Spark Overlay Welding Repair of Power Plant Steam Turbine Casing Sealing Surface

Literature Overview

This technical report, published in "New Technology and New Process" in 2003 (No. 1, pp. 33-34), was authored by Wang Ruijun, Xu Lin, Zhang Tianjian, and Huang Xiao'ou from the Surface Engineering Technology Research Institute of the Chinese Academy of Agricultural Machinery Sciences. The study documents the successful application of spark overlay welding (also known as electric spark surfacing or electrical discharge surface alloying) for repairing the sealing surface of a steam turbine casing at a power plant in Shandong Province, China.

Technical Background

Spark overlay welding is a non-traditional surface engineering technique that uses controlled electrical discharges to transfer electrode material onto a substrate surface. Unlike conventional arc welding processes, spark overlay welding operates at lower heat input levels, resulting in minimal thermal distortion and minimal dilution of the deposited layer with the base metal. This makes it particularly suitable for repairing precision-machined surfaces where dimensional accuracy and low thermal stress are critical requirements.

Steam turbine casings in power plants are subjected to extreme operating conditions including high temperatures (up to 600 degrees Celsius for supercritical units), high pressures, cyclic thermal loading, and corrosive steam environments. The sealing surfaces of turbine casings are precision-machined to tight tolerances and are susceptible to wear, corrosion, and thermal fatigue cracking. When these surfaces become damaged, traditional repair methods often require disassembly, resurfacing, and re-machining, which is extremely costly and time-consuming.

Process Description and Application

The spark overlay welding process used in this repair involved the following key characteristics:

Electrode material selection matched to the turbine casing alloy composition and service environment
Controlled discharge parameters (voltage, current, pulse frequency) to achieve uniform deposition
Minimal heat input to prevent thermal distortion of the casing
Post-deposition machining to restore the sealing surface to original dimensional specifications

The successful repair of the steam turbine casing sealing surface at the Shandong power plant demonstrated the economic viability of spark overlay welding as a repair technique. The cost savings compared to conventional replacement or extensive machining procedures were significant, making this approach attractive for power plant maintenance operations.

Comparative Analysis of Repair Methods

Repair Method	Heat Input	Dilution	Distortion Risk	Dimensional Control	Cost
Spark overlay welding	Very low	Minimal	Very low	Excellent	Low
Conventional arc welding	High	Moderate to high	High	Poor	Moderate
Laser cladding	Low	Low	Low	Good	High
HVOF spraying	Very low	None	Very low	Good	High
Full replacement	N/A	N/A	N/A	N/A	Very high

Spark overlay welding occupies a favorable position in this comparison, offering low heat input, minimal dilution, and low cost, making it an attractive option for on-site repair of critical power plant components.

Engineering Practice Considerations

For power plant maintenance engineers, spark overlay welding offers several advantages for turbine casing repair:

On-site applicability: The equipment can be transported to the repair location, avoiding the need to transport large casing components to a workshop
Minimal disassembly: The process can be applied with the component largely in place, reducing downtime
Dimensional restoration: The deposited layer can be machined to precise tolerances, restoring the sealing surface geometry
Thermal compatibility: Low heat input prevents thermal distortion that could affect the fit and alignment of casing halves

However, engineers must also consider limitations including the relatively thin deposit thickness achievable per pass, the need for multiple passes to build up sufficient material, and the requirement for precise control of discharge parameters to achieve uniform surface quality.

Study Insights and Reflections

This case study demonstrates the practical value of spark overlay welding in power plant maintenance. The economic benefits achieved through the successful repair of the steam turbine casing are compelling evidence for the adoption of this technology in power plant repair programs. The technique's ability to restore precision surfaces with minimal thermal impact makes it uniquely suited for critical rotating equipment where dimensional accuracy directly affects performance and safety.

The broader implication is that surface engineering technologies should be considered as viable alternatives to component replacement in power plant maintenance. As power plants extend the operational life of aging equipment, the availability of effective repair techniques becomes increasingly important. Spark overlay welding, with its combination of low heat input, material flexibility, and economic efficiency, represents a valuable tool in the maintenance engineer's toolkit for extending component service life.