Study on High-Temperature Wear Resistance of Valve Sealing Surface Overlay Materials

Literature Overview

This 1996 study published in the Journal of Shanghai Jiao Tong University by Yao Shoushan, Lu Hao, Zhu Yanping, Hu Wenzheng, Gu Pujin, and Pan Dayou from the Department of Materials Engineering at Shanghai Jiao Tong University and Shanghai Power Plant Auxiliary Machinery Factory addresses a critical engineering problem in the power generation industry: the degradation of valve sealing surfaces under high-temperature operating conditions. The research was conducted in direct collaboration with industry, reflecting the applied engineering orientation of the work.

Core Technical Content

Valve Sealing Surface Operating Environment

Steam valves in power plants operate under extreme conditions: temperatures ranging from 350°C to 600°C depending on the valve position in the steam cycle, pressures up to 25 MPa, and cyclic thermal loading. The sealing surfaces experience a combination of abrasive wear from steam-borne particulates, adhesive wear from metal-to-metal contact during opening and closing, and thermal degradation of the overlay material. The combination of these mechanisms makes conventional hardfacing materials inadequate for long-term service.

Materials Evaluated

The study evaluates several candidate overlay materials for valve sealing surfaces, including:

• Cast iron-based hardfacing alloys (high-carbon, high-silicon compositions)
• Cobalt-based hardfacing alloys (Stellite-type)
• Nickel-based overlay alloys
• Manganese bronze and other copper-based alloys

The cobalt-based alloys (analogous to Stellite 6 or domestic CoCrW alloys) demonstrated superior high-temperature wear resistance, maintaining hardness values above 300 HV at 600°C, while cast iron-based materials experienced significant softening above 500°C.

High-Temperature Wear Mechanisms

The study identifies three dominant wear mechanisms at elevated temperatures:

1. Thermally activated abrasive wear: At temperatures above 400°C, the matrix material softens, reducing the embedment resistance of hard carbide particles. The carbides become more susceptible to ploughing and fracture.
2. Oxidative wear: At temperatures above 500°C, rapid oxidation of the overlay surface forms a brittle oxide layer that spalls off, exposing fresh material to continued oxidation and material loss. The presence of chromium in cobalt-based alloys is critical for forming a protective Cr2O3 scale.
3. Thermal fatigue cracking: Repeated thermal cycling induces residual stresses in the overlay layer, leading to microcracking that accelerates wear through material removal along crack paths.

Microstructural Stability at Elevated Temperatures

A key finding of the study is that the microstructural stability of the overlay material at operating temperature is more important than room-temperature hardness for predicting long-term wear performance. Cobalt-based alloys maintain their metastable carbide structure (Co3W, Co2W, and Cr7C3) at elevated temperatures due to the high melting point of cobalt and the thermodynamic stability of the carbide phases. In contrast, cast iron-based alloys experience graphitization and pearlite decomposition above 500°C, leading to rapid loss of hardness and wear resistance.

Process Considerations for Valve Seat Overlay

The application of overlay welding to valve seat surfaces presents unique challenges:

• Geometry constraints: The sealing surface is often a narrow band (10–20 mm wide) on a curved surface, requiring precise weld deposition.
• Heat input control: Excessive heat input can cause distortion of the valve body and compromise the dimensional accuracy of the sealing surface.
• Post-weld machining: The overlay layer typically requires machining to achieve the required surface finish (Ra ≤ 0.8 μm for sealing surfaces), which means the overlay must be applied with sufficient excess thickness (typically 2–3 mm).
• Bond strength: The overlay must remain firmly bonded to the valve body material (typically carbon steel or low-alloy steel) throughout the thermal cycling service life.

Engineering Practice Integration

For power plant valve manufacturers, this study provides the technical basis for material selection in high-temperature service. The recommended approach is to use cobalt-based hardfacing alloys (such as CoCrW-119 or Stellite 6 equivalents) for valves operating above 500°C, with careful attention to the welding procedure to minimize dilution and ensure adequate bond strength. For valves operating below 450°C, nickel-based or high-silicon cast iron overlays may be acceptable with proper qualification.

The study's emphasis on the industry-academia collaboration model is noteworthy: the direct involvement of Shanghai Power Plant Auxiliary Machinery Factory ensured that the research addressed real engineering problems rather than purely academic questions. This approach remains a model for applied materials research.

The practical lesson for engineers is that high-temperature wear resistance cannot be evaluated solely by room-temperature hardness or wear tests. Elevated-temperature wear testing and thermal cycling qualification are essential for reliable material selection in power plant applications.