the following elements:

The ultrasonic testing for bond strength verification is particularly important for large tube sheets because any lack of bond can lead to delamination during service, which is a catastrophic failure mode. The UT technique must be capable of detecting small areas of lack of bond across the entire surface area of the tube sheet, which for large diameters can exceed several square meters.

Study Insights and Engineering Implications

This work demonstrates the complexity of manufacturing large cladded components and the importance of integrating design, process, and quality considerations. The key insight is that distortion control is not a single technique but a system of complementary strategies that must be applied in combination. No single method is sufficient for large tube sheets; rather, the welding sequence, constraint methods, thermal management, and post-weld correction must all be optimized together.

The economic implications of this technology are significant. Large tube sheets are expensive components, and the cost of cladding adds substantially to the total manufacturing cost. However, the use of cladding to provide corrosion resistance extends the service life of the component and reduces the need for replacement, resulting in lower lifecycle costs. The ability to manufacture large cladded tube sheets in-house, rather than sourcing them from specialized manufacturers, provides significant supply chain advantages.

For engineers working on pressure vessel and heat exchanger design, this work highlights the importance of considering manufacturing feasibility during the design phase. The cladding material, thickness, and process must be specified with sufficient detail to allow the manufacturer to achieve the required quality. Ambiguous specifications can lead to disputes, rework, and ultimately to components that do not meet the intended performance requirements.