CLADDING TECHNOLOGY SHANXI CO., LTDStrip Cladding Technology for Large Diameter Tube Sheets of 4000 mm
Literature Overview
This 2011 technical paper published in China Chemical Equipment documents the application of strip cladding (electroslag welding overlay) technology for manufacturing large-diameter tube sheets with a diameter of 4000 mm. Authored by Guan Yunsheng from Daqing Oilfield Construction Group Building Materials Company, this work addresses the practical challenges of applying strip cladding technology to very large diameter components where thermal distortion control, geometric accuracy, and metallurgical quality become increasingly difficult to achieve simultaneously.
Technical Background and Component Requirements
Large tube sheets are critical components in heat exchangers used in petroleum refining and chemical processing. A tube sheet with a diameter of 4000 mm represents a substantial engineering challenge due to:
- Material requirements: The base plate is typically low-alloy steel (e.g., 16MnR, 18MnMoNbR per GB 1591) for structural strength, while the tube-side surface requires stainless steel (typically 304 or 316L) for corrosion resistance against process fluids.
- Dimensional tolerances: Flatness tolerance of ≤ 1 mm/m and overall flatness ≤ 2 mm for a 4000 mm diameter component.
- Thickness requirements: Typical total thickness of 60-100 mm including the cladding layer (6-12 mm stainless steel overlay).
- Mechanical properties: The cladding layer must maintain adequate toughness while providing corrosion resistance, and the bond strength between layers must exceed specified minimum values.
Strip Cladding Process Description
The electroslag welding (ESW) strip cladding process is the preferred method for large tube sheets due to its high deposition rate and excellent metallurgical quality. The process involves:
Process Parameters:
| Parameter | Specification | Notes |
|---|---|---|
| Welding process | ESW strip cladding | High deposition rate, low dilution |
| Electrode type | Consumable electrode (stainless steel strip) | ER308L or ER316L equivalent |
| Electrode width | 25-40 mm | Matched to cladding layer width |
| Electrode thickness | 4-6 mm | Single pass or multi-pass |
| Welding current | 800-1200 A | DC, electrode positive |
| Arc voltage | 30-45 V | Controls slag pool fluidity |
| Travel speed | 200-400 mm/min | Inverse relationship with current |
| Flux composition | Basic flux with deoxidizers | Controls slag chemistry |
| Preheat temperature | 150-250 °C | Reduces cracking risk |
| Interpass temperature | 200-350 °C | Maintains thermal gradient |
Process Sequence:
- Base plate preparation: The base plate surface is ground clean to remove scale, oil, and contaminants. A transition layer may be deposited using a compatible filler (e.g., E309L) to bridge the metallurgical gap between the carbon steel base and the austenitic stainless overlay.
- First layer deposition: The first strip cladding pass establishes the bond between the base material and the overlay. This pass typically uses a higher current and slower travel speed to ensure complete fusion with the base metal.
- Subsequent layers: Additional cladding passes are deposited with progressively refined parameters to achieve the final composition and microstructure. Each subsequent layer dilutes the previous layer, gradually approaching the target composition.
- Final dressing: The final surface layer may be deposited using a slightly different composition or process parameters to optimize surface quality and corrosion resistance.
Challenges Specific to Large Diameter Components
The 4000 mm diameter presents unique challenges that distinguish this application from smaller tube sheets:
1. Thermal Distortion Control
The large plate area creates significant differential thermal expansion during welding. Without proper thermal management, the plate can develop bowing, warping, or angular distortion that exceeds dimensional tolerances. Countermeasures include:
- Symmetric welding sequence (starting from center and working outward in a spiral or zigzag pattern)
- Use of backing plates and clamping fixtures to restrain movement
- Controlled preheating to reduce thermal gradient
- Interpass temperature monitoring to prevent excessive heat accumulation
2. Weld Quality Maintenance Across Large Area
Maintaining consistent weld quality over the entire 4000 mm diameter requires:
- Rigorous consumable lot control and traceability
- Welder certification and periodic performance verification
- Automated or semi-automated welding equipment with parameter monitoring
- In-process inspection at regular intervals
3. Metallurgical Uniformity
The composition and microstructure of the cladding layer must be uniform across the entire surface to ensure consistent corrosion resistance. Variations in dilution rate across the large area can result in localized regions of substandard composition. This is managed through:
- Multi-layer deposition with composition convergence
- Regular metallographic sampling from different locations
- Chemical analysis of coupons taken from various positions
Inspection and Quality Control
Given the criticality of tube sheets in heat exchanger service, comprehensive inspection is mandatory:
| Inspection Method | Coverage | Acceptance Criteria |
|---|---|---|
| Visual testing (VT) | 100% | No surface defects, uniform profile |
| Penetrant testing (PT) | 100% | No linear indications |
| Ultrasonic testing (UT) | 100% of bond line | No lack of fusion, no delamination |
| Magnetic particle testing (MT) | 100% of base metal surface | No cracks in HAZ |
| Hardness testing | Grid pattern across surface | 150-250 HV for 304/316L overlay |
| Bond strength test | Coupon specimens | Tensile strength ≥ base metal |
| Chemical analysis | Multiple locations | Within specification limits |
| Intergranular corrosion test | Coupon specimens | Per ASTM A262 Practice E |
Engineering Practice Integration
From the perspective of engineering practice, the successful application of strip cladding to 4000 mm tube sheets demonstrates several important principles:
- Process scalability: Strip cladding technology, when properly designed and controlled, can be scaled to very large components without fundamental changes to the metallurgical approach.
- Equipment capability: The welding equipment must be capable of maintaining stable parameters over long weld lengths. Automated or semi-automated systems with consumable feeding, wire straightening, and parameter monitoring are essential for consistent quality.
- Thermal management philosophy: The approach to thermal control must shift from local weld management to global plate thermal management, incorporating preheating, interpass temperature control, and post-weld thermal treatment as integral parts of the process design.
- Cost-benefit analysis: For large tube sheets, the strip cladding approach offers significant cost advantages over through-thickness clad plate, as only the corrosion-resistant surface layer requires the expensive alloy material.
Summary and Technical Reflections
This technical documentation provides valuable insight into the practical challenges and solutions associated with applying strip cladding technology to very large diameter tube sheets. The successful execution of such projects requires careful integration of metallurgical design, process engineering, thermal management, and quality control into a cohesive manufacturing approach. For engineers involved in the design and fabrication of large heat exchangers, this work demonstrates that strip cladding remains a viable and economical alternative to through-thickness cladding, provided that the specific challenges of large-area fabrication are addressed through systematic process design and rigorous quality management.