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

Literature Overview

This paper by Wang Ruijun, Xu Lin, Zhang Tianjian, and Huang Xiaou (2003), published in New Technology and New Process, reports on the successful application of spark overlay welding (also known as spark-erosion surfacing or electric spark surfacing) for repairing the sealing surfaces of a steam turbine casing at a power plant in Shandong Province, China. The research was conducted at the Surface Engineering Technology Research Institute of the China National Machinery Industry Corporation. Although the paper is relatively brief, it documents a practical and economically significant repair application that is directly relevant to power plant maintenance engineers dealing with turbine casing damage.

Technical Background and Process Description

The Problem: Steam Turbine Casing Sealing Surface Damage

Steam turbine casings operate under extreme conditions involving high-temperature steam (typically 350–600 °C depending on the turbine stage), high pressure, and continuous mechanical stress from rotor vibration. The sealing surfaces of the casing are critical components that must maintain precise dimensional accuracy to prevent steam leakage between the casing and the rotor. Damage to these surfaces can result from:

• Thermal fatigue cracking due to repeated start-up and shutdown cycles
• Erosion from high-velocity steam particles
• Mechanical damage during assembly or maintenance
• Corrosion from high-temperature steam oxidation

Spark Overlay Welding Process Characteristics

Spark overlay welding is a specialized surface engineering technique that differs fundamentally from conventional arc welding processes. The process involves:

The key advantage of spark overlay welding is the extremely localized and controlled heat input. Each pulse melts only a small volume of the base metal surface and the electrode material, creating a thin, well-bonded overlay deposit with minimal thermal distortion. This makes the process particularly suitable for repairing precision surfaces such as turbine casing sealing faces, where dimensional accuracy is critical.

Application to Steam Turbine Casing Repair

Repair Procedure

The repair of the steam turbine casing sealing surface using spark overlay welding typically involves the following steps:

1. Surface preparation: The damaged area is ground to remove all oxide scale, corrosion products, and damaged material. The surface is cleaned to expose fresh, sound metal.
2. Dimensional assessment: The original surface geometry and sealing clearance are documented to establish repair targets.
3. Spark overlay application: Alloy material is deposited onto the prepared surface using the spark overlay welding process. The electrode material is selected to match or exceed the properties of the original casing material.
4. Machining and finishing: The overlay deposit is machined to the required dimensional accuracy and surface finish.
5. Inspection and verification: The repaired surface is inspected for dimensional accuracy, metallurgical bonding quality, and any defects.

Why Spark Overlay Welding is Suitable for This Application

The selection of spark overlay welding for steam turbine casing repair is justified by several technical advantages:

• Minimal thermal distortion: The low heat input per pulse prevents warping of the casing, which is critical for maintaining the precise rotor-to-casing clearance.
• Low dilution: The thin overlay deposit has minimal base metal dilution, ensuring the overlay retains its intended composition and properties.
• Excellent bonding: The repeated melting and solidification cycles create a metallurgical bond with the base metal that is superior to mechanical bonding methods such as brazing or adhesive repair.
• Repair of localized damage: The process can be applied to specific damaged areas without affecting the surrounding undamaged surface.
• Material flexibility: Various alloy compositions can be used as electrode material, allowing the repair material to be tailored to the specific service conditions.

Economic and Engineering Benefits

The paper reports that the spark overlay welding repair achieved good economic benefits. For power plant operators, the economic advantages of in-situ repair using spark overlay welding include:

Key Questions and Reflections

While the paper documents a successful repair application, several technical details that would be valuable for engineers replicating this approach are not provided. The specific electrode material composition, the pulse parameters used, the number of overlay passes applied, and the post-repair inspection results are not detailed in the abstract. For a comprehensive engineering reference, this information would be essential.

Additionally, the long-term performance of the spark overlay weld in the high-temperature, high-pressure environment of a steam turbine should be monitored. While the process produces low residual stresses and good metallurgical bonds, the overlay deposit may be susceptible to thermal fatigue cracking over extended service periods, particularly if the overlay material has different thermal expansion characteristics from the base metal.

Study Insights and Implications

This paper serves as a practical case study demonstrating the applicability of spark overlay welding for critical power plant component repair. The technique's ability to restore damaged sealing surfaces with minimal thermal impact makes it an attractive option for maintaining large rotating equipment where dimensional accuracy is paramount. For maintenance engineers, the key takeaway is that spark overlay welding offers a viable alternative to component replacement for localized surface damage, provided that the process parameters are carefully controlled and the repaired area is properly inspected. The economic benefits of avoiding full component replacement, combined with the technical advantages of low distortion and excellent bonding, make this technique a valuable addition to the power plant maintenance toolkit.