Zhuojin Pipe Fitting Co., LtdFlexible Strip Overlay Material Development for Internal Surface Cladding
Literature Overview
This 2010 paper by Song Dan and colleagues from Shenyang University of Technology, published in Welding, presents the development of a flexible strip overlay material designed for internal surface cladding of small-diameter tubing and similar confined geometries. Funded by the Liaoning Provincial Department of Education Science and Technology Fund, this research addresses a practical challenge in the welding industry: how to apply hardfacing or corrosion-resistant overlay deposits to internal surfaces where conventional electrode or wire feeding is impractical or impossible.
Material Design and Fabrication
The flexible strip material is fabricated by combining hardfacing alloy powders, specifically Ni60 and WC (tungsten carbide) powder, with various adhesive materials including silicone rubber and other organic binders. The mixture is formed into strip shape, dried, and then applied to the substrate using TIG welding, where the strip acts as a self-fluxed or semi-self-fluxed consumable.
Adhesive Material Comparison
| Adhesive Material | Forming Performance | Drying Characteristics | Welding Compatibility | Overall Rating |
|---|---|---|---|---|
| Silicone rubber | Excellent flexibility | Good | Good | Best |
| Other organic binders | Moderate | Variable | Variable | Acceptable |
The study systematically compared different adhesive materials and found that silicone rubber provided the best overall performance in terms of strip forming flexibility, drying behavior, and welding compatibility. The flexibility of the silicone rubber adhesive allows the strip to conform to curved internal surfaces of small tubes, which is impossible with rigid strip materials.
Microstructure and Hardness Analysis
Metallographic examination of the TIG-welded overlay deposits revealed a microstructure characteristic of Ni60-based hardfacing alloys, consisting of austenite matrix with M7C3 carbide precipitates. The addition of WC powder to the Ni60 matrix creates a composite microstructure where the exogenous WC particles are distributed throughout the deposit, providing additional wear resistance beyond what the Ni60 matrix alone could achieve.
The hardness profile across the deposit typically shows a gradient from the substrate surface outward, with the highest hardness achieved in the region where the WC particles are most concentrated. The Ni60 matrix provides excellent toughness and corrosion resistance, while the WC particles provide the wear resistance needed for sliding or abrasive wear applications.
Microstructural Considerations
The microstructure of the flexible strip overlay deposit differs from that of conventionally applied Ni60 hardfacing in several important ways. First, the pre-mixed composition ensures a more uniform distribution of WC particles throughout the deposit compared to electrode or wire feeding, where particle segregation can occur due to fluid flow in the weld pool. Second, the strip geometry provides a controlled deposit shape that can be tailored to the specific application geometry. Third, the organic binder in the strip can act as a mild flux, helping to remove surface oxides and improve wetting during welding.
Process Parameters and Application Methodology
The TIG welding process was selected for its low heat input, clean arc, and excellent weld pool control, all of which are essential for achieving good bonding and microstructure in overlay applications on thin-walled or small-diameter components. The following process considerations are critical:
- Base metal preparation: The internal surface must be cleaned to remove scale, rust, and contamination. For thin-walled tubing, the back side should be supported to prevent collapse.
- Strip placement: The flexible strip must be positioned carefully against the internal surface before welding. In some cases, a temporary fixture may be needed to hold the strip in place.
- Welding parameters: Low current (typically 60-120 A depending on tube diameter and wall thickness), moderate travel speed, and proper torch angle are essential to avoid excessive penetration and burn-through.
- Cooling management: For thin-walled tubing, water cooling or copper backing bars may be needed to control the heat input and prevent distortion.
- Multi-pass application: Thick overlay builds require multiple strips applied sequentially, with interpass cleaning between passes to ensure good bonding.
Engineering Applications
The flexible strip overlay material is particularly suited for the following applications:
- Internal cladding of small-diameter tubing in heat exchangers, chemical reactors, and process piping where corrosion or erosion resistance is required
- Repair of worn internal surfaces in cylindrical components where access for conventional welding tools is limited
- Localized hardfacing of specific internal areas in pump casings, valve bodies, and similar components
- Surface treatment of thin-walled components where low heat input is essential to prevent distortion
The key advantage of this approach over conventional overlay welding methods is the ability to apply overlay material to internal surfaces that are inaccessible to standard electrodes or wires. The flexibility of the strip allows it to be inserted through small openings and positioned against curved internal surfaces.
Quality Control Considerations
Quality assurance for flexible strip overlay welding requires attention to several critical parameters:
- Bonding strength: Verified by microstructural examination of the interface and, if possible, by shear or peel testing
- Porosity: The organic binder in the strip can generate gas during welding, which may lead to porosity if not properly controlled by welding parameters
- Cracking: Ni60-based deposits are generally crack-resistant, but the presence of WC particles can increase susceptibility if the matrix is not sufficiently ductile
- Thickness uniformity: Critical for functional applications such as corrosion-resistant linings where thin areas could be preferentially attacked
FMEA for Flexible Strip Overlay Welding
| Failure Mode | Severity | Occurrence | Detection | RPN | Mitigation |
|---|---|---|---|---|---|
| Poor bonding | 9 | 5 | 4 | 180 | Surface preparation, proper strip placement |
| Porosity from binder | 6 | 6 | 4 | 144 | Adequate drying, proper welding parameters |
| Burn-through | 8 | 4 | 3 | 96 | Low current, copper backing, water cooling |
| Incomplete melting | 7 | 5 | 4 | 140 | Sufficient current, proper travel speed |
| Distortion | 6 | 5 | 3 | 90 | Backing support, controlled heat input |
Study Insights and Implications
This research demonstrates an innovative approach to solving the practical problem of internal surface overlay welding. The use of flexible strips as a consumable form factor is a creative adaptation of existing hardfacing alloy chemistry to a new application scenario. The systematic comparison of adhesive materials and the metallographic characterization of the resulting deposits provide a solid scientific basis for the technology.
The engineering implication is significant: this technology opens up new possibilities for protecting and repairing internal surfaces of components that were previously difficult or impossible to overlay using conventional methods. The combination of Ni60 with WC provides a versatile material system that can be adapted to different service requirements by varying the WC content and particle size.
Future development should focus on automated application of flexible strips, improved binder formulations that minimize gas generation during welding, and standardized welding procedure specifications that ensure consistent quality across different operators and production facilities. The technology has particular promise in the oil and gas, chemical processing, and power generation industries, where internal surface protection of small-diameter tubing is a recurring maintenance challenge.