Zhuojin Pipe Fitting Co., LtdInternal Surface Automatic Overlay Welding of Small-Diameter Pipes
Literature Overview
The article by Zhang Wenfeng, published in China Chemical Equipment (Vol. 10, No. 1, 2008, pp. 37–38), describes the automatic overlay welding of the internal surface of small-diameter pipes. The author, affiliated with Lanzhou Lanchi Industrial Development Co., Ltd., presents the composition of the welding wire, the overlay welding process parameters, and the configuration and functionality of the welding equipment. The study is accompanied by practical examples that demonstrate the feasibility and effectiveness of the technique. This topic is particularly relevant to the chemical and petrochemical industries, where internal corrosion protection of pipes is a critical requirement for service life and safety.
Core Technical Content
Small-diameter pipes, typically with inner diameters less than 50 mm, present unique challenges for internal surface overlay welding. The confined space limits the access of welding equipment, the visibility of the weld seam is poor, and the heat input must be carefully controlled to avoid distortion and excessive dilution. Automatic welding is therefore preferred over manual welding to ensure consistency and quality.
Welding Wire Composition
The welding wire used for internal overlay welding must be specifically formulated to provide the desired corrosion resistance or wear resistance in the service environment. For chemical service applications, the wire typically contains alloying elements such as chromium, nickel, and molybdenum to provide resistance to specific corrosive media. The wire composition must be compatible with the base pipe material to ensure good metallurgical bonding and to prevent cracking during welding.
| Wire Component | Typical Content | Function |
|---|---|---|
| Carbon | 0.05–0.15% | Base strength |
| Manganese | 1.0–2.0% | Deoxidation, strength |
| Silicon | 0.3–0.8% | Deoxidation |
| Chromium | 20–30% | Corrosion resistance |
| Nickel | 8–12% | Toughness, corrosion resistance |
| Molybdenum | 2–4% | Pitting corrosion resistance |
| Sulfur | < 0.01% | Cleanliness |
| Phosphorus | < 0.01% | Cleanliness |
The wire is typically a solid wire with a core of flux or a flux-cored wire with a metal powder core. The flux or powder provides additional alloying elements and helps to stabilize the arc and control the weld pool geometry.
Process Parameters
The overlay welding process parameters must be optimized to achieve good penetration, adequate overlay thickness, and minimal dilution with the base metal. The key parameters include:
| Process Parameter | Typical Value | Notes |
|---|---|---|
| Welding current | 80–150 A | Depends on wire diameter |
| Arc voltage | 18–25 V | Stable arc required |
| Travel speed | 50–150 mm/min | Depends on pipe diameter |
| Wire feed speed | 3–8 m/min | Matched to current |
| Shielding gas | Ar or Ar/CO2 mix | Flow rate 8–15 L/min |
| Preheat temperature | 50–150°C | If required by base metal |
| Interpass temperature | ≤ 150°C | To prevent cracking |
| Number of passes | 2–4 | To achieve required thickness |
| Overlay thickness per pass | 1–2 mm | Controlled by parameters |
Welding Equipment Configuration
The automatic welding equipment for internal surface overlay welding of small-diameter pipes consists of several key components:
- Welding head: A compact torch assembly designed to fit within the pipe bore, with precise positioning of the wire electrode and gas nozzle.
- Wire feeding mechanism: A precision wire feeder that maintains constant wire feed speed and tension.
- Travel mechanism: A carriage or turntable that moves the welding head along the pipe axis or rotates the pipe, ensuring uniform travel speed.
- Gas supply system: A regulator and flow meter to maintain constant shielding gas flow.
- Power supply: A constant voltage or constant current welding power source with stable arc characteristics.
- Control system: A controller that coordinates the wire feed, travel speed, and power supply to maintain consistent welding conditions.
The equipment must be designed for compactness and precision, as the internal space of small-diameter pipes is severely limited. The welding head must be able to maintain a consistent arc length and travel speed throughout the weld, which requires careful mechanical design and control.
Engineering Practice Integration
The internal overlay welding of small-diameter pipes is a specialized technique used in applications such as:
- Chemical process pipes where internal corrosion protection is required
- Heat exchanger tubes where erosion-corrosion resistance is needed
- Catalyst support pipes where wear resistance is critical
- Piping systems in the pharmaceutical and food industries where surface integrity is essential
The technique requires careful preparation of the pipe internal surface, including cleaning, degreasing, and possibly machining to remove any surface defects. The welding procedure must be qualified through test coupons that simulate the actual pipe geometry and service conditions. Non-destructive examination of the completed overlay weld is challenging due to the confined space, and alternative methods such as eddy current testing or visual inspection through borescopes may be required.
Defect Analysis and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Porosity | Inadequate gas shielding, wire moisture | Ensure proper gas flow, dry wire |
| Lack of fusion | Insufficient heat input | Increase current, reduce travel speed |
| Excessive dilution | High heat input, thin overlay | Reduce current, increase travel speed |
| Arc instability | Poor wire feed consistency | Maintain wire feeder, check tension |
| Uneven overlay thickness | Travel speed variation | Calibrate travel mechanism |
| Undercut | Excessive arc length | Maintain consistent arc length |
Key Questions and Reflections
The study highlights the challenges of automatic welding in confined spaces and the importance of equipment design and process parameter optimization. Several questions arise for engineering practice:
- How does the pipe rotation speed affect the quality of the overlay weld, and what is the optimal rotation speed for different pipe diameters?
- What are the limitations of the technique in terms of minimum and maximum pipe diameter, and how can these be extended?
- How can the quality of the internal overlay weld be verified without dismantling the pipe assembly?
These questions point to the need for further research and development in the area of internal welding technology, particularly for increasingly demanding applications in the chemical and petrochemical industries.
Study Insights and Implications
This literature provides a valuable reference for engineers involved in the internal surface protection of small-diameter pipes. The systematic approach to welding wire selection, process parameter optimization, and equipment configuration demonstrates the importance of a comprehensive understanding of the welding process. The practical examples presented in the paper confirm the feasibility of the technique and provide a basis for its implementation in industrial applications. For engineers in the chemical and petrochemical industries, this study underscores the potential of automatic overlay welding as a reliable method for extending the service life of critical piping systems.