CLADDING TECHNOLOGY SHANXI CO., LTDDevelopment of a Positioner for Cladding Technology Applications
Literature Overview
This 2013 publication in Mechanical and Electrical Engineering Engineering reports on the development of a specialized positioner for cladding welding applications, authored by Zhang Jianxin, Zhang Guosheng, Liu Junying, and Jiang Boping from the Tianjin Engineering Machinery Research Institute. The project was supported by the National Science and Technology Support Program (2011BAF11B08), reflecting its role in advancing manufacturing capabilities for heavy engineering equipment in China.
The positioner described in this study addresses a fundamental challenge in overlay welding of large and heavy components: the need to precisely control the workpiece orientation and rotation to ensure uniform weld quality around complex geometries. Unlike conventional welding positioners designed for butt welding or fillet welding, cladding positioners must accommodate the unique requirements of multi-pass overlay welding, including consistent torch-to-workpiece distance, controlled rotation speeds for uniform heat input distribution, and the ability to handle components with significant mass and irregular shapes.
Core Technical Content
Design Requirements and Engineering Challenges
The development of a cladding positioner involves addressing several distinct engineering challenges that differentiate it from standard welding positioners. First, the rotational speed must be precisely controllable to maintain a constant linear travel speed at the welding point, which varies with the radius of rotation. Second, the positioner must support heavy loads, often exceeding 500 kg for large pressure vessel shells or heat exchanger tubesheets. Third, the clamping mechanism must provide secure fixation without damaging the surface preparation of the component, which is critical for achieving good bond strength in the overlay layer.
The following table outlines the key design parameters and requirements identified in the study:
| Design Parameter | Specification Range | Rationale |
|---|---|---|
| Maximum load capacity | 500-2000 kg | Accommodate large pressure vessel components |
| Rotation speed range | 0.1-10 rpm | Match welding travel speed requirements |
| Speed control accuracy | ±0.5% | Ensure uniform heat input distribution |
| Clamping force | 5-20 kN | Secure workpiece without surface damage |
| Positioning accuracy | ±0.1 mm | Maintain consistent torch-to-workpiece distance |
| Drive system | AC servo motor with reducer | High torque at low speed with precise control |
Mechanical Design and Drive System
The positioner employs an AC servo motor coupled with a precision reducer to achieve the required torque and speed control characteristics. The servo control system enables closed-loop speed control with high accuracy, which is essential for maintaining consistent welding conditions throughout the rotation. The mechanical structure incorporates a robust base frame with vibration-damping features to minimize the transmission of welding-induced vibrations to the workpiece.
The clamping mechanism is designed with adjustable jaws that can accommodate cylindrical workpieces of varying diameters. The jaws are lined with soft material to prevent marring of the prepared surface. For large flat components, the positioner can be configured with a turntable arrangement that allows the welding torch to traverse the surface at a controlled speed.
Integration with Welding Equipment
The positioner is designed for integration with various overlay welding processes, including plasma transferred arc (PTA) welding, submerged arc welding (SAW), and gas metal arc welding (GMAW). The study discusses the interface between the positioner and the welding torch mount, emphasizing the need for synchronized motion control to maintain the optimal torch angle and stand-off distance.
For PTA welding, the positioner must provide smooth and continuous rotation to avoid interruptions in the powder feed and plasma arc stability. For SAW overlay, the positioner speed must be coordinated with the wire feed speed to maintain the desired bead profile and dilution ratio. The study highlights the importance of programmable speed profiles that allow acceleration and deceleration at the start and end of each pass to minimize crater defects and spatter accumulation.
Process and Standards Analysis
Procedure Qualification Considerations
When using a positioner for overlay welding, the welding procedure specification (WPS) must account for the positional effects introduced by the rotation. The heat input per unit length varies with the rotation speed, and this variation must be controlled within the qualified range. According to ASME Section IX, the travel speed is a limiting variable that must be qualified, and the positioner speed directly determines this parameter for rotary welding.
The following table presents the relationship between positioner rotation speed and welding parameters:
| Rotation Speed (rpm) | Workpiece Diameter (mm) | Linear Travel Speed (mm/min) | Heat Input (kJ/mm) | Bead Width (mm) |
|---|---|---|---|---|
| 0.5 | 500 | 157 | 0.8-1.2 | 12-15 |
| 1.0 | 500 | 314 | 0.5-0.8 | 10-12 |
| 2.0 | 500 | 628 | 0.3-0.5 | 8-10 |
| 0.5 | 1000 | 314 | 0.5-0.8 | 10-12 |
| 1.0 | 1000 | 628 | 0.3-0.5 | 8-10 |
Quality Control and Inspection
The use of a positioner introduces specific quality control considerations. The uniformity of the overlay layer around the circumference of the workpiece must be verified through dimensional measurements and hardness testing at multiple locations. Ultrasonic testing (UT) should be performed at intervals around the circumference to detect lack of bond, cracks, and porosity. The positioner's speed control accuracy directly affects the consistency of these quality characteristics.
A typical inspection plan for positioner-applied overlay includes:
| Inspection Method | Coverage | Acceptance Criteria | Standard Reference |
|---|---|---|---|
| Visual testing (VT) | 100% | No visible defects | ASME V, NB/T 47013 |
| Magnetic particle testing (MT) | 100% | No linear indications | ASME V, JB/T 4730 |
| Ultrasonic testing (UT) | 100% of bond line | No indications above threshold | ASME V, NB/T 47013 |
| Hardness testing | 12 locations around circumference | Within specified range | ASTM B600, ISO 6507 |
| Dimensional check | 12 locations | Within tolerance | Drawing specification |
Integration with Engineering Practice
In the context of pressure vessel fabrication, positioner-based overlay welding is particularly important for the manufacture of heat exchanger tubesheets, reactor shells, and large-diameter piping spools. These components often require overlay layers of stainless steel or nickel-based alloys on carbon steel substrates to provide corrosion resistance while maintaining the structural strength of the base material.
A practical application involves the overlay welding of a heat exchanger tubesheet made of SA-516 Gr. 70 carbon steel, requiring a 316L stainless steel overlay on the tube-side surface. The positioner is used to rotate the tubesheet while a PTA torch applies the overlay in multiple passes. The rotation speed is programmed to provide a travel speed of 400 mm/min, which, combined with a wire feed rate of 6 m/min and a plasma current of 300 A, produces a bead width of approximately 10 mm with a dilution ratio of 15-20%. The resulting overlay layer exhibits a hardness of 180-220 HV and passes 100% UT inspection for bond integrity.
Key Questions and Reflections
The study raises an important question about the scalability of positioner technology for even larger components, such as spherical storage tanks or large hydrogenation reactor shells with diameters exceeding 5 meters. The load capacity, clamping mechanism, and speed control system must be scaled accordingly, and the structural rigidity of the positioner must be sufficient to prevent deflection under the weight of the workpiece.
Another reflection concerns the integration of positioner technology with advanced welding monitoring systems. Real-time monitoring of welding parameters such as arc voltage, current, and travel speed, combined with in-situ defect detection techniques such as acoustic emission or infrared thermography, could significantly improve the quality and reliability of positioner-applied overlay layers. The positioner control system could potentially be linked to the welding power source to enable adaptive control that adjusts rotation speed based on real-time feedback from the welding process.
The study also highlights the importance of operator training and procedural discipline. Even with an advanced positioner, the quality of the overlay layer depends on proper surface preparation, correct torch alignment, and adherence to the qualified welding procedure. Any deviation in the setup or operation of the positioner can lead to inconsistent weld quality and potential defects.
Study Insights and Implications
This research contributes to the advancement of overlay welding capabilities in heavy engineering by providing a purpose-built positioner design that addresses the specific needs of cladding applications. The emphasis on precision speed control, robust load capacity, and flexible clamping mechanisms reflects a deep understanding of the practical challenges faced in industrial overlay welding. Engineers involved in the fabrication of bimetal components should consider the positioner as a critical piece of equipment that directly influences the quality, consistency, and productivity of the overlay welding process.
The work also highlights the interdisciplinary nature of modern welding engineering, which requires expertise in mechanical design, control systems, welding metallurgy, and quality assurance. Future developments in this area should focus on the integration of digital control systems, automated parameter optimization, and real-time quality monitoring to further enhance the capabilities of positioner-based overlay welding.