CLADDING TECHNOLOGY SHANXI CO., LTDAdaptive Torch Height Tracking for Internal Vessel Overlay Welding
Literature Overview
This study by Zhang Ning, Xue Ruilei, Xia Lei, and Kang Feng from Xinjiang University addresses a persistent challenge in the fabrication of clad pressure vessels: maintaining consistent torch-to-workpiece distance during overlay welding on the internal surfaces of cylindrical and spherical vessels. The research was supported by the Xinjiang Uygur Autonomous Region Natural Science Foundation (2022D01C391) and Xinjiang University Excellent Doctoral Innovation Program (XJU2022BS092), published in Manufacturing Technology and Machine Tools in 2025. The core problem is that internal vessel cladding—whether by SAW, GTAW, or FCAW—requires the torch to travel along curved, often inaccessible surfaces where conventional fixed-offset torch holders cannot maintain the critical arc length.
Core Technical Viewpoints
The fundamental insight of this work is that a static torch height setting inevitably leads to arc length drift as the torch traverses the curvature of a vessel interior. This drift directly causes variations in heat input, bead geometry, dilution rate, and ultimately the metallurgical quality of the overlay layer. The proposed adaptive system employs real-time sensing to detect the actual torch-to-surface distance and continuously adjusts the torch position through a servo-controlled actuator.
Sensing Mechanism and Control Architecture
The adaptive torch height control system typically integrates one or more of the following sensing modalities:
| Sensing Method | Operating Principle | Typical Accuracy | Suitability for Internal Cladding |
|---|---|---|---|
| Contact-type (mechanical probe) | Physical contact with workpiece surface | ±0.5 mm | High, but risk of contamination |
| Non-contact optical (laser triangulation) | Laser beam reflection geometry | ±0.1 mm | Moderate, affected by spatter |
| Arc voltage sensing | Voltage proportional to arc length | ±1.0 mm | High, but affected by consumable type |
| Arc current sensing | Current-voltage coupling | ±0.8 mm | Moderate, indirect measurement |
The adaptive algorithm processes the sensor signal and drives a servo motor or hydraulic actuator to maintain the torch at the target standoff distance. The control loop must respond within milliseconds to compensate for surface irregularities such as weld bead transitions, pre-existing corrosion, or surface roughness.
Engineering Significance for Pressure Vessel Fabrication
In the context of bimetal pressure vessels governed by GB/T 150 or ASME VIII Div.1, internal overlay welding is commonly required for corrosion resistance in hydrogenation reactors, acid storage vessels, and chemical process equipment. The overlay layer thickness is typically specified at 1.5 to 3.0 mm for single-layer applications or up to 5.0 mm for multi-layer builds. Any deviation in torch height during the build causes:
- Excessive height: Arc lengthening, reduced arc density, wider and flatter beads, increased dilution from the base metal, and potential lack of fusion at the bond line.
- Insufficient height: Arc shortening, excessive heat concentration, narrow deep beads, potential burn-through on thin cladding plates, and increased residual stress.
For a vessel with an internal diameter of 2000 mm and a wall thickness of 30 mm, the curvature presents a significant geometric challenge. A conventional manual torch holder would produce height variations of 5 to 15 mm over a single circumferential pass, which is unacceptable for producing a uniform overlay layer meeting the requirements of NB/T 47002 or ASME IX QW-451.
Process Parameters and Control Windows
The following table summarizes typical process windows for internal vessel overlay welding where adaptive torch height tracking is critical:
| Process | Wire/Consumable | Current (A) | Voltage (V) | Travel Speed (mm/min) | Target Standoff (mm) |
|---|---|---|---|---|---|
| FCAW (internal) | ER309L flux-cored | 350–500 | 28–34 | 200–400 | 15–20 |
| SAW (internal) | ER309L + flux | 500–700 | 30–36 | 150–350 | 10–15 |
| GTAW (internal) | ER309L | 100–200 | 12–18 | 80–200 | 3–5 |
| Hot-wire TIG | ER309L | 80–150 | 10–16 | 300–600 | 4–6 |
The adaptive system must maintain standoff within ±10% of the target value to ensure acceptable bead geometry and metallurgical properties. For GTAW overlay, where the standoff tolerance is only 3 to 5 mm, the control response time must be under 50 ms.
Integration with Engineering Practice
In actual vessel fabrication shops, adaptive torch height tracking is most valuable for:
- Spherical vessels: The continuously varying curvature makes manual height control virtually impossible. Adaptive systems have been successfully applied to hydrogen storage spheres where 304L overlay layers are required.
- Large-diameter cylindrical vessels: For vessels exceeding 3000 mm in diameter, the curvature over a single torch pass is small, but the cumulative effect over multiple passes still demands adaptive control.
- Nozzle-to-shell transitions: The geometric discontinuity at nozzle connections creates abrupt surface changes that only an adaptive system can handle reliably.
The study's approach aligns with the quality control philosophy embedded in ASME IX and NB/T 47014, where welder qualification requires demonstration of consistent bead geometry. An adaptive system essentially eliminates the operator variability that contributes to qualification failures.
Key Questions and Reflections
A critical question arises: how does the adaptive system perform when the internal surface is contaminated with scale, rust, or residual flux from previous operations? Contact-type sensors may be obstructed, and optical sensors may provide erroneous readings. The engineering answer is that thorough surface preparation per NB/T 47002 Section 4 is a prerequisite, and the adaptive system should include outlier rejection logic to ignore transient sensor spikes.
Another consideration is the interaction between torch height and the multi-layer build sequence. When building up an overlay layer of 4 mm total thickness in 4 passes, each subsequent pass traverses the previous bead. The adaptive system must distinguish between the intended surface profile (the previous bead) and any defect (undercut, porosity) that could cause inappropriate height adjustment. This requires either a pre-scan mapping function or a threshold-based filtering algorithm.
Study Insights and Implications
The research by Zhang and colleagues represents a practical advancement in addressing one of the most common quality issues in internal vessel cladding: inconsistent overlay thickness and bond line quality caused by torch height drift. From a standards perspective, the ability to maintain uniform arc length directly supports compliance with the dilution control requirements of API 934 and the metallurgical acceptance criteria of GB/T 150. The adaptive approach also reduces the need for post-weld grinding to achieve the required overlay thickness, thereby minimizing the risk of removing the overlay layer entirely at thin spots.
In my assessment, the next logical step for this technology is the integration of adaptive height control with real-time bead geometry monitoring using vision systems, creating a fully closed-loop overlay welding process. Such a system would simultaneously control arc length, travel speed, and wire feed rate to maintain both geometric and metallurgical quality throughout the entire build sequence. This would be particularly transformative for the fabrication of large hydrogenation reactors where overlay layers of Hastelloy C276 or Inconel 625 must be deposited on carbon steel substrates with strict dilution limits of less than 30%.