High-Temperature Wear-Resistant Overlay Welding Electrodes for Power Station Valve Sealing Surfaces

Literature Overview

The research by Yang Genxi, Shi Duanhu, Yin Yousheng, and Huang Chuanhui, published in the Journal of Hefei University of Technology (Natural Science Edition) in 2010, addresses the development of specialized overlay welding electrodes for power station valve sealing surfaces operating at elevated temperatures. This work, conducted jointly by Xuzhou Institute of Technology and Shenyang University of Technology, responds to a critical industry need: valve seats and sealing surfaces in power generation equipment suffer severe wear and erosion at operating temperatures often exceeding 400 degrees Celsius, leading to frequent maintenance outages and safety concerns.

Core Technical Content

Power station valves, particularly those in steam lines, feedwater systems, and turbine bypass circuits, operate under extreme conditions combining high temperature, high pressure, and erosive flow. The sealing surfaces must maintain dimensional accuracy and surface integrity over extended service periods. Conventional overlay materials often fail at elevated temperatures due to softening of the binder phase, oxidation of carbides, and thermal fatigue cracking.

Design Requirements for High-Temperature Overlay Electrodes

Alloy Design Strategy

The electrode composition must balance multiple competing requirements. Hardness at elevated temperatures requires stable carbide phases that resist softening and coarsening. Chromium-rich carbides such as Cr7C3 and Cr3C2 provide excellent thermal stability but may be too brittle. Molybdenum and tungsten additions enhance high-temperature strength but increase cost. The binder phase must retain sufficient toughness to accommodate thermal expansion differences and prevent cracking during thermal cycling.

A typical alloy design for this application might include:

• Carbon: 4.0-5.5 wt% for carbide formation
• Chromium: 12-18 wt% for oxidation resistance and carbide stability
• Molybdenum: 3-6 wt% for high-temperature strength
• Nickel: 2-5 wt% for toughness improvement and dilution compensation
• Silicon: 1-2 wt% for deoxidation and minor hardening

Process Analysis and Welding Considerations

Electrode Coating Design

The electrode coating must be formulated to provide a stable arc, proper slag protection, and controlled dilution. For high-carbon, high-alloy compositions, the coating often includes ferroalloy additions to compensate for burn-off losses. The coating thickness and composition directly influence the chemical composition of the deposited layer.

Welding Process Parameters

The welding process for valve sealing surfaces typically employs manual shielded metal arc welding (SMAW) or submerged arc welding (SAW). Manual welding provides flexibility for complex geometries, while SAW offers higher deposition rates for large valve seats. Key parameters include:

• Current: 100-200 A (SMAW), depending on electrode diameter
• Arc voltage: 22-28 V (SMAW)
• Deposition rate: 0.5-2.0 kg/h (SMAW)
• Layer thickness: 3-6 mm per pass
• Interpass temperature: < 150 degrees C to minimize cracking
• Number of passes: 2-4 layers for final thickness of 6-12 mm

Quality Control and Inspection

Quality assurance for high-temperature overlay welds on valve sealing surfaces is critical. The inspection regime should include:

1. Visual examination: Check for surface cracks, undercuts, and excessive reinforcement
2. Magnetic particle testing (MT): Detect surface and near-surface cracks in the overlay and heat-affected zone
3. Ultrasonic testing (UT): Verify bond strength at the overlay-base interface and detect internal defects
4. Hardness testing: Profile hardness across the overlay thickness and verify values meet specifications
5. Metallographic examination: Verify microstructure, carbide distribution, and absence of interfacial cracks
6. Chemical analysis: Confirm overlay composition meets specification requirements
7. Thermal cycling test: Subject coupon samples to simulated service thermal cycles and verify crack resistance

Engineering Practice and Field Application

In power station maintenance practice, valve overlay repair is typically performed during scheduled outages. The procedure involves grinding the worn sealing surface, cleaning to bare metal, applying the overlay in multiple passes, and then machining to the final dimensional specification. The critical challenge is achieving a flat, smooth sealing surface with uniform hardness across the entire sealing area.

Post-overlay machining is essential for achieving the required surface finish and dimensional accuracy. The overlay material must be machinable despite its high hardness. Grindability is influenced by carbide size and distribution; excessively large or coarse carbides can damage grinding wheels and produce poor surface finishes. Optimizing the carbide morphology through controlled solidification is therefore important from a manufacturing perspective.

Study Insights and Implications

This research addresses a genuine and persistent problem in power generation maintenance. The development of specialized electrodes that maintain wear resistance at elevated temperatures represents a significant advance over generic hardfacing consumables. The systematic approach to alloy design, combining thermal stability requirements with practical welding considerations, provides a model for developing other specialized overlay materials for extreme service conditions.

Engineers should note that the performance of any overlay material is highly dependent on the quality of the welding process and post-weld treatment. Even an optimally designed electrode can underperform if applied with improper technique. Therefore, welding procedure qualification (WPQ) and welder performance qualification (WPQ) in accordance with applicable standards such as NB/T 47014 or ASME IX are essential prerequisites for production use.