Manufacturing Technology of Large-Diameter Porous Double-Sided Overlay Tubesheet

Literature Overview

This study by Wang Han, Jia Xiaobin, Wang Jinxia, Zheng Weixin, Wang Zhigang, and Zhou Caiyun from Lanzhou Lanchi Heavy Equipment Co., Ltd. and the Gansu Provincial Key Laboratory of Special Materials Welding for Pressure Vessels, published in the journal of China Chemical Equipment in 2019, addresses the manufacturing challenges of large-diameter tubesheets with double-sided overlay (cladding) and porous structure. The research is particularly relevant to the fabrication of heat exchangers and reactors used in petrochemical and refining industries where corrosion resistance on both sides of the tubesheet is required.

Background and Technical Significance

Large-diameter tubesheets (typically exceeding 1000 mm in diameter) with double-sided overlay present unique manufacturing challenges due to the combination of factors: large size, complex geometry (numerous tube holes), requirement for corrosion-resistant overlay on both sides, and the need to maintain structural integrity for pressure containment. The porous structure refers to the tube holes that penetrate the tubesheet, creating a complex stress state during welding and post-weld heat treatment.

The primary applications for such components include:

• Large-scale heat exchangers in refinery and petrochemical units
• Hydrogenation reactor tubesheets
• Crude oil distillation column tubesheets
• Acid gas cooler tubesheets in gas processing plants

The overlay material is typically a nickel-based alloy (Inconel 625, Monel 400) or austenitic stainless steel (316L, 321), while the base material is a low-alloy steel (1.25Cr-0.5Mo, 2.25Cr-1Mo, 9Cr-1Mo).

Manufacturing Process Challenges

The fabrication of large-diameter porous double-sided overlay tubesheets involves several critical process steps, each presenting unique challenges:

Overlay Welding Procedure Development

The welding procedure for double-sided overlay of a large-diameter tubesheet requires careful consideration of the following factors:

First Side Overlay:

• The first side overlay is applied to the as-rolled surface of the base plate.
• Multi-pass welding is required to achieve the specified overlay thickness (typically 6-12 mm).
• The first pass (root pass) requires careful control of dilution from the base metal to ensure adequate corrosion resistance in the overlay.
• Heat input should be controlled to minimize the risk of hydrogen-induced cracking in the base metal HAZ, particularly for Cr-Mo steels.

Tube Hole Drilling:

• After the first side overlay, tube holes are drilled through the overlay and base metal.
• The drilling process must avoid damaging the overlay layer, particularly at the hole edges.
• A pilot hole is drilled, followed by gradual enlargement to the final diameter.
• The surface finish at the tube hole edge should be Ra 6.3 micrometers or better to prevent stress concentration.

Second Side Overlay:

• The second side overlay is applied to the drilled surface, requiring welding around each tube hole.
• The heat input must be carefully controlled to avoid thermal cracking of the first side overlay.
• A welding sequence that minimizes distortion is critical, typically using a symmetrical pattern starting from the center or edges.
• The interpass temperature must be maintained below 250°C to prevent sensitization of the overlay.

Residual Stress and Distortion Control

Residual stress and distortion are the primary concerns in the fabrication of large-diameter tubesheets. The following strategies are employed:

1. Symmetrical welding sequence: Welding is performed in a symmetrical pattern to minimize net bending moments.
2. Low heat input: Heat input is kept within the range of 15-25 kJ/mm to minimize the thermal gradient and associated residual stresses.
3. Interpass temperature control: The interpass temperature is maintained below 250°C to limit the size of the heat-affected zone and reduce the risk of hydrogen-induced cracking.
4. Post-weld stress relief: After the second side overlay, the tubesheet undergoes stress relief at the temperature specified for the base metal (typically 590-650°C for low-alloy steels).
5. Final machining: A machining allowance of 3-5 mm is provided to correct any residual distortion after stress relief.

Quality Control and Inspection

The inspection program for double-sided overlay tubesheets is comprehensive and includes:

• Visual inspection: 100% inspection of all overlay weld surfaces for cracks, undercuts, and other surface defects.
• Dye penetrant testing (PT): 100% inspection of all overlay weld surfaces to detect surface-breaking defects.
• Magnetic particle testing (MT): 100% inspection of all overlay weld surfaces (if ferromagnetic) to detect surface and near-surface defects.
• Ultrasonic testing (UT): 100% inspection of the overlay/base metal interface to verify bond quality and detect lack of fusion or delamination.
• Radiographic testing (RT): 10-20% of the overlay weld volume, depending on the criticality of the application.
• Hardness testing: Verification of overlay hardness within the specified range (typically 180-260 HV for Inconel 625).
• Chemical analysis: Spectroscopic analysis of the overlay surface to verify composition and detect excessive dilution.
• Intergranular corrosion testing: ASTM A262 Practice E or A to verify the corrosion resistance of the overlay.

Engineering Practice Implications

The successful fabrication of large-diameter porous double-sided overlay tubesheets requires integration of multiple disciplines: welding engineering, materials science, mechanical design, and quality assurance. The following practical recommendations emerge from this study:

• Pre-fabrication planning: A detailed fabrication plan should be developed before starting production, including the welding sequence, inspection plan, and contingency measures for potential problems.
• Procedure qualification: The welding procedure should be qualified per NB/T 47014 or ASME IX, with specific attention to the effects of tube holes on weld quality and residual stress.
• Welder training: Welders should receive specific training for the unique challenges of overlay welding on perforated plates, including welding around tube holes and maintaining consistent weld quality.
• Equipment capability: The welding equipment should be capable of precise heat input control, including automatic or semi-automatic welding systems with current and voltage monitoring.

Key Questions and Reflections

The most challenging aspect of this fabrication is the interaction between the tube holes and the overlay welds. Each tube hole creates a stress concentration point that can initiate cracking under the residual stresses from welding. The welding sequence must be carefully designed to minimize the residual stress at these critical locations.

Another important consideration is the long-term performance of the overlay at the tube hole edges. The stress concentration at the tube hole edge, combined with the cyclic thermal loading during service, can lead to fatigue cracking of the overlay. This is particularly relevant for heat exchangers operating under thermal cycling conditions.

The study also highlights the importance of the post-weld heat treatment in relieving residual stresses while maintaining the corrosion resistance of the overlay. As discussed in the companion study on Inconel 625 overlay heat treatment, the PWHT temperature must be carefully selected to balance stress relief and sensitization risk.

Study Insights and Practical Recommendations

The fundamental insight from this research is that the fabrication of large-diameter porous double-sided overlay tubesheets is a complex, multi-step process that requires careful planning, precise execution, and comprehensive quality control. The success of the fabrication depends on the integration of welding procedure development, residual stress control, distortion management, and quality inspection.

For production implementation, I recommend adopting a systematic approach that includes: first, developing and qualifying the welding procedure with full consideration of the tube hole geometry; second, implementing a detailed welding sequence that minimizes residual stress and distortion; third, performing comprehensive non-destructive testing at each critical stage; and fourth, maintaining detailed records of all process parameters for traceability and future reference.

The experience gained from this type of fabrication is directly transferable to other large-scale overlay welding applications, including reactor shells, heat exchanger channels, and large-diameter piping spools. Engineers who master the challenges of double-sided overlay on perforated plates will be well-equipped to tackle similar challenges in other complex pressure vessel fabrication projects.