Manufacturing Technology for Large-Diameter Multi-Hole Double-Sided Overlay Tubesheets

Literature Overview

This paper by Wang Han, Jia Xiaobin, Wang Jinxia, Zheng Weixin, Wang Zhigang, and Zhou Caiyun from Lanzhou Lanshi Heavy Equipment Co., Ltd. and the Gansu Provincial Key Laboratory for Special Materials Welding in Pressure Vessels, published in China Chemical Equipment (2019, Vol. 21, Issue 6, pp. 9-13), addresses the manufacturing challenges of large-diameter, relatively thin 12Cr2Mo1 forged tubesheets with double-sided overlay weld cladding and multiple tube holes.

Technical Challenge

The tubesheet described in this paper presents a unique combination of manufacturing difficulties:

• Large diameter: Increases the risk of distortion during welding and machining
• Relatively thin section: Reduces the structural rigidity, making the component more susceptible to deformation
• 12Cr2Mo1 base material: A low-alloy martensitic steel requiring careful heat treatment control
• Double-sided overlay: Cladding on both faces multiplies the thermal input and distortion risk
• Multiple tube holes: Machining thousands of precisely located holes after overlay welding introduces additional deformation concerns

Manufacturing Process Flow

Key Technical Measures

Machining Allowance Strategy

Adequate machining allowance is critical for distortion compensation:

• Overlay thickness + machining allowance: Typically 3-5 mm overlay + 2-3 mm machining allowance per side
• Flatness tolerance: The final tubesheet face must meet tight flatness requirements (typically 0.5-1.0 mm across the diameter)
• Allowance distribution: Must account for both welding distortion and thermal distortion from PWHT

Welding Method and Parameters

The selection of welding method is critical for minimizing distortion:

Deformation Prevention Measures

1. Rigid backing: Use of heavy backing plates or fixtures to constrain deformation during welding
2. Symmetrical welding: Welding both sides alternately in a balanced sequence to cancel distortion
3. Welding sequence optimization: Starting from the center and working outward, or using a spiral pattern
4. Thermal compensation: Pre-bending or thermal pre-distortion of the blank to compensate for expected welding distortion
5. Intermediate stress relief: Performing stress relief between overlay passes on opposite sides to reduce accumulated residual stresses

Machining Sequence

The machining sequence is critical to maintaining dimensional accuracy:

1. Machine one face to within 2-3 mm of final thickness
2. Weld overlay on the other face
3. Stress relieve
4. Machine both faces to final dimensions simultaneously (if possible)
5. Drill tube holes using a rigid drilling rig
6. Re-check flatness and hole positions

Quality Control Considerations

Engineering Practice Insights

From my experience with tubesheet manufacturing, this paper highlights several practical lessons:

1. Distortion is cumulative: Each welding operation adds to the total distortion. The manufacturing strategy must account for cumulative effects, not just individual weld distortion.
2. The thin section paradox: While thin tubesheets are lighter and cheaper to forge, they are significantly more difficult to manufacture with overlay welds because they have less inherent rigidity to resist distortion.
3. Machining allowance is not optional: Insufficient machining allowance is the most common cause of tubesheet rejection. The cost of additional machining time is far less than the cost of scrapping a component.
4. Welding sequence is a design decision: The welding sequence should be determined during the design phase, not left to the welding operator. It should be documented in the welding procedure specification (WPS).
5. Interference fit considerations: The tube holes must be machined to precise dimensions to achieve the required interference fit with tubes. Any distortion after hole machining will compromise the fit.

Critical Analysis

Several challenges remain unaddressed in this paper:

• Residual stress measurement: The paper does not discuss quantitative residual stress measurement (e.g., X-ray diffraction or hole drilling) to validate the effectiveness of the deformation prevention measures.
• Long-term service performance: The paper focuses on manufacturing but does not address the long-term performance of the overlay welds in service, particularly under thermal cycling.
• Cost analysis: A comparison of the cost of the proposed manufacturing approach versus alternative approaches (e.g., thicker tubesheet, different material selection) would strengthen the engineering justification.
• Digital simulation: Modern finite element analysis (FEA) can predict welding distortion before manufacturing. The paper does not discuss whether simulation was used to optimize the welding sequence.

Study Insights

This paper provides a practical manufacturing methodology for a challenging component that combines multiple technical difficulties. The systematic approach to deformation control—integrating machining allowance, welding sequence, stress relief, and machining order—represents the kind of holistic engineering thinking required for complex pressure vessel manufacturing. For engineers involved in tubesheet design and manufacturing, the key lesson is that deformation control must be planned from the beginning of the manufacturing process, not addressed as a corrective measure after distortion has occurred. The paper also underscores the importance of having adequate machining allowance as a fundamental design principle for components requiring overlay weld cladding.