CLADDING TECHNOLOGY SHANXI CO., LTDResearch on Electric Spark Overlay Welding Technology and Its Application in Power Shaft Repair
Literature Overview and Research Context
Electric spark overlay welding (also known as electric arc surfacing with pulse current or spark erosion welding) represents a specialized technique for surface repair and overlay application that differs fundamentally from conventional arc welding processes. This study examines the principles, parameters, and practical applications of electric spark overlay welding, with particular focus on its use in repairing power generation shaft components such as turbine shafts, generator rotors, and pump shafts. The technique is particularly valued in power industry maintenance operations where minimal heat input and precise material deposition are critical requirements.
Core Technical Findings
Working Principle and Process Characteristics
Electric spark overlay welding operates on the principle of controlled electrical discharge between the electrode and workpiece, creating localized molten pools that solidify rapidly to form deposited material. Unlike conventional arc welding, the process uses short-duration, high-current pulses (typically 1-10 ms duration at 500-2000 A peak current) that create individual deposited droplets rather than a continuous weld bead.
Key process parameters include:
| Parameter | Typical Range | Effect |
|---|---|---|
| Pulse current | 500-2000 A | Controls droplet size and penetration |
| Pulse duration | 1-10 ms | Affects heat input per pulse |
| Pulse frequency | 5-50 Hz | Controls deposition rate |
| Electrode-to-workpiece distance | 0.5-2.0 mm | Affects arc stability |
| Electrode material | Ni-based, Co-based, Cu-based | Determines deposit properties |
| Travel speed | 5-30 mm/s | Controls layer thickness uniformity |
Microstructural Features of Spark Overlay Deposits
The rapid solidification inherent in spark overlay welding produces distinctive microstructural features:
- Fine-grained cellular or dendritic structure with grain sizes typically 10-50 μm
- High volume fraction of retained austenite in stainless steel deposits (15-35%)
- Reduced carbide coarsening compared to conventional welding
- Minimal heat-affected zone (HAZ) in the base metal, typically <0.5 mm
- Lower residual stress levels due to the pulsed nature of heat input
The study demonstrates that the dilution rate in spark overlay is significantly lower (2-8%) compared to conventional arc welding (10-25%), resulting in deposit composition that more closely matches the electrode material specification.
Application to Power Shaft Repair
In power generation applications, shaft repair presents unique challenges:
- Large diameter components (500-2000 mm) with significant mass
- Critical dimensional tolerances (often ±0.05 mm)
- High cyclic stress levels requiring fatigue-resistant repair deposits
- Limited access for conventional welding equipment
- Requirement for minimal distortion to maintain shaft alignment
The study documents successful applications on:
- High-pressure turbine shafts with bearing journal wear
- Generator rotor shafts with keyway damage
- Pump shafts with corrosion pitting
- Turbine blade root repair (in situ)
Engineering Practice Integration
Comparison with Alternative Repair Methods
| Repair Method | Heat Input | Dilution | Applicable Defect Size | Equipment Portability |
|---|---|---|---|---|
| Spark overlay welding | Very Low | Low (2-8%) | 0.5-5 mm depth | High |
| GTAW (TIG) welding | Moderate | Moderate (10-15%) | 1-10 mm depth | Moderate |
| SAW welding | High | Moderate (15-25%) | 2-20 mm depth | Low |
| Laser cladding | Low | Low (3-10%) | 0.5-3 mm depth | Moderate |
| Cold spray | None | None | 0.5-2 mm depth | High |
Quality Assurance for Spark Overlay Repairs
Quality control of spark overlay repairs requires specialized attention:
- Pre-repair assessment: Dimensional measurement, magnetic particle inspection for crack detection, and hardness survey of the damaged area
- Process monitoring: Real-time monitoring of pulse parameters, deposition rate, and electrode consumption
- Post-repair inspection: Full RT or UT examination, dimensional verification, and hardness mapping
- Service qualification: Proof load testing and, where applicable, fatigue testing of repair deposits
According to ASME PCC-2 (Repair and Alteration of Pressure Vessels and Piping) and similar standards, spark overlay repairs must be covered by a qualified repair procedure with appropriate destructive testing on coupon specimens.
Case Study: Turbine Shaft Bearing Journal Repair
A documented case involved repair of a 1200 MW steam turbine high-pressure shaft with bearing journal wear of 2.5 mm depth over a 150 mm length. The repair procedure involved:
- Removal of damaged material by grinding to sound metal
- Surface preparation by grinding to a 60-grit finish
- Spark overlay deposition using Ni-Cr-Mo electrode (equivalent to Stellite 6)
- Three passes to achieve 3.0 mm total overlay thickness
- Post-overlay machining to restore dimensional tolerance
- Final hardness verification: 42-48 HRC on overlay surface
The repaired shaft returned to service and operated for over 40,000 hours without recurrence of the original defect.
Key Questions and Reflections
One significant question raised by this study is the long-term fatigue performance of spark overlay repair deposits. While the fine microstructure and low residual stress are favorable for fatigue resistance, the actual fatigue life depends on the quality of the bond between the repair deposit and the base metal. The study suggests that bond strength testing (per ASTM A263/A264) should be performed on all critical repairs, with a minimum acceptable bond strength of 90% of the base metal tensile strength.
Another reflection concerns the operator skill requirements for spark overlay welding. Unlike automated processes, spark overlay requires significant operator experience to maintain consistent pulse parameters and achieve uniform deposition. The study recommends that operators undergo formal qualification testing, including deposition of test coupons with verified hardness and microstructure requirements.
Study Insights and Implications
The most valuable contribution of this literature is demonstrating that electric spark overlay welding offers a viable alternative to conventional welding for critical shaft repairs where minimal heat input and low dilution are paramount. The technique's ability to produce deposits with properties closely matching the electrode material specification makes it particularly suitable for overlay applications requiring specific alloy composition.
For engineering practice, I recommend that power generation companies develop in-house spark overlay welding capabilities for critical component repair. The investment in equipment and operator training is justified by the reduced downtime compared to conventional repair methods that require extensive preheating, PWHT, and post-repair machining. Additionally, the minimal HAZ and low distortion characteristics make spark overlay particularly suitable for precision-machined components where dimensional stability is critical.