Zhuojin Pipe Fitting Co., LtdDevelopment of Vertical 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 designed for the deposition of corrosion-resistant overlay layers in the sealing grooves of flanges mounted on pressure vessels. The equipment addresses a specific and challenging welding scenario: the welding of stainless steel overlay material into narrow, deep sealing grooves in flanges that are positioned in the vertical orientation on large pressure vessels. The equipment integrates seven coordinated motion axes, arc voltage tracking, stepping systems, AC servo motors, planetary reducers, and PLC control to achieve high-quality welds in all positions.
Technical Challenges of Flange Sealing Groove Overlay Welding
Geometric Constraints
Flange sealing grooves are typically narrow (2–5 mm wide) and relatively deep (3–8 mm deep), with a V-shaped or U-shaped profile. The overlay material must be deposited in multiple passes to fill the groove completely, with each pass requiring precise positioning and consistent weld geometry. The vertical orientation of the flange introduces additional challenges:
- Gravity effects: Molten weld metal tends to sag or drip in the vertical position, leading to poor bead shape and incomplete groove fill.
- Heat distortion: The asymmetric heat input in vertical welding can cause distortion of the flange, affecting the sealing surface geometry.
- Access limitations: The groove may be partially obstructed by the pressure vessel body, limiting the approach angle of the welding torch.
Material Requirements
The overlay material is typically austenitic stainless steel (e.g., 308L, 316L) or nickel-base alloy (e.g., Inconel 625, Hastelloy C-276) to provide corrosion resistance against the process fluid. The overlay must:
- Be metallurgically compatible with the base flange material (typically carbon steel or low-alloy steel).
- Exhibit low dilution to maintain the corrosion resistance of the overlay.
- Be free of cracks, porosity, and lack of fusion to ensure sealing integrity.
- Have a smooth surface finish to ensure proper gasket contact.
Equipment Architecture and Control System
Seven-Axis Motion System
The equipment employs a seven-axis motion system to achieve full positional welding capability:
| Axis | Function | Actuator Type |
|---|---|---|
| Axis 1 | Torch vertical position (Z-axis) | AC servo motor with planetary reducer |
| Axis 2 | Torch horizontal position (X-axis) | AC servo motor with planetary reducer |
| Axis 3 | Torch angular orientation (tilt) | AC servo motor |
| Axis 4 | Torch rotation around its own axis | AC servo motor |
| Axis 5 | Workpiece rotation (flange rotation) | Stepping motor |
| Axis 6 | Workpiece vertical translation | Stepping motor |
| Axis 7 | Torch retraction for pass sequencing | Linear actuator |
The coordination of these seven axes enables the torch to maintain the optimal welding position throughout the groove, regardless of the groove geometry and the orientation of the flange.
Arc Voltage Tracking System
The arc voltage tracking system continuously monitors the arc voltage and adjusts the torch position to maintain a constant arc length. This is critical for consistent weld quality because variations in arc length lead to variations in heat input, penetration, and bead geometry. The tracking system operates at a sampling rate of 100 Hz or higher to respond to rapid changes in arc voltage caused by workpiece surface irregularities or groove geometry variations.
PLC Control and Process Sequencing
The PLC (Programmable Logic Controller) system manages the following process sequences:
- Groove alignment: The torch is positioned at the start of the groove using pre-programmed coordinates.
- Arc striking: The torch is lowered to the workpiece surface, and the arc is initiated.
- Welding pass: The torch traverses the groove with the arc voltage tracking system maintaining constant arc length.
- Pass sequencing: After each pass, the torch retracts, and the next pass is initiated at the appropriate offset position.
- Post-weld inspection: The torch retracts, and the groove is inspected (manual or automated).
Process Parameters
| Parameter | Value | Notes |
|---|---|---|
| Welding process | GTAW (Tungsten Inert Gas Welding) | Preferred for narrow groove overlay |
| Shielding gas | Argon or Argon-Helium mixture (80:20) | Argon for base metal; Ar-He for overlay |
| Current | 80–150 A | Depends on groove geometry and overlay thickness |
| Travel speed | 200–400 mm/min | Adjusted based on groove fill rate |
| Arc length | 2–3 mm | Maintained by arc voltage tracking |
| Wire feed rate | 1.5–3.0 m/min | For wire-fed GTAW; none for non-consumable |
| Interpass temperature | ≤ 150°C | Prevents overheating of base metal |
| Number of passes | 3–6 | Depends on groove depth and overlay thickness |
Quality Control and Defect Prevention
| Defect | Cause | Prevention |
|---|---|---|
| Incomplete groove fill | Insufficient travel speed or wire feed rate | Calibrate pass parameters based on groove geometry |
| Excessive dilution | High heat input or large bead width | Use lower current and higher travel speed |
| Porosity | Inadequate shielding gas coverage | Ensure gas flow rate of 15–20 L/min with back purge |
| Cracks | Excessive cooling rate or hydrogen contamination | Preheat to 100°C; use dry electrodes and clean workpiece |
| Surface irregularities | Torch misalignment or arc length variation | Use arc voltage tracking and regular torch calibration |
Study Insights and Implications
This paper demonstrates the power of multi-axis automation in solving complex welding challenges that are infeasible with manual or semi-automatic methods. The seven-axis system provides the flexibility needed to handle the geometric complexity of flange sealing grooves in all orientations, while the arc voltage tracking and PLC control ensure consistent weld quality. The integration of planetary reducers and AC servo motors provides the precision and repeatability required for high-quality overlay welding in critical pressure vessel applications.
One observation I would add is that the equipment described in this paper could be adapted for other challenging overlay welding applications, such as the repair of turbine blades, the overlay of corrosion-resistant layers on heat exchanger tubes, or the welding of dissimilar metal joints in nuclear power plant components. The core technology—multi-axis motion control with arc voltage tracking—is broadly applicable to any welding scenario that requires precise torch positioning in complex geometries.
The paper also highlights the importance of process integration in modern welding operations. The equipment is not merely a welding machine but a complete system that integrates motion control, process monitoring, and quality assurance. This systems-level approach is essential for achieving the reliability and consistency required in critical applications such as pressure vessel fabrication. Future developments should focus on integrating real-time weld quality monitoring (e.g., acoustic emission, optical monitoring) into the control loop to enable adaptive process control and predictive quality assurance.