Zhuojin Pipe Fitting Co., LtdVertical Automatic Overlay Welding Equipment for Flange Sealing Grooves
Literature Overview
The paper by Su Bozhong (Welding Technology, Vol. 41, Issue 8, 2012, pp. 47–49) describes the development of a specialized automatic overlay welding equipment for the filling and overlay welding of sealing grooves on vertical-position flanges in pressure vessel applications. The equipment is designed to address the challenge of applying corrosion-resistant overlay layers to sealing grooves that are located in vertical or overhead positions on large pressure vessels. The equipment employs a total of seven motion axes working in coordination, combined with arc voltage tracking, stepping systems, AC servo systems, planetary reducers, and a PLC control system, to achieve high-quality stainless steel overlay welds in all positions. This work is significant because it demonstrates how specialized equipment can solve complex welding challenges that are difficult or impossible to address with conventional manual or semi-automatic welding methods.
Core Technical Points
The paper describes a sophisticated multi-axis welding system designed for a specific and challenging application:
1. Application Requirements
The sealing grooves on flanges in pressure vessels are critical areas that require corrosion-resistant overlay layers to ensure long-term sealing integrity. The challenges of this application include:
| Challenge | Description |
|---|---|
| Position | Vertical or overhead orientation, difficult for manual welding |
| Geometry | Groove geometry may be complex, requiring precise torch positioning |
| Material | Stainless steel overlay on carbon steel base, requiring control of dilution |
| Quality | Zero tolerance for defects, as leaks can be catastrophic |
| Accessibility | Large pressure vessels may have limited access for welding equipment |
2. Equipment Design
The equipment is designed with seven motion axes that work in coordination:
| Axis | Function |
|---|---|
| Axis 1 | Vertical movement of the welding head |
| Axis 2 | Horizontal movement of the welding head |
| Axis 3 | Rotation of the welding head around the groove |
| Axis 4 | Tilt of the welding head to accommodate groove geometry |
| Axis 5 | Vertical movement of the workpiece (if applicable) |
| Axis 6 | Horizontal movement of the workpiece (if applicable) |
| Axis 7 | Rotation of the workpiece (if applicable) |
The coordination of these seven axes allows the welding torch to follow the groove geometry precisely, regardless of the groove's orientation and position on the pressure vessel. The equipment can handle both vertical and horizontal groove orientations, making it versatile for different pressure vessel designs.
3. Control Systems
The equipment employs several advanced control systems:
- Arc Voltage Tracking System: This system monitors the arc voltage in real time and adjusts the torch position to maintain a consistent arc length. This is critical for maintaining a stable welding process and ensuring consistent weld quality. The system can detect variations in the groove geometry and adjust the torch position accordingly.
- Stepping System: The stepping system provides precise positioning of the welding head along the groove. It uses stepping motors to move the welding head in discrete increments, allowing for precise control of the welding path.
- AC Servo System: The AC servo system provides high-speed, high-precision movement of the welding head. It is used for the axes that require rapid movement and precise positioning, such as the rotation and tilt axes.
- Planetary Reducer: The planetary reducer provides high torque and high precision at the output shaft. It is used to drive the rotation and tilt axes of the welding head.
- PLC System: The programmable logic controller (PLC) system coordinates all the motion axes and control systems. It executes the welding program, which defines the welding path, welding parameters, and sequence of operations. The PLC also monitors the welding process and can stop the welding if a fault is detected.
4. Welding Process Parameters
The equipment is designed for use with gas metal arc welding (GMAW) or flux-cored arc welding (FCAW) processes. The recommended welding parameters are:
| Parameter | Value |
|---|---|
| Process | GMAW or FCAW |
| Wire diameter | 1.2–1.6 mm |
| Current | 150–250 A |
| Voltage | 22–28 V |
| Travel speed | 200–500 mm/min |
| Shielding gas | Argon + CO2 (80:20) or pure argon |
| Preheating | 100–200 °C (depending on base material thickness) |
| Interpass temperature | ≤ 200 °C |
| Post-weld treatment | Solution treatment or stress relief (depending on application) |
Defect Analysis and Prevention
The paper addresses several potential defects that can occur during the overlay welding of flange sealing grooves:
1. Lack of Fusion
Lack of fusion is a critical defect that can compromise the integrity of the overlay weld. It can occur at the weld-metal/bond interface or between weld passes. The equipment's arc voltage tracking system helps prevent lack of fusion by maintaining a consistent arc length and ensuring that the torch is positioned correctly relative to the groove. The multi-pass technique, with each pass overlapping the previous pass by 50%, also helps prevent lack of fusion.
2. Porosity
Porosity can occur due to atmospheric contamination, insufficient shielding gas coverage, or porosity-forming elements in the weld metal. The equipment's precise torch positioning and consistent arc length help ensure that the shielding gas covers the weld pool effectively. The use of a high-purity shielding gas and a clean wire also helps prevent porosity.
3. Cracking
Cracking can occur in the weld metal or at the weld-metal/bond interface. The equipment's control of preheat and interpass temperatures helps prevent cracking by reducing the cooling rate and minimizing residual stresses. The multi-pass technique with alternating directions also helps reduce residual stresses. The selection of a compatible overlay material with good crack resistance is also critical.
4. Dilution
Dilution from the base material into the overlay layer can reduce the corrosion resistance of the overlay. The equipment's precise control of the welding parameters and torch positioning helps minimize dilution by ensuring that the weld pool is properly confined within the groove. The use of a multi-pass technique with a transition layer can also help minimize dilution.
5. NDT Verification
The paper emphasizes the importance of NDT to verify the quality of the overlay weld. The recommended NDT methods include:
| NDT Method | Purpose |
|---|---|
| Magnetic Particle Testing (MT) | Surface and near-surface cracks |
| Ultrasonic Testing (UT) | Bond defects and internal cracks |
| Dye Penetrant Testing (PT) | Surface cracks |
| Radiographic Testing (RT) | Internal defects (if accessible) |
| Hardness Testing | Verify overlay hardness profile |
| Chemical Analysis | Verify overlay composition |
Engineering Practice Integration
The practical significance of this paper lies in the development of specialized equipment that can solve a specific and challenging welding problem. The equipment's multi-axis design and advanced control systems allow it to weld sealing grooves in positions that would be difficult or impossible to weld manually. This is particularly important for large pressure vessels where the sealing grooves may be located in hard-to-reach positions.
From a manufacturing perspective, the equipment can be used in various applications:
- Pressure vessel fabrication: Overlay welding of sealing grooves on flanges
- Pipeline fabrication: Overlay welding of sealing grooves on flanges
- Heat exchanger fabrication: Overlay welding of sealing grooves on tube sheets
- Nuclear power plant components: Overlay welding of sealing grooves on reactor vessel nozzles
The equipment's versatility in handling both vertical and horizontal groove orientations makes it suitable for a wide range of pressure vessel designs. The PLC-based control system allows for easy programming of different welding paths, making the equipment adaptable to different groove geometries and sizes.
Study Insights and Implications
This paper provides a valuable example of how specialized equipment can be developed to solve complex welding challenges. The multi-axis design and advanced control systems demonstrate the potential of automation and robotics in welding applications. The paper's emphasis on the coordination of multiple control systems (arc voltage tracking, stepping, AC servo, planetary reducer, PLC) highlights the complexity of modern welding equipment and the importance of integrated system design. Engineers should note that the development of such specialized equipment requires a deep understanding of both the welding process and the control systems, and that the equipment must be carefully calibrated and maintained to ensure consistent weld quality. The paper also highlights the importance of NDT verification in ensuring the quality of overlay welds, which is critical for applications where leaks can be catastrophic. Overall, this work serves as a useful reference for engineers involved in the design and application of specialized welding equipment for challenging applications.