Electric Spark Cladding Repair of Power Plant Steam Turbine Casing

Literature Overview

This study documents the application of electric spark cladding (ESC) technology for the repair of damaged steam turbine casings in power plants. Steam turbine casings are critical components that experience high temperatures, thermal cycling, and mechanical stresses during operation. Damage to casing surfaces can result from thermal fatigue cracking, corrosion, erosion, or manufacturing defects. The study evaluates the effectiveness of ESC as a repair method, examining the metallurgical quality, mechanical properties, and service performance of the repaired areas.

Core Technical Findings

Electric spark cladding, also known as electric spark overlay or spark welding, is a process that uses repeated electric arc discharges to transfer small amounts of electrode material onto the substrate surface. Each spark creates a small molten pool that rapidly solidifies, producing a layer of overlay material with a unique microstructure. The process is characterized by low heat input per cycle, high deposition rates, and the ability to repair large areas relatively quickly.

Process Parameters

Parameter	Typical Range	Effect on Repair Quality
Arc voltage	20–40 V	Controls spark energy and penetration
Current	50–200 A	Affects deposition rate and dilution
Electrode feed rate	0.5–3 m/min	Controls layer thickness and composition
Electrode material	Ni-based, Co-based, Fe-based	Determines overlay properties
Layer thickness	0.1–0.5 mm per pass	Multiple passes for thicker builds
Travel speed	50–200 mm/min	Affects overlap and uniformity
Substrate preheat	100–250 °C	Reduces cracking tendency

The study used nickel-based electrodes (similar to Inconel 625 composition) for repairing the steam turbine casing, which was made of low-alloy steel (ASTM A217 or similar). The overlay layer was built up in multiple passes to achieve a total thickness of 1.5–3 mm, depending on the depth of the damage being repaired.

Microstructural Characteristics

The ESC overlay layer exhibited a unique microstructure characterized by fine, equiaxed grains with a size of 10–30 μm, which is significantly finer than what is typically observed in conventional arc welding overlays. The rapid solidification rate of each spark (estimated at 10³–10⁴ K/s) promotes this fine grain structure. The overlay also contained dispersed carbides and intermetallic phases that contribute to the hardness and wear resistance of the layer.

The dilution at the interface between the ESC overlay and the substrate was measured at 8–15%, which is comparable to laser cladding and significantly lower than conventional arc welding (20–40%). This low dilution is achieved because each spark deposits a small amount of material with minimal interaction with the underlying layer.

Mechanical Properties

Property	ESC Overlay	Substrate (Low-Alloy Steel)	Conventional GTAW Overlay
Hardness (HV)	320–420	180–220	280–350
Tensile strength (MPa)	650–780	450–550	550–650
Elongation (%)	12–18	15–22	10–15
Intergranular corrosion resistance	Excellent	Poor	Good
Thermal fatigue resistance	Excellent	Moderate	Good

The ESC overlay demonstrated excellent thermal fatigue resistance, which is critical for steam turbine casing applications where the component undergoes repeated heating and cooling cycles during startup, load changes, and shutdown. The fine grain structure and low dilution contribute to this superior performance.

Non-Destructive Testing Results

The repaired areas were inspected using multiple NDT methods:

NDT Method	Inspection Result	Acceptance Criteria
Visual examination (VT)	No visible cracks or defects	No cracks, porosity, or lack of fusion
Magnetic particle testing (MT)	No indications	No linear indications > 1 mm
Penetrant testing (PT)	No indications	No linear indications > 0.5 mm
Ultrasonic testing (UT)	No delamination or cracks	No reflections from defects
Hardness mapping	Uniform hardness within range	Within 250–450 HV

All inspection results met the acceptance criteria specified in the applicable codes and standards, confirming the quality of the ESC repair.

Engineering Practice Implications

For power plant maintenance programs, ESC offers a practical solution for repairing steam turbine casings without requiring the entire component to be replaced. The process can be performed on-site or in a workshop, and the equipment is relatively compact compared to laser cladding systems. The deposition rate of ESC (0.5–2 kg/h) is significantly higher than laser cladding (0.1–0.5 kg/h), making it more economical for large-area repairs.

Application Scenarios

Damage Type	ESC Suitability	Recommended Electrode	Notes
Thermal fatigue cracking	Excellent	Ni-based (Inconel 625 type)	Remove cracks first by grinding
Corrosion pitting	Good	Ni-based or Co-based	Fill pits with overlay
Erosion damage	Good	Co-based (Stellite type)	Harder overlay for erosion
Manufacturing defects	Excellent	Ni-based	Can build up significant thickness
Wear damage	Excellent	Co-based or Fe-based	Hardfacing for wear resistance

Defect Analysis and Countermeasures

Defect	Cause	Detection	Countermeasure
Cracking in overlay	High residual stress, incompatible electrode	MT, PT	Preheat, use compatible electrode, reduce current
Excessive dilution	High current, slow travel speed	Metallography, EDS	Reduce current, increase speed
Poor adhesion	Surface contamination, oxide	UT (tapping)	Clean surface thoroughly before cladding
Non-uniform layer thickness	Inconsistent travel speed	Visual, thickness gauge	Use automated feed and travel control
Crater formation	Inconsistent arc parameters	Visual, UT	Stabilize power supply, maintain electrode alignment

Key Questions and Reflections

A key question is the long-term durability of ESC repairs under actual service conditions. The study provides excellent laboratory and short-term service data, but the long-term performance over multiple thermal cycles and years of operation needs to be validated through field experience. Engineers should consider implementing a monitoring program for ESC-repaired casings, including periodic NDT inspections and hardness checks, to track the performance of the repair over time.

Another consideration is the qualification of ESC as a repair process according to applicable codes. ASME Section VIII Div. 1 and Div. 2 have specific requirements for repair procedures, and ESC may not be explicitly covered in all code editions. Engineers need to verify code compliance and may need to develop a qualified repair procedure with appropriate performance qualifications.

Study Insights and Reference Value

This literature demonstrates that electric spark cladding is a viable and effective method for repairing steam turbine casings in power plants. The combination of low dilution, fine microstructure, excellent thermal fatigue resistance, and high deposition rate makes ESC a compelling alternative to conventional repair methods. The comprehensive NDT results confirm that the repair quality meets code requirements, providing confidence for engineering application. The practical guidance on process parameters, electrode selection, and defect prevention provides engineers with actionable information for implementing ESC repairs in their own facilities.