High Temperature Adhesive Wear Resistance of Valve Sealing Surface Overlay Materials

Literature Overview

This study by Yao Shoushan et al. from Shanghai Jiao Tong University and Shanghai Power Plant Auxiliary Machinery Factory (published in 1996, Journal of Shanghai Jiao Tong University, Vol. 30, No. 8, pp. 120-124) investigates the high-temperature adhesive wear behavior of four overlay materials applied to valve sealing surfaces. The materials examined include cobalt-based Stellite D802 electrode, nickel-based NDG-2 powder, D547Mo electrode, and D507Mo electrode. The wear testing was conducted from room temperature up to 600°C using an adhesive wear configuration. This work is particularly relevant to engineers working on power plant valve maintenance and metallurgical design of sealing surfaces subjected to thermal cycling and sliding contact.

Core Technical Content

The researchers evaluated the wear resistance of four overlay systems under varying conditions of counterface material, contact configuration, and test temperature. The fundamental finding is that the combination of overlay material, counterface material, and test temperature collectively govern the wear performance, and no single variable dominates the outcome independently.

Test Materials and Configuration

Material	Type	Typical Application	Key Alloying Elements
D802 (Stellite)	Co-based electrode	High-temp sealing surfaces	Co, Cr, W, Mo
NDG-2	Ni-based powder	Thermal barrier overlays	Ni, Cr, B, Si
D547Mo	Ni-based electrode	Wear-resistant coatings	Ni, Cr, Mo
D507Mo	Ni-based electrode	General overlay welding	Ni, Cr, Mo

Key Findings and Wear Mechanisms

The adhesive wear mechanism at elevated temperatures involves several interacting factors:

Temperature effect: As temperature increases from room temperature to 600°C, the hardness of all overlay materials decreases, reducing their resistance to adhesive transfer. The softening of the matrix phase accelerates material removal through micro-ploughing and micro-cutting mechanisms.
Counterface material influence: The selection of the counterface (typically the mating valve seat or disc) critically affects the severity of adhesive wear. A dissimilar counterface generally reduces cold welding and adhesion compared to a similar material pair.
Microstructural evolution: High-temperature metallographic and electron microscopy analysis revealed that the Co-based Stellite D802 retains superior wear resistance at elevated temperatures due to the stability of its face-centered cubic (FCC) matrix and the presence of dispersed carbides (Cr7C3, WC, Mo2C). The Ni-based systems exhibit greater susceptibility to oxidation-induced softening above 400°C.
Oxidation-assisted wear: At temperatures above 400°C, oxide scale formation on the wear surface plays a dual role—protective in some cases (when a stable oxide layer forms) and detrimental in others (when the oxide is brittle and spalls off, exposing fresh metal for adhesion).

Engineering Practice Integration

In power plant service, valve sealing surfaces experience temperatures ranging from 200°C to 600°C depending on the service location (feedwater valves, main steam valves, reheater outlet valves). The selection of overlay material must balance the following considerations:

Co-based Stellite D802 is the preferred choice for main steam and hot reheat valves operating above 500°C due to its excellent hot hardness retention and resistance to galling. However, cost and welding consumable availability may limit its use in lower-temperature applications.
Ni-based D547Mo offers a good compromise between cost and performance for valves operating in the 300-500°C range. The Mo addition improves hot hardness compared to plain D507Mo.
NDG-2 powder overlay provides excellent corrosion resistance but may be less suitable for severe adhesive wear conditions due to its relatively lower hardness at elevated temperatures.

Key Questions and Reflections

This study raises important questions for modern engineering practice:

Multi-pass overlay optimization: The wear performance reported assumes a standard overlay configuration. In practice, multi-pass overlay with graded compositions (harder surface layer, tougher transition layer) may provide superior performance. The study does not address this, which is a significant gap for engineering application.
Wear rate quantification: While the study discusses wear mechanisms qualitatively, quantitative wear rate data (mm³/N·m or mg/km) would be more directly applicable to life prediction models used in maintenance planning.
Thermal cycling effects: The study examines steady-state elevated temperature wear but does not address thermal cycling fatigue, which is prevalent in power plant valves experiencing frequent start-stop operations.
Counterface hardness matching: The optimal hardness ratio between overlay and counterface (typically 1:1 to 3:1) should be specified in valve design specifications. This study provides some guidance but does not establish a definitive ratio.

Study Insights and Implications

The fundamental insight from this work is that high-temperature wear resistance cannot be assessed by hardness alone. The combination of hot hardness, oxidation resistance, and microstructural stability under sliding contact determines the actual wear life of valve sealing surfaces. For engineers involved in valve refurbishment or new valve specification, this study reinforces the importance of matching the overlay material to the actual service temperature and counterface material. The Co-based Stellite alloys remain the benchmark for high-temperature sealing applications, while Ni-based systems offer cost-effective alternatives for moderate temperature service. The emphasis on adhesive wear mechanisms also highlights the importance of surface finish and lubrication strategies in reducing wear severity.