Electro-Spark Overlay Welding Application in Power Plant Critical Component Repair

Literature Overview

This paper by Wang Ruijun, Sun Ying, Huang Xiaou, and Lu Jianhua, published in 2003 in China Electric Power, documents the application of electro-discharge (spark) overlay welding technology for the repair of critical components in power plant equipment. The research was conducted collaboratively by the Surface Engineering Technology Research Institute of the Chinese Academy of Agricultural Mechanization Sciences and the Gansu Electric Power Test Research Institute. The study addresses the practical challenges of repairing worn or damaged power plant components such as turbine blades, valve seats, pump impellers, and generator rotor surfaces using electro-discharge overlay welding as a surface engineering technique.

Core Technical Content

Electro-discharge overlay welding (EDOW), also known as spark overlay welding or electric spark welding, is a solid-state bonding process that deposits material onto a base substrate through repetitive electric discharges between a consumable electrode and the workpiece. The process differs fundamentally from conventional arc welding in that the material transfer occurs through explosive plasma jets generated by rapid vaporization of the electrode tip during each discharge cycle.

The authors describe the application of EDOW for repairing specific power plant components, including turbine shaft journals, valve stem surfaces, pump wear rings, and generator bearing surfaces. The key advantages of EDOW for power plant repair include minimal thermal input (reducing distortion and residual stress), the ability to deposit hardfacing materials with low dilution, and the capability to repair components without complete disassembly.

Process Mechanism

The EDOW process operates on the principle of controlled electric discharge. The electrode is fed toward the workpiece at a controlled rate, and when the gap between the electrode tip and the workpiece surface reaches a critical distance (typically 0.05–0.5 mm), an electric discharge occurs. The discharge generates a plasma channel with temperatures exceeding 20,000 K, which rapidly vaporizes the electrode tip material. The vaporized material is propelled by the explosive expansion of the plasma onto the workpiece surface, where it solidifies as a deposited layer.

The process parameters include discharge current (typically 100–500 A), discharge frequency (50–500 Hz), electrode feed rate, and electrode-to-workpiece distance. These parameters directly control the deposit thickness, dilution rate, and microstructure of the overlay layer.

Microstructural Characteristics

The overlay layer produced by EDOW exhibits a distinctive microstructure characterized by a fine-grained, rapidly solidified matrix with embedded unmelted or partially melted particles from the electrode material. The rapid solidification rates (10^3–10^6 K/s) result in fine grain structures with reduced segregation compared to conventional arc welding. The dilution rate in EDOW is typically 5–15%, significantly lower than the 20–40% dilution rates observed in conventional arc welding overlay processes.

Process and Standards Analysis

The EDOW process is governed by standards including GB/T 19418 (Welding — Electro-discharge welding) and related surface engineering specifications. For power plant applications, the repair procedures must also comply with relevant equipment manufacturer specifications and industry standards such as API 579 (Fitness-for-Service) for pressure equipment assessment.

The key process parameters for EDOW in power plant repair applications are summarized below:

Quality Control Considerations

Quality control for EDOW overlay deposits includes visual inspection, dimensional measurement, hardness testing, and bond strength verification. The typical acceptance criteria for power plant component repair include:

• Deposit thickness: within ±0.5 mm of specified value
• Hardness: meeting minimum specified value (typically 40–60 HRC for hardfacing applications)
• Bond strength: minimum 200 MPa (shear test) or equivalent
• Surface quality: no cracks, porosity, or excessive spatter
• Dilution rate: verified through spectrometric analysis of the fusion boundary

Integration with Engineering Practice

The application of EDOW in power plant repair offers several practical advantages over conventional arc welding methods. First, the minimal thermal input reduces the risk of distortion and residual stress in precision components such as turbine shafts and generator rotors, where dimensional accuracy is critical. Second, the low dilution rate allows for the deposition of specialized overlay materials (such as Stellite, Hastelloy, or Inconel) without significant compositional contamination from the base material.

In a documented case study, a turbine shaft journal in a coal-fired power plant boiler feed pump was repaired using EDOW with a Cr-based hardfacing electrode. The original shaft exhibited wear grooves of 0.8 mm depth, which were repaired by building up the surface to the original diameter with an EDOW deposit of 1.2 mm thickness. The deposit hardness was measured at 52 HRC, exceeding the base material hardness of 32 HRC. Post-repair service monitoring showed no recurrence of wear for over 12,000 hours of continuous operation.

Another application involved the repair of a valve seat in a high-pressure steam valve. The valve seat, originally made of carbon steel, exhibited erosion damage after 8,000 hours of service. EDOW was used to deposit a 2.0 mm layer of Stellite-type alloy on the valve seat surface, followed by precision machining to the required seating geometry. The repaired valve demonstrated improved sealing performance and extended service life compared to the original uncladded valve seat.

Key Questions and Reflections

A critical question regarding EDOW application in power plant repair is the long-term durability of the overlay deposit under cyclic thermal and mechanical loading. Power plant components experience significant thermal cycling (startup/shutdown cycles, load following) and mechanical fatigue (rotating machinery vibrations). The rapidly solidified microstructure of EDOW deposits, while initially fine-grained, may undergo coarsening and phase transformation under prolonged thermal exposure. The dilution zone at the fusion boundary is also a potential site for crack initiation under cyclic loading.

Another important consideration is the repairability of EDOW deposits. If a component repaired with EDOW requires subsequent repair, the interface between the original EDOW deposit and the new overlay layer may exhibit different properties than the original deposit. The composition and microstructure of the re-melted interface zone depend on the process parameters of the second repair, and may not match the original deposit properties. This cumulative repair effect should be evaluated for components with multiple repair histories.

Study Insights and Implications

The work by Wang et al. demonstrates the practical viability of EDOW for power plant component repair, providing documented case studies with quantifiable performance data. The technology offers a valuable alternative to conventional arc welding for applications where thermal input must be minimized, dilution must be controlled, and precision deposition is required.

From a standards perspective, EDOW repair procedures should be qualified under NB/T 47014 or equivalent welding procedure qualification standards, with specific attention to the unique process parameters and their effects on deposit properties. The qualification coupon should include both the base material and the overlay material, with mechanical testing performed on the fusion boundary and the deposit interior.

The findings have significant implications for the maintenance and repair practices of power plants, where component availability and repair time directly impact plant availability and operating costs. EDOW offers a rapid, on-site repair capability that can reduce component downtime and extend service life, contributing to improved plant reliability and economic performance. Engineers should consider EDOW as a viable repair option when evaluating repair strategies for critical power plant components, particularly for applications where conventional arc welding is not suitable due to thermal sensitivity or dilution concerns.