Development of High-Temperature Wear-Resistant Overlay Electrodes for Power Plant Valve Sealing Surfaces

Literature Overview

Power plant valves — particularly steam turbines, feedwater valves, and control valves operating at temperatures ranging from 200 °C to 600 °C — face severe combined wear and corrosion conditions. The sealing surfaces of these valves must maintain dimensional accuracy, sealing integrity, and wear resistance over thousands of operating cycles. The literature reviewed addresses the development of specialized overlay welding electrodes designed specifically for these demanding applications, combining high-temperature strength, oxidation resistance, and tribological durability.

Core Technical Viewpoints

The fundamental challenge in developing high-temperature wear-resistant overlay electrodes for power plant valves is achieving a balance between hardness (for wear resistance), toughness (to prevent cracking under thermal cycling), and oxidation resistance (to survive steam and hot gas environments). Conventional hard-facing electrodes based on carbide particles (WC, Cr₃C₂) often suffer from thermal softening above 400 °C and are prone to spalling under thermal fatigue. The developed electrode composition addresses these limitations through a matrix alloy approach with controlled carbide precipitation.

Electrode Composition Design

Component	Content (wt%)	Function
Base Matrix	Fe-Cr-Ni-C	Structural integrity and weldability
Chromium	20–30%	Oxidation resistance, solid solution strengthening
Molybdenum	4–8%	High-temperature strength, pitting resistance
Vanadium	2–5%	Carbide formation, wear resistance
Tungsten	1–3%	High-temperature hardness retention
Carbon	0.8–1.5%	Carbide precipitation for hardness
Nickel	5–10%	Toughness, hot cracking resistance

Performance Characteristics

Property	Typical Value	Test Condition
Hardness (as-welded)	45–55 HRC	Room temperature
Hardness at 500 °C	38–45 HRC	After 1000 h exposure
Hot hardness retention	≥85% of RT value	At 600 °C
Wear rate (pin-on-disk)	1–3 × 10⁻⁴ mm³/N·m	Against steel counterface
Thermal cycling resistance	>500 cycles	200 °C to 600 °C
Cracking resistance	No cracks after 500 cycles	Thermal fatigue test

Microstructural Analysis

The overlay deposit microstructure consists of a tough austenitic-ferritic matrix with uniformly distributed fine carbides (primarily M₇C₃ and M₂C type carbides containing V, W, and Cr). The grain size is controlled to be fine (ASTM 8–10) to provide both strength and toughness. Metallographic examination reveals no sigma phase or intermetallic precipitates after prolonged exposure at 600 °C for 1000 hours, indicating excellent high-temperature stability.

Process Parameters and Application

The electrode is designed for manual SMAW (shielded metal arc welding) application with the following recommended parameters:

Parameter	Value
Electrode diameter	3.2 mm, 4.0 mm
Welding current	90–130 A (3.2 mm), 140–180 A (4.0 mm)
Arc voltage	22–28 V
Preheat temperature	150–250 °C
Interpass temperature	≤250 °C
Overlay thickness	3–6 mm (typically 2–3 passes)
Post-weld treatment	Stress relief at 650–700 °C for 2 h

The electrode is particularly suitable for overlaying on carbon steel and low-alloy steel valve bodies (e.g., WCB, LCB, A182 F91). The bond strength to these substrates exceeds 250 MPa in shear testing, well above the minimum requirement of 200 MPa specified in ASME Section IX.

Defect Prevention and Quality Control

Potential Defect	Prevention Strategy
Hot cracking	Control S and P in electrode coating, maintain interpass temperature
Cold cracking	Adequate preheat, low hydrogen electrode, controlled cooling
Incomplete fusion	Proper surface preparation, adequate current, correct travel speed
Excessive dilution	Multi-pass overlay, use of transition layer if needed
Surface porosity	Proper electrode drying (300 °C for 2 h), clean workpiece

Engineering Practice and Validation

The developed electrode has been validated through field trials on power plant control valves operating at 550 °C steam conditions. The overlay life was extended from approximately 6 months (with conventional hard-facing) to over 24 months, representing a fourfold improvement. No spalling, cracking, or excessive wear was observed during the extended service period. The overlay surface maintained a roughness of Ra 3.2–6.3 μm after grinding, which is suitable for valve seat sealing requirements.

Study Insights and Implications

The most significant insight from this study is the demonstration that high-temperature wear resistance can be achieved through matrix alloy design rather than relying solely on hard carbide particles. This approach provides superior toughness and thermal fatigue resistance compared to traditional carbide-based hard-facing materials. The controlled precipitation of fine M₇C₃ carbides provides effective wear resistance while maintaining the ductility necessary to withstand thermal cycling.

From a practical standpoint, the development of such specialized electrodes requires close collaboration between materials scientists, welding engineers, and field operators. The feedback loop between laboratory testing and field performance is essential for optimizing the electrode composition and coating formulation. Engineers should note that electrode storage and handling conditions significantly affect performance — proper drying procedures and storage at controlled humidity are non-negotiable for maintaining low-hydrogen characteristics.