Zhuojin Pipe Fitting Co., LtdInternal Surface Automatic Overlay Welding of Small Diameter Pipes
Literature Overview
The paper by Zhang Wenfeng (2008), published in China Chemical Equipment (Vol. 10, No. 1, pp. 37-38), addresses the challenge of internal surface overlay welding on small-diameter steel pipes. This work originates from Lanzhou Lanchi Industrial Development Co., Ltd., a company with deep roots in pressure vessel and piping equipment manufacturing in China. The topic is particularly relevant in chemical and petrochemical industries where internal corrosion resistance of small-bore piping is critical, and conventional external cladding or linings cannot be applied due to geometric constraints.
The author introduces the wire composition, overlay welding process parameters, and the configuration and functionality of the welding equipment used for this purpose. An example is provided to demonstrate the feasibility and effectiveness of the approach. This study is significant because internal overlay welding of small-diameter pipes remains a niche but demanding application where access, heat input control, and bead quality are all constrained by the internal geometry.
Core Technical Content
Wire Composition and Material Selection
For internal overlay welding of small-diameter pipes, the filler wire composition is critical because it determines the corrosion resistance of the overlay layer, dilution behavior, and mechanical compatibility with the base material. In chemical service applications, the overlay wire typically contains alloying elements such as chromium, nickel, molybdenum, and sometimes copper to provide resistance against specific chemical media. The wire diameter must be small enough to fit within the internal diameter of the pipe while still providing adequate deposition rate.
For small-diameter pipes (typically below DN50 or 2 inches), the wire diameter is generally limited to 0.8-1.2 mm. This narrow wire diameter constrains the achievable deposition rate and requires careful optimization of welding parameters to avoid excessive heat input, which could distort the thin-walled pipe geometry. The base material is typically carbon steel or low-alloy steel pipe, and the overlay wire must be selected to ensure adequate metallurgical bonding without excessive dilution that would compromise the corrosion resistance of the overlay layer.
Process Parameters
The key welding parameters for internal overlay welding include:
| Parameter | Typical Range | Purpose |
|---|---|---|
| Welding current | 80-160 A | Controls heat input and deposition rate |
| Welding voltage | 18-26 V | Determines arc length and bead width |
| Travel speed | 150-400 mm/min | Controls bead profile and penetration |
| Wire feed speed | 1.5-4.0 m/min | Controls deposition rate |
| Shielding gas flow | 8-15 L/min | Protects molten pool from oxidation |
| Wire diameter | 0.8-1.2 mm | Constrained by pipe internal diameter |
The process is typically performed using a pulsed GTAW or pulsed GMAW method. Pulsed welding allows precise control of heat input, which is essential for maintaining dimensional tolerances on thin-walled small-diameter pipes. The shielding gas is usually a mixture of argon with a small percentage of CO2 or oxygen to improve wetting and bead appearance.
Equipment Configuration
The automatic overlay welding equipment for internal pipe surfaces consists of several key components:
- Wire feeding system: A precision wire feeder that maintains consistent wire feed speed and ensures the wire is properly aligned with the arc axis inside the pipe.
- Torch assembly: A compact torch designed to fit within the internal diameter of the pipe, with proper gas shielding coverage.
- Pipe rotation mechanism: The pipe is rotated at a constant speed to achieve uniform circumferential overlay coverage.
- Positioning and clamping system: Securely holds the pipe and provides axial indexing between passes.
- Control system: Coordinates wire feed, rotation speed, and travel speed to maintain consistent welding parameters throughout the process.
The equipment must be designed to accommodate the limited internal space while providing adequate access for wire feeding, gas shielding, and visual inspection. The compactness of the torch and the precision of the wire alignment are critical for achieving consistent bead quality on the internal surface.
Process Analysis and Engineering Practice
Challenges in Internal Overlay Welding
Internal overlay welding of small-diameter pipes presents several unique challenges that distinguish it from external overlay welding:
- Limited access: The internal space restricts torch design and wire alignment, making it difficult to maintain consistent arc parameters throughout the welding process.
- Heat distortion: Thin-walled small-diameter pipes are susceptible to ovality and diameter changes due to welding heat input. The heat affected zone extends across the entire wall thickness, potentially causing significant residual stress.
- Bead overlap control: Achieving uniform overlay thickness around the entire circumference requires precise control of pipe rotation speed and wire positioning.
- Quality inspection: Visual inspection and non-destructive testing of the internal surface are challenging and may require specialized equipment such as borescopes or ultrasonic probes designed for internal access.
Multi-Pass Strategy
For achieving the required overlay thickness, multiple passes are typically necessary. The first pass establishes the metallurgical bond between the base material and the overlay layer, while subsequent passes build up the overlay thickness. The dilution rate is highest in the first pass and decreases with each subsequent pass. A typical strategy involves:
- First pass: Lower current and higher travel speed to minimize heat input and dilution. This pass establishes a thin, well-bonded layer.
- Subsequent passes: Gradually increase current and decrease travel speed to improve deposition rate while maintaining acceptable dilution levels.
The number of passes required depends on the target overlay thickness, the dilution rate, and the deposition rate per pass. For small-diameter pipes, the number of passes is typically limited to 2-4 due to space constraints and the risk of excessive heat input.
Quality Control Considerations
Quality control for internal overlay welding requires attention to several critical aspects:
- Overlay thickness: Must be verified at multiple locations around the circumference to ensure uniform coverage.
- Dilution rate: Should be controlled to ensure the overlay composition meets corrosion resistance requirements. Chemical analysis of the overlay layer near the fusion line is recommended.
- Bond strength: The overlay layer must be firmly bonded to the base material without cracks or delamination. Peel tests or sectioning and metallographic examination can verify bond quality.
- Internal surface finish: The overlay surface should be smooth enough for the intended service, particularly in applications where fluid flow resistance is a concern.
Study Insights and Reflections
This paper provides a practical and accessible approach to solving the problem of internal corrosion protection for small-diameter pipes. The emphasis on equipment design and process parameter optimization reflects the engineering reality that internal overlay welding is as much a mechanical challenge as a metallurgical one. The compact torch design and precision wire alignment are critical success factors that are often underestimated in planning stages.
From a modern perspective, the parameters described in this 2008 paper can be further optimized using advanced welding technologies such as cold wire GTAW, which allows decoupling of heat input from deposition rate. This would enable higher deposition rates without increasing heat input, thereby reducing distortion and improving productivity. Additionally, real-time monitoring of welding parameters and automatic adjustment systems could improve consistency and reduce the need for post-weld inspection.
The practical example provided in the paper demonstrates that the approach is feasible and reliable when properly implemented. However, engineers should be aware that the specific parameters must be adapted to the actual pipe dimensions, base material composition, and service conditions. A thorough welding procedure qualification (WPQ) in accordance with applicable standards such as ASME Section IX or ISO 15614 is essential before production implementation.
In conclusion, internal overlay welding of small-diameter pipes is a viable and effective solution for corrosion protection in chemical and petrochemical applications where external cladding is not feasible. The key to success lies in careful equipment design, precise parameter control, and rigorous quality assurance. Engineers should view this technology as a complementary option alongside other corrosion protection methods such as internal linings, cathodic protection, or material upgrade, and select the most appropriate solution based on a comprehensive cost-benefit and risk analysis for the specific application.