Zhuojin Pipe Fitting Co., LtdManufacturing Technology of Large-Diameter Multi-Hole Double-Sided Overlay Tube Sheets
Literature Overview
This paper by Wang Han, Jia Xiaobin, Wang Jinxia, Zheng Weixin, Wang Zhigang, and Zhou Caiyun from Lanzhou Lanchuan Heavy Equipment Co., Ltd. and the Gansu Provincial Key Laboratory of Special Materials Welding for Pressure Vessels, published in China Chemical Equipment (2019, Vol. 21, No. 6, pp. 9–13), presents the manufacturing technology for large-diameter, relatively thin 12Cr2Mo1 forged tube sheets with overlay welding and multiple hole drilling. The study addresses the significant deformation challenges associated with overlay welding and hole drilling on large-diameter, thin-section tube sheets.
Core Technical Challenges
Geometric Characteristics and Deformation Risk
The tube sheets described in this paper present unique manufacturing challenges due to their combination of large diameter and relatively thin section thickness. The key geometric parameters and their implications are:
| Parameter | Typical Value | Engineering Implication |
|---|---|---|
| Diameter | Large (typically > 2000 mm) | High aspect ratio, low bending stiffness |
| Section thickness | Relatively thin (D/t ratio high) | Low resistance to bending deformation |
| Base material | 12Cr2Mo1 forging | Creep-resistant, requires overlay for corrosion resistance |
| Overlay material | Austenitic stainless steel | Large thermal expansion mismatch with base |
| Hole pattern | Multiple holes (tens to hundreds) | Stress concentration, deformation sensitivity |
The fundamental challenge is that the thermal expansion mismatch between the 12Cr2Mo1 base material (a low-alloy creep-resistant steel with a thermal expansion coefficient of approximately 12.5 × 10⁻⁶/°C) and the austenitic stainless steel overlay (typically 17–18 × 10⁻⁶/°C) generates significant residual stresses during welding. On a large-diameter, thin-section component, these residual stresses manifest as macroscopic deformation in the form of warping, bowing, or local buckling.
Manufacturing Process Sequence Optimization
The authors propose a carefully optimized manufacturing sequence to minimize deformation:
- Forging and rough machining: Establish adequate machining allowance to accommodate post-weld deformation.
- Overlay welding: Apply overlay welds on both sides of the tube sheet using controlled welding parameters.
- Stress relief heat treatment: Reduce residual stresses before hole drilling.
- Hole drilling: Drill tube holes after stress relief to prevent stress concentration from triggering deformation.
- Final machining and inspection: Complete dimensional finishing and quality verification.
Anti-Deformation Measures
The study proposes multiple anti-deformation strategies:
| Measure | Method | Effectiveness |
|---|---|---|
| Backing plate support | Rigid backing plate during welding | High - prevents local deformation |
| Welding sequence optimization | Symmetric, balanced welding pattern | High - minimizes directional distortion |
| Interpass temperature control | Limited interpass temperature (≤ 200°C) | Medium - reduces thermal cycling |
| Welding speed optimization | Moderate to high travel speed | Medium - reduces heat input |
| Current clamping | Mechanical clamping during welding | High - constrains deformation |
| Post-weld straightening | Induction heating or mechanical correction | Medium - corrective rather than preventive |
Engineering Practice Integration
Welding Process Parameters
For 12Cr2Mo1 + austenitic stainless steel overlay welding, the following process parameters are recommended:
- Welding method: GTAW for root and first pass, followed by SMAW or FCAW for fill and cap passes
- Current type: DCEP (Direct Current Electrode Positive) for GTAW to promote deep penetration
- Shielding gas: Pure argon (GTAW) or CO₂/Ar mixture (FCAW)
- Travel speed: 80–150 mm/min depending on wire diameter and current
- Interpass temperature: ≤ 200°C for overlay layers, ≤ 150°C between overlay and base
- Heat input: 0.8–1.5 kJ/mm for overlay layers
Deformation Monitoring and Control
A systematic deformation monitoring approach is essential:
- Pre-weld baseline: Measure flatness and geometry before welding begins.
- In-process monitoring: Measure deformation after each welding sequence or weld zone.
- Post-weld assessment: Document final deformation and compare with specifications.
- Corrective action: Apply corrective measures if deformation exceeds tolerance.
Typical acceptance criteria for tube sheet flatness:
| Tube Sheet Diameter | Maximum Allowable Flatness Deviation |
|---|---|
| 2000–3000 mm | 1.5 mm over any 1000 mm span |
| 3000–4000 mm | 2.0 mm over any 1000 mm span |
| 4000–5000 mm | 2.5 mm over any 1000 mm span |
| > 5000 mm | 3.0 mm over any 1000 mm span |
Hole Drilling Considerations
Hole drilling on overlay-welded tube sheets requires special attention:
- Drilling sequence: Start from the center and work outward, or use a symmetric pattern, to prevent progressive deformation.
- Drilling speed: Moderate speed to avoid excessive heat generation that could affect the overlay weld.
- Coolant application: Adequate coolant to prevent thermal damage to the overlay weld.
- Tool selection: Carbide drills or solid carbide inserts for the hard overlay material.
- Hole size verification: Check hole size and position after drilling to detect any deformation-induced misalignment.
Study Insights and Reflections
This paper provides practical manufacturing guidance for a challenging and common component in pressure vessel fabrication. The emphasis on manufacturing sequence optimization and anti-deformation measures reflects the practical engineering approach of addressing problems at their source rather than relying solely on post-weld correction. The integration of multiple anti-deformation strategies—backing plates, welding sequence optimization, interpass temperature control, and mechanical clamping—demonstrates the systems engineering approach required for successful large-component fabrication.
One aspect that deserves further emphasis in practice is the importance of the machining allowance. Insufficient machining allowance is a common cause of tube sheet rejection, as deformation correction consumes material that was not budgeted. Engineers should ensure that the forging specification includes adequate allowance for post-weld and post-drilling machining operations. Additionally, the use of finite element analysis (FEA) for deformation prediction during the design phase would complement the empirical approach presented here, enabling more proactive deformation management. The work underscores the critical role of welding process knowledge in pressure vessel manufacturing, where geometric accuracy is essential for both functional performance and regulatory compliance.