CLADDING TECHNOLOGY SHANXI CO., LTDCladding Repair of Pure Copper Bearing Bush Surface Wear
Literature Overview
The study by Kong Lingxin, Han Yu, and Zhang Yunlai (2009), conducted at the Pressure Vessel Inspection Institute of the Xinjiang Dushanzi Petrochemical Company Research Institute, addresses the practical challenge of repairing worn pure copper bearing bush surfaces through weld overlay (cladding) technology. Pure copper bearing bushes are critical components in large-scale rotating machinery, particularly in the petrochemical industry where they serve as journal bearings in compressors, pumps, and turbines. Wear of these bearing surfaces can result from inadequate lubrication, misalignment, or extended service beyond design life. The research explores the feasibility and effectiveness of restoring worn copper bearing surfaces through overlay welding, providing a cost-effective alternative to complete component replacement.
Technical Background and Challenges
Pure copper (Cu-99.9%, conforming to GB/T 5231 or ASTM B189) is widely used for bearing applications due to its excellent conformability, low friction coefficient, good thermal conductivity, and compatibility with various lubricants. However, pure copper has relatively low hardness (approximately 40–60 HV in annealed condition) and limited wear resistance, making it susceptible to surface degradation under heavy loading or poor lubrication conditions.
The primary challenges in cladding repair of pure copper bearing bushes include:
| Challenge | Technical Difficulty | Impact |
|---|---|---|
| High thermal conductivity | Rapid heat dissipation from weld zone | Poor wetting, incomplete fusion |
| Low melting point (1085 °C) | Excessive thermal input causes base melting | Distortion, microstructure coarsening |
| Oxide formation | Cu₂O and CuO formation at high temperatures | Porosity, hot cracking |
| Hydrogen embrittlement | Hydrogen pickup from flux or moisture | Delayed cracking |
| Dimensional accuracy | Bearing surfaces require tight tolerances (IT6–IT7) | Post-weld machining required |
| Material compatibility | Dissimilar metal welding considerations | Intermetallic compound formation |
Welding Process Selection and Parameters
The selection of welding process for copper bearing bush repair is critical. Based on the technical requirements and the nature of the substrate material, the following processes are most commonly considered:
Suitable Welding Processes
- Gas Tungsten Arc Welding (GTAW/TIG): The most widely used process for copper cladding repair due to its precise heat input control, good weld quality, and suitability for thin overlay layers. The process employs a non-consumable tungsten electrode with a copper matching filler wire.
- Plasma Transferred Arc (PTA) Powder Cladding: Offers high deposition rates with narrow heat-affected zones, making it suitable for precision repair of bearing surfaces.
- Oxy-Acetylene Welding: Sometimes used for thicker repair deposits where high heat input is acceptable and the geometry allows for easy access.
Typical GTAW Parameters for Copper Cladding Repair
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Welding current | 120–200 A | DCEN polarity for deep penetration |
| Arc voltage | 10–14 V | Controls bead width and profile |
| Travel speed | 80–150 mm/min | Balances deposition rate and heat input |
| Shielding gas | 100% argon or Ar-5% N₂ | Prevents oxide inclusion |
| Preheat temperature | 200–300 °C | Reduces thermal gradient, improves wetting |
| Interpass temperature | ≤ 300 °C | Prevents grain coarsening |
| Filler wire | Cu-Cr or pure copper wire | Matching or enhanced wear resistance |
Metallurgical Considerations
The metallurgical behavior of copper weld overlays presents unique challenges compared to steel welding. Copper does not undergo a phase transformation during solidification, so the microstructure is primarily determined by cooling rate and grain growth kinetics. However, several metallurgical phenomena must be considered:
Solidification Behavior
Pure copper solidifies through a simple liquid-to-face-centered cubic (FCC) transformation. The solidification crack susceptibility is related to the solidification range and the coefficient of thermal contraction. Copper has a narrow solidification range, which generally reduces hot cracking susceptibility compared to aluminum alloys. However, the presence of impurities such as oxygen, sulfur, and phosphorus can significantly increase cracking tendency.
Intermetallic Compound Formation
When copper is welded onto steel substrates (as may occur when repairing a copper bush that has a steel backing plate), intermetallic compounds such as FeCu, FeCu₃, and Fe₂Cu form at the interface. These compounds are brittle and can compromise the bond strength and fatigue resistance of the repair. The formation of intermetallics is governed by the Kirkendall effect and diffusion kinetics, with the thickness of the intermetallic layer increasing with time and temperature exposure.
Hardness and Wear Performance
The hardness of the cladding layer directly influences the wear resistance of the repaired bearing surface. Pure copper in the as-welded condition typically exhibits a hardness of 60–80 HV, which is insufficient for heavy-duty bearing applications. The following strategies can be employed to enhance hardness:
- Use of Cu-Cr or Cu-Al alloy filler wires (hardness up to 150–200 HV)
- Post-weld aging treatment at 400–500 °C to promote precipitation hardening
- Surface treatment such as electroless nickel plating or chemical hardening
- Multi-pass welding with controlled interpass temperatures to refine grain structure
Quality Control and Inspection
The quality of the cladding repair is critical to the safe operation of rotating machinery. The following quality control measures should be implemented:
| Inspection Type | Method | Acceptance Criteria |
|---|---|---|
| Visual examination | Direct observation | No cracks, porosity, undercut, or excess reinforcement |
| Penetrant testing (PT) | Dye penetrant per ASTM E709 | No linear indications; round indications ≤ 1 mm |
| Ultrasonic testing (UT) | Contact UT per ASTM E164 | No lack of fusion or delamination |
| Hardness survey | Vickers hardness per ASTM E18 | Uniform hardness within ±15 HV of target |
| Dimensional inspection | Coordinate measuring machine or optical comparator | Surface flatness ≤ 0.02 mm; roundness ≤ 0.01 mm |
| Metallographic examination | Cross-section per ASTM E3 | No cracks, porosity, or excessive intermetallics |
Engineering Practice and Case Study Insights
In the context of the Dushanzi Petrochemical Company, bearing bush repairs are performed under maintenance shutdown conditions where minimizing downtime is a primary concern. The cladding repair approach offers the advantage of in-situ repair without complete disassembly of the bearing housing, significantly reducing maintenance time compared to replacement with a new bush.
A typical repair procedure would follow these steps:
- Surface preparation: Remove worn material by grinding to expose sound substrate; clean with acetone or alcohol to remove oil and debris.
- Preheating: Apply controlled preheat using induction heating or oxy-fuel torches to reach the target temperature uniformly across the repair area.
- Overlay welding: Apply the cladding layer using GTAW with matching or alloyed filler wire, building up the deposit in multiple thin passes.
- Post-weld treatment: Apply aging treatment if required; perform stress relief annealing at 300–400 °C for 1–2 hours.
- Machining: Machine the repaired surface to the required dimensional tolerances and surface finish (typically Ra ≤ 0.8 μm for bearing surfaces).
- Final inspection: Perform dimensional, hardness, and non-destructive testing to verify compliance with specifications.
The key insight from this research is that successful copper bearing bush repair requires a holistic approach that integrates welding process optimization, metallurgical understanding, and precision machining. The repair procedure must be qualified through a welding procedure qualification (WPQ) per relevant standards, with production welders certified to the qualified procedure.
Study Insights and Conclusions
The research on pure copper bearing bush cladding repair provides practical guidance for maintenance engineers in the petrochemical industry. The GTAW-based overlay welding approach, when properly controlled, offers a reliable method for restoring worn bearing surfaces with minimal downtime. The key success factors are precise thermal input control, appropriate filler metal selection, rigorous surface preparation, and meticulous post-weld machining. This work demonstrates that even relatively soft and thermally conductive materials like pure copper can be effectively repaired through weld overlay technology, provided that the specific metallurgical and process challenges are properly addressed. The economic benefits of repair over replacement, particularly for large and expensive bearing assemblies, make this technology highly valuable for industrial maintenance programs.