Large-Diameter Tube Sheet Weld Overlay Manufacturing Technology for Chemical Equipment

Literature Overview

This 2016 publication by Zheng Weixin, Xu Wenkui, Ning Xingsheng, Duo Yuanchai, and Wang Zhigang from Lanzhou Lansi Heavy Equipment Co., Ltd. addresses the manufacturing challenges of large-diameter tube sheets requiring weld overlay cladding for corrosion resistance. Large tube sheets are critical components in shell-and-tube heat exchangers used in petrochemical, refining, and chemical processing industries. They must withstand high internal pressures while resisting corrosion from process media such as sour gas, hydrochloric acid, and various organic solvents. The tube sheet diameter can exceed 3000 mm, and the thickness can range from 50 to 200 mm, presenting significant challenges for achieving uniform, defect-free overlay coverage across the entire surface.

Core Technical Challenges

The primary challenges in large-diameter tube sheet overlay manufacturing include:

1. Geometric complexity: Tube sheets contain thousands of tube holes, which create stress concentration sites and complicate the weld overlay path planning.
2. Thermal distortion control: The large mass and thickness of tube sheets create differential cooling rates between the overlay region and the bulk material, leading to significant warpage.
3. Overlay uniformity: Maintaining consistent overlay thickness across a large diameter while avoiding burn-through at tube hole edges requires precise process control.
4. Bond strength assurance: The overlay must maintain metallurgical bonding across the entire surface without delamination, which is particularly challenging at the edges of tube holes and at the outer periphery.

The following table presents the typical process parameters for large tube sheet overlay:

Process Strategy and Implementation

The authors adopted a multi-pass submerged arc welding (SAW) strategy with a single-layer multi-pass approach for the first layer and a multi-layer multi-pass approach for subsequent layers. The first layer, known as the transition layer, is critical because it establishes the metallurgical bond between the carbon steel base and the stainless steel overlay. To minimize carbon dilution from the base metal, the first pass is applied at a high travel speed with low heat input, resulting in a thin first layer with minimal base metal melting. The subsequent overlay layers are deposited at lower travel speeds to build up the required thickness while maintaining a low carbon content in the overlay.

The welding sequence is carefully planned to minimize thermal distortion. A spiral or circumferential welding pattern starting from the center and progressing outward is typically employed. This approach ensures that the residual stresses from each successive weld are partially relieved by the thermal expansion of the previously deposited metal. For tube sheets with a diameter exceeding 3000 mm, intermediate cooling periods of 30-60 minutes are introduced between weld sections to allow stress relaxation.

Tube hole protection is a critical aspect of the process. Each tube hole is sealed with a removable plug or covered with a heat-resistant ceramic cap before overlay welding begins. After the overlay is completed, the plugs are removed, and the tube hole edges are ground flush with the overlay surface. The grinding must be performed carefully to avoid damaging the overlay layer or exposing the base metal at the tube hole rim.

Heat Treatment and Stress Relief

Post-weld heat treatment (PWHT) is mandatory for large tube sheet overlays to relieve residual stresses and improve the metallurgical properties of the weld zone. The PWHT temperature is typically set at 620-680 °C for a duration of 2 hours per 25 mm of thickness, with a minimum of 4 hours for the entire component. The heating and cooling rates are limited to 150 °C/hour during the temperature range of 300-600 °C to prevent thermal shock cracking.

The following table compares the mechanical properties before and after PWHT:

Quality Assurance and Inspection Protocol

The quality assurance program for large tube sheet overlays follows a comprehensive inspection regime:

• Visual inspection (VT): 100% examination of the entire overlay surface for surface defects, porosity, undercut, and incomplete coverage.
• Magnetic particle testing (MT): 100% examination of the overlay surface and tube hole edges for surface and near-surface cracks.
• Ultrasonic testing (UT): 100% examination of the overlay-to-base metal interface for lack of fusion and delamination. The UT technique employs a dual-probe contact method with a frequency of 2-5 MHz to detect planar defects at the interface.
• Dye penetrant testing (PT): 100% examination of tube hole edges after grinding for cracks introduced during the grinding process.
• Hardness testing: Grid-pattern testing across the overlay surface to verify hardness uniformity and identify any areas of excessive dilution.
• Intergranular corrosion testing: A representative coupon from the overlay layer is subjected to ASTM A262 Practice A (5% citric acid, 60 °C, 24 hours) to verify resistance to sensitization cracking.

Engineering Practice and Case Study

In a practical application, a heat exchanger tube sheet with a diameter of 3200 mm and a thickness of 120 mm was overlaid with 316L stainless steel using the SAW process. The tube sheet contained 2,400 tube holes with a diameter of 25 mm. The overlay consisted of 4 passes with a total thickness of 5 mm. The total welding time was approximately 48 hours, including preheating, welding, cooling, and PWHT. The overlay achieved a carbon content of less than 0.03% in the top layer, well below the 0.08% threshold specified in NB/T 47002. Post-PWHT hardness values ranged from 185 to 210 HV across the entire surface, demonstrating excellent uniformity.

The key lesson from this case is that successful large tube sheet overlay requires not only skilled welders and proper equipment but also a systematic approach to thermal management, sequence planning, and quality verification. The investment in a comprehensive PWHT cycle and thorough non-destructive testing is justified by the catastrophic consequences of overlay failure in service, which can lead to tube leak, process contamination, and safety incidents.

Study Insights and Outlook

This publication provides a valuable engineering reference for the fabrication of large-diameter overlaid tube sheets in the chemical and petrochemical industries. The systematic treatment of process parameters, sequence planning, and quality assurance reflects the maturity of Chinese heavy equipment manufacturing capabilities. The technology described here aligns with international standards such as ASME VIII Div.1, EN 10028-7, and ASTM A263/A264, ensuring that components manufactured using this approach can be certified for international markets.

The future development of this technology should focus on further reducing thermal distortion through advanced simulation-guided welding sequences, exploring the use of hybrid processes such as submerged arc plus gas metal arc for improved first-layer quality, and developing automated inspection systems capable of real-time monitoring of overlay bond quality during production. These advancements will further enhance the reliability and efficiency of large tube sheet manufacturing for next-generation chemical processing equipment.