CLADDING TECHNOLOGY SHANXI CO., LTDHigh-Speed Strip Electrode Overlay Welding Process and Equipment
Literature Overview
The 1991 paper by Li Chunxu, Wang Xijing, Wei Jikun, and Li Heqi from Gansu University of Technology (now Lanzhou University of Technology) addressed the development and characterization of a high-speed strip electrode overlay welding process and the associated equipment. Published in the Journal of Welding (Han Jie Xue Bao), one of the most respected welding research journals in China, this work represents a significant advancement in strip cladding technology during a period of rapid industrial growth in China. The research focused on improving the deposition rate, productivity, and quality consistency of strip electrode cladding processes, which are widely used for manufacturing bimetallic products and corrosion-resistant overlay layers on pressure vessels and heat exchangers.
Core Technical Content and Interpretation
Strip electrode overlay welding, also known as strip cladding or twin-wire submerged arc welding (SAW) overlay, is a highly productive process for depositing thick cladding layers on large structural components. The process involves feeding two strips of the cladding alloy material as the consumable electrodes, which are submerged beneath a layer of granular flux during welding. The high deposition rate, typically ranging from 5 to 15 kg/h, makes this process particularly suitable for industrial-scale cladding operations where large volumes of overlay material need to be deposited efficiently.
The key innovation described in this paper was the development of high-speed strip electrode welding equipment that could operate at significantly higher travel speeds than conventional strip cladding machines. The researchers addressed several critical engineering challenges associated with increasing the welding speed, including maintaining stable arc conditions, ensuring adequate flux coverage, controlling dilution from the base metal, and preventing defects such as undercut, porosity, and incomplete fusion at the high deposition rates.
Process Parameters and Equipment Configuration
The following table presents the typical process parameters and equipment characteristics for high-speed strip electrode overlay welding:
| Parameter | Conventional Process | High-Speed Process |
|---|---|---|
| Travel speed | 200 to 400 mm/min | 400 to 800 mm/min |
| Deposition rate | 3 to 8 kg/h | 8 to 15 kg/h |
| Welding current | 800 to 1500 A | 1000 to 2000 A |
| Arc voltage | 25 to 35 V | 28 to 40 V |
| Strip thickness | 3 to 5 mm | 2 to 4 mm |
| Strip width | 15 to 30 mm | 15 to 25 mm |
| Flux type | Rutile or basic granular flux | Optimized basic flux with controlled moisture |
| Flux preheating | 200 to 250 °C | 250 to 350 °C |
| Flux recycling | Yes | Enhanced with moisture monitoring |
| Shielding gas | None (flux-shielded) | Optional CO₂ or Ar backing gas |
Equipment Design and Engineering Challenges
The high-speed strip electrode welding equipment developed by the research team incorporated several innovative features to address the challenges of high-speed operation. The wire feed mechanism was designed with high precision and low backlash to ensure consistent strip feeding rates, which is critical for maintaining stable arc conditions at elevated travel speeds. The torch design incorporated a flux delivery system with adjustable nozzle geometry to optimize flux distribution and coverage, ensuring adequate protection of the molten pool even at high speeds.
One of the most significant challenges in high-speed strip cladding is the control of dilution from the base metal. At higher travel speeds, the heat input per unit length decreases, which can lead to increased dilution due to the reduced time available for the cladding alloy to mix with the base metal melt. The researchers addressed this challenge by optimizing the welding current density, arc voltage, and flux composition to achieve a balance between deposition rate and dilution control. The use of high-current, low-voltage welding parameters was found to be particularly effective for minimizing dilution while maintaining high deposition rates.
Quality Control and Defect Prevention
Maintaining weld quality at high speeds requires rigorous process control and monitoring. The research team emphasized the importance of flux moisture control, as excessive moisture content in recycled flux leads to hydrogen-induced porosity and cracking. The equipment incorporated a flux drying system with continuous moisture monitoring and automated flux replacement to ensure consistent flux quality throughout the welding operation.
The following table summarizes the common defects encountered in high-speed strip cladding and their countermeasures:
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Porosity | High flux moisture, inadequate arc shielding | Flux drying to below 0.5% moisture; ensure proper flux coverage |
| Undercut | Excessive arc voltage, high travel speed | Reduce arc voltage; optimize torch angle |
| Incomplete fusion | Insufficient heat input, excessive travel speed | Increase welding current; reduce travel speed |
| Cracking | High hydrogen content, rapid cooling | Preheat base metal; use low-hydrogen flux |
| Excessive dilution | High heat input, long arc length | Reduce arc length; optimize current and voltage |
| Surface irregularities | Uneven strip feeding, flux bridging | Calibrate wire feed; ensure uniform flux delivery |
Integration with Engineering Practice
High-speed strip electrode overlay welding is extensively used in the fabrication of bimetallic pressure vessels, heat exchangers, and storage tanks where corrosion-resistant cladding layers are required on large-diameter vessels. The process is particularly advantageous for cladding operations on cylindrical shells and heads of pressure vessels, where the large surface area and the need for thick cladding layers make conventional single-wire SAW overlay impractical.
In the context of pressure vessel fabrication governed by standards such as GB/T 150, NB/T 47002, and ASME VIII Div.1, the qualification of high-speed strip cladding procedures requires careful attention to several critical aspects. The procedure qualification test (PQT) must demonstrate that the welding parameters produce a sound cladding layer with adequate bond strength, acceptable dilution, and the required corrosion resistance. The non-destructive testing (NDT) requirements for strip cladding typically include ultrasonic testing (UT) for bond quality and magnetic particle testing (MT) or dye penetrant testing (PT) for surface defect detection.
The productivity advantages of high-speed strip cladding make it economically attractive for large-scale production, but the initial investment in specialized equipment and the need for skilled operators must be considered. The equipment developed by the research team represents a step toward making high-speed strip cladding more accessible to Chinese manufacturers, reducing the reliance on imported equipment and enabling domestic development of high-quality bimetallic products.
Study Insights and Implications
The 1991 research by Li Chunxu and colleagues is historically significant as it contributed to the maturation of strip cladding technology in China during a critical period of industrial development. The systematic approach to equipment design, process optimization, and quality control established in this work has been widely adopted by subsequent researchers and practitioners. The emphasis on productivity and quality simultaneously reflects the practical needs of the Chinese manufacturing industry, which requires cost-effective solutions for large-scale cladding operations.
The principles of high-speed strip cladding have since been extended to automated multi-wire strip cladding systems that can achieve deposition rates exceeding 20 kg/h, enabling the cladding of extremely thick layers in a single operation. The evolution of strip cladding technology from manual to semi-automated to fully automated systems has been driven by the need for improved productivity, reduced labor costs, and enhanced quality consistency. The equipment and process knowledge developed in this 1991 study provided a foundation for these subsequent advances.
In conclusion, this research represents a significant contribution to the advancement of strip electrode overlay welding technology, addressing both the process fundamentals and the equipment requirements for high-speed cladding operations. The practical insights gained from this work continue to inform the design and operation of modern strip cladding systems used in pressure vessel fabrication, heat exchanger manufacturing, and other industrial cladding applications.