Strip Cladding Technology for 4000mm Large Diameter Tube Sheets

Literature Overview and Background

This study note examines the strip cladding (electroslag welding overlay) technology applied to large-diameter tube sheets with a nominal diameter of 4000 mm. Tube sheets are critical components in heat exchangers, reactors, and other pressure vessels, and they are often required to have a corrosion-resistant surface to withstand the process fluid. For large-diameter tube sheets, conventional weld overlay methods such as submerged arc welding (SAW) or gas metal arc welding (GMAW) face significant challenges due to the large surface area, geometric constraints, and the need for uniform cladding thickness. Strip cladding, also known as electroslag welding (ESW) overlay, offers a viable solution by enabling high deposition rates, deep penetration, and uniform cladding over large areas.

The literature under review focuses on the specific challenges and solutions associated with strip cladding of 4000 mm diameter tube sheets, including the selection of cladding material, welding procedure development, distortion control, and quality assurance. The study provides valuable insights into the practical implementation of strip cladding for large-scale pressure vessel components.

Core Technical Points

Principles of Strip Cladding

Strip cladding is a specialized form of electroslag welding (ESW) in which a continuous strip of cladding material is fed into a slag pool formed on the base material surface. The heat source is generated by the electrical resistance of the molten slag, which melts the strip and the base material simultaneously. The molten metal and slag are then removed from the back of the strip by a cooling plate or a mechanical device, leaving a uniform cladding layer on the front surface. The key advantages of strip cladding include:

• High deposition rate (typically 5–15 kg/h, depending on the strip thickness and welding parameters)
• Deep penetration (typically 3–8 mm per pass, depending on the base material thickness)
• Uniform cladding thickness (typically controlled within ±0.5 mm)
• Low dilution rate (typically 5–15%, depending on the welding parameters)
• Ability to clad large areas without the need for multiple passes

Material Selection for Tube Sheet Cladding

The selection of cladding material for a tube sheet depends on the process fluid, operating conditions, and the base material. For hydrogenation reactors and heat exchangers exposed to H2S-containing environments, common cladding materials include:

• Austenitic stainless steels (e.g., 304L, 316L, 321, 347): These materials provide good corrosion resistance and are widely used for tube sheet cladding. The low-carbon grades (304L, 316L) are preferred to minimize the risk of sensitization.
• Nickel-based alloys (e.g., Inconel 625, Hastelloy C-276): These materials offer superior corrosion resistance in aggressive environments but are significantly more expensive. They are typically used for high-temperature, high-pressure applications.
• Duplex stainless steels (e.g., 2205): These materials combine the corrosion resistance of austenitic stainless steels with the strength of duplex steels, making them suitable for applications requiring both properties.

The base material for the tube sheet is typically a carbon or low-alloy steel, such as SA-266 Gr.2 or SA-516 Gr.70. The metallurgical compatibility between the base material and the cladding material is a critical consideration, as it affects the dilution rate, the properties of the cladding layer, and the risk of cracking.

Welding Procedure Development

The development of a welding procedure for strip cladding of a 4000 mm diameter tube sheet requires careful consideration of several factors:

1. Preheat temperature: The preheat temperature is selected to reduce the risk of cracking in the base material and the cladding layer. For carbon and low-alloy steels, a preheat temperature of 100–200°C is typically used, depending on the carbon equivalent and the thickness of the base material.
2. Interpass temperature: The interpass temperature is controlled to maintain the desired microstructure in the cladding layer and to prevent overheating. For austenitic stainless steel cladding, the interpass temperature should be kept below 150°C to minimize sensitization.
3. Welding sequence: The welding sequence is critical for controlling distortion in a large-diameter tube sheet. The literature recommends a spiral or zigzag welding sequence that starts from the center of the tube sheet and progresses outward, or vice versa. The sequence should be designed to minimize the cumulative effect of thermal stresses and to maintain the flatness of the tube sheet within acceptable tolerances.
4. Backing and support: The tube sheet must be adequately supported to prevent distortion during welding. The literature recommends the use of a rigid backing plate or a custom fixture that provides uniform support across the entire surface of the tube sheet.

Distortion Control

Distortion is a major challenge in strip cladding of large-diameter tube sheets. The thermal expansion and contraction of the base material during welding can cause significant warping, which may exceed the flatness tolerance specified by the applicable code (e.g., ASME VIII Div.1 requires a flatness tolerance of 1/1000 of the diameter, but not more than 6 mm for a 4000 mm diameter tube sheet). The literature discusses several strategies for controlling distortion:

• Symmetric welding sequence: Welding in a symmetric pattern that alternates between opposite sides of the tube sheet to balance thermal stresses.
• Mechanical clamping: Using clamps or fixtures to constrain the tube sheet during welding and prevent warping.
• Post-weld straightening: If distortion exceeds the tolerance, the tube sheet can be straightened by mechanical or thermal means. However, this should be avoided if possible, as it may introduce residual stresses and compromise the integrity of the cladding layer.
• Multi-pass welding: Using multiple thin passes instead of a single thick pass to reduce the heat input per pass and minimize distortion.

Process and Standards Analysis

The fabrication of a strip-clad tube sheet must comply with the applicable codes and standards, which provide the framework for material selection, welding procedure qualification, inspection, and acceptance criteria.

Applicable Codes and Standards

Welding Procedure Qualification

The welding procedure for strip cladding must be qualified per ASME Section IX or the equivalent national standard. The qualification includes:

• Essential variables: These include the base material, cladding material, welding process, preheat temperature, interpass temperature, and welding parameters (current, voltage, speed). Changes to essential variables require requalification.
• Non-essential variables: These include the welding position, the number of passes, and the backing material. Changes to non-essential variables do not require requalification but must be documented.
• Performance qualifications: For strip cladding, performance qualifications may include bond strength tests, corrosion tests, and mechanical property tests to verify the integrity of the cladding layer.

Inspection and Acceptance Criteria

The inspection of a strip-clad tube sheet includes:

• Visual inspection (VT): To check for surface defects, such as porosity, cracks, and undercuts.
• Radiographic testing (RT) or ultrasonic testing (UT): To detect volumetric defects in the weld and the base material.
• Magnetic particle testing (MT): To detect surface-breaking defects in the cladding layer and the HAZ.
• Hardness testing: To verify that the hardness of the HAZ and the cladding layer are within the specified limits (typically ≤ 22 HRC for SSC resistance).
• Metallographic examination: To assess the microstructure of the cladding layer and the HAZ.
• Bond strength testing: To verify the bond strength between the cladding layer and the base material (typically ≥ 35 ksi or 240 MPa per ASME VIII Div.1).
• Corrosion testing: To verify the corrosion resistance of the cladding layer (e.g., intergranular corrosion test per ASTM A262 Practice E).

Integration with Engineering Practice

The application of strip cladding to 4000 mm diameter tube sheets has been successfully implemented in several engineering projects, including hydrogenation reactors, heat exchangers, and storage tanks. The literature provides several case studies that illustrate the practical implementation of this technology.

Case Study 1: Hydrogenation Reactor Tube Sheet

A hydrogenation reactor tube sheet with a diameter of 4000 mm and a thickness of 120 mm was clad with 6 mm of 316L stainless steel using strip cladding. The base material was SA-266 Gr.2, and the welding procedure was qualified per ASME IX. The welding sequence was a spiral pattern starting from the center and progressing outward, with a preheat temperature of 180°C and an interpass temperature of 200°C. The distortion was controlled to within 3 mm, well within the ASME tolerance of 6 mm. The cladding layer was inspected by RT, MT, and hardness testing, and all results were within the acceptance criteria. The tube sheet passed a hydrostatic test at 1.5 times the design pressure and was put into service without any issues.

Case Study 2: Heat Exchanger Tube Sheet

A heat exchanger tube sheet with a diameter of 4000 mm and a thickness of 100 mm was clad with 5 mm of Inconel 625 using strip cladding. The base material was SA-516 Gr.70, and the welding procedure was qualified per ASME IX. The welding sequence was a zigzag pattern, with a preheat temperature of 150°C and an interpass temperature of 200°C. The dilution rate was controlled to 10% by adjusting the welding speed and the slag flux ratio. The cladding layer was inspected by RT, UT, and metallographic examination, and the microstructure showed a uniform austenitic structure with no signs of sensitization. The tube sheet passed all inspections and was put into service.

FMEA Analysis

A failure mode and effects analysis (FMEA) was conducted for the strip cladding process of a 4000 mm diameter tube sheet. The following failure modes were identified:

Key Questions and Reflections

The study of strip cladding technology for 4000 mm diameter tube sheets raises several important questions:

1. How does the size of the tube sheet affect the welding procedure? The larger the tube sheet, the greater the challenge of controlling distortion and maintaining uniform cladding thickness. The welding sequence must be carefully designed to minimize the cumulative effect of thermal stresses, and the welding parameters must be optimized to ensure consistent deposition rates and penetration depths across the entire surface.
2. What is the impact of the base material thickness on the cladding process? The base material thickness affects the heat input, the dilution rate, and the risk of cracking. For thick base materials, a higher preheat temperature and a slower welding speed may be required to ensure adequate penetration and fusion. However, this also increases the risk of distortion and sensitization, which must be carefully managed.
3. How can the dilution rate be controlled? The dilution rate is a critical parameter that affects the composition and properties of the cladding layer. A high dilution rate can lead to a cladding layer with insufficient corrosion resistance, while a low dilution rate can result in poor bond strength. The dilution rate can be controlled by adjusting the welding speed, the slag flux ratio, and the strip thickness. The literature recommends using a combination of these parameters to achieve a dilution rate of 5–15%, which provides a good balance between corrosion resistance and bond strength.

Study Insights and Implications

The study of strip cladding technology for 4000 mm diameter tube sheets provides several valuable insights for engineering practice. First, it demonstrates that strip cladding is a viable and efficient method for cladding large-diameter tube sheets, with deposition rates that are significantly higher than those of conventional weld overlay methods. Second, it highlights the importance of careful planning and execution, particularly in terms of welding sequence, distortion control, and quality assurance. Third, it emphasizes the need for a thorough understanding of the metallurgical interactions between the base material and the cladding material, as these interactions determine the integrity and performance of the cladding layer.

The literature also points to several areas for future improvement. The development of new cladding materials with improved corrosion resistance and higher temperature capability could further enhance the performance of strip-clad tube sheets. The use of advanced monitoring and control systems, such as real-time thickness measurement and automated welding speed control, could improve the consistency and quality of the cladding layer. Finally, the development of new welding procedures that combine strip cladding with other overlay methods, such as laser cladding or plasma transferred arc (PTA) cladding, could provide additional flexibility and performance.

In conclusion, the study of strip cladding technology for 4000 mm diameter tube sheets is a complex and multifaceted topic that requires a deep understanding of welding metallurgy, materials science, and engineering practice. The engineer must carefully balance the economic, technical, and quality considerations to arrive at an optimal welding procedure that meets the operational requirements while ensuring long-term integrity. The key takeaway is that strip cladding is a powerful and versatile technology that can be effectively applied to large-diameter tube sheets, provided that the welding procedure is carefully designed and rigorously executed.