Application of Electric Spark Overlay Welding in Repair of Critical Power Plant Equipment

Literature Overview

The paper by Wang Ruijun, Sun Ying, Huang Xia'ou, and Lu Jianhua (2003), published in Electric Power, presents case studies of electric spark overlay welding (also known as electric spark cladding or discharge plasma welding) applied to repair worn critical components in power plants. The authors are affiliated with the China Academy of Agricultural Mechanization Sciences and the Gansu Electric Power Testing Research Institute. Three distinct repair cases are documented: a 100 MW steam turbine main shaft journal with wear grooves at a Lanzhou power plant, a turbine casing sealing surface with erosion grooves at a Shandong power plant, and a hot network circulation pump main shaft with worn surfaces at a Taiyuan power plant. The paper emphasizes the unique advantages of electric spark overlay welding for precision repair of hardened and wear-resistant components where conventional arc welding is impractical.

Core Technical Features of Electric Spark Overlay Welding

Electric spark overlay welding is fundamentally different from conventional arc welding processes. The process operates by generating a controlled electrical discharge between a consumable electrode (typically a rod or wire of the desired overlay material) and the workpiece. The discharge creates a localized molten pool of extremely small volume, resulting in minimal heat input into the base material. This is the process's defining characteristic: the thermal input is orders of magnitude lower than that of SMAW, GTAW, or FCAW. As a direct consequence, residual stresses in the base material are negligible, and distortion is essentially eliminated. This makes the process uniquely suited for repairing hardened components such as turbine shafts, where pre-existing hardness from heat treatment must be preserved.

Case Study Analysis

The three repair cases described in the paper illustrate the versatility of the process across different component geometries and failure modes. The 100 MW turbine main shaft journal repair involved filling wear grooves on a hardened shaft surface. The low heat input of electric spark overlay welding ensured that the pre-existing hardness profile of the shaft was not compromised, which is critical for maintaining bearing load capacity. The turbine casing sealing surface repair addressed erosion damage caused by high-velocity steam flow; the process allowed precise filling of erosion grooves without warping the thin casing wall. The hot network circulation pump shaft repair dealt with a wetted surface subject to cavitation and abrasive wear; the overlay material selection and process parameters were tailored to restore both dimensional accuracy and surface hardness.

In all three cases, the repair was successful in restoring the components to serviceable condition, with the repaired areas performing comparably to the original material in subsequent operation. The paper notes that the application of electric spark overlay welding in the power industry for shaft-type critical component repair holds significant promise, given the process's ability to handle hardened surfaces without the need for preheating or post-weld tempering.

Engineering Practice Considerations

From a practical standpoint, electric spark overlay welding requires specialized equipment and operator skill. The consumable electrode must be carefully matched to the base material and the service environment. For turbine shaft repairs, materials such as high-carbon chromium steels or nickel-aluminum bronze overlays are commonly selected to provide enhanced wear and corrosion resistance. The process is inherently slow compared to conventional welding, and the build-up rate is limited, which means that deep or wide wear areas may require multiple passes and extended repair time. However, for precision repair of critical components where dimensional accuracy and preservation of base material properties are paramount, the slower rate is an acceptable trade-off.

Quality assurance for electric spark overlay welds typically involves visual inspection, dimensional verification, and hardness testing of the overlay layer. Since the process produces minimal dilution and no significant HAZ, the primary quality concern is the adhesion of the overlay layer to the base metal. Proper surface preparation, including thorough cleaning and removal of oxide layers, is essential to ensure metallurgical bonding. Engineers should also consider the long-term performance of the overlay under cyclic loading and thermal cycling conditions, as these are the dominant failure modes in turbine and pump service.

Study Insights and Implications

This paper serves as a practical reference for engineers evaluating repair options for critical power plant components. The key takeaway is that electric spark overlay welding fills a niche that conventional welding cannot address: precision repair of hardened, wear-damaged surfaces where thermal distortion and residual stress must be avoided. The documented success in three different repair scenarios across different power plants provides confidence in the process's reliability. For maintenance planning, the paper suggests that electric spark overlay welding should be considered as a first-line repair option for shaft journals, sealing surfaces, and wetted pump components, potentially extending component life and reducing unplanned outages. The process's ability to restore critical dimensions without disassembly or extensive machining makes it particularly valuable for minimizing downtime in power generation operations.