High-Temperature Wear Resistance of Valve Sealing Surface Cladding Materials

Literature Overview

This study, published in the Journal of Shanghai Jiao Tong University in 1996 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 in collaboration with Shanghai Power Station Auxiliary Machinery Factory, addresses a critical engineering challenge in power generation equipment: the degradation of valve sealing surfaces under high-temperature service conditions. The research is particularly relevant to steam turbine valves, control valves, and safety valves operating in coal-fired power stations where temperatures routinely exceed 500°C and where abrasive particles from steam carryover or combustion products accelerate surface degradation.

The core motivation for this work stems from the observation that conventional valve seat materials, whether carbon steel or standard austenitic stainless steels, suffer from rapid wear at elevated temperatures due to thermal softening, oxidation-assisted wear, and thermal fatigue cracking. The authors investigated a range of overlay materials specifically designed to maintain hardness and wear resistance at temperatures up to 650°C, which is the upper service limit for supercritical steam turbine inlet valves.

Core Technical Findings

The study examined multiple cladding material systems including cobalt-based alloys (Stellite type), nickel-chromium alloy systems, and martensitic stainless steel hardfacing alloys. The key findings can be summarized as follows:

Material Systems Evaluated

Key Technical Points

1. Thermal stability of carbide-bearing alloys: The cobalt-based system demonstrated superior hardness retention at elevated temperatures because the Co-Cr carbides (Cr₇C₃, Cr₂₃C₆) exhibit minimal softening below 800°C, owing to the high cohesive energy of the Co matrix and the thermodynamic stability of chromium carbides.
2. Oxidation resistance as a wear mechanism: At temperatures above 500°C, oxidation becomes the dominant wear mechanism. The Cr₂O₃ protective oxide layer forms preferentially on cobalt-chromium and nickel-chromium alloys, reducing oxidative wear rates by an order of magnitude compared to martensitic stainless steels.
3. Weldability and dilution effects: The study emphasized that the actual performance of the overlay layer depends heavily on the dilution rate during welding. For cobalt-based alloys applied by gas tungsten arc welding (GTAW), the dilution from a carbon steel substrate typically ranges from 15% to 35%, which can reduce the hardness of the final overlay by 20-40 HV compared to the pure filler metal.
4. Multi-pass overlay strategy: To achieve the required hardness at the sealing surface, the authors recommended a multi-pass approach with a transition layer. A typical sequence involves: first pass with a nickel-iron transition alloy (to reduce dilution and improve bonding), followed by 2-3 passes of the final cobalt-based hardfacing alloy.

Process Analysis and Engineering Practice

The study employed a combination of laboratory testing and field validation. The overlay process used was primarily gas tungsten arc welding (GTAW) with solid wire fillers, supplemented by submerged arc welding (SAW) for thicker overlay layers on large valve bodies.

Recommended Process Parameters

Common Defects and Countermeasures

The study identified several recurring defects in valve seat cladding applications:

• Cracking at the overlay/substrate interface: This occurs when the thermal expansion mismatch between the cobalt-based overlay and carbon steel substrate exceeds the yield strength of the interface. Countermeasures include using a nickel-iron transition layer (e.g., Ni-20Fe or Ni-30Fe), controlling the preheat temperature at 150-200°C, and limiting the single-pass dilution to below 25%.
• Porosity in the overlay layer: Caused by hydrogen absorption from flux or moisture contamination. The recommended countermeasure is to dry the flux at 300°C for 2 hours and use a low-hydrogen flux type.
• Hardness variation across the overlay: This results from inconsistent travel speed or wire feed rate. The authors recommended mechanized welding with CNC-controlled parameters to ensure uniformity.

Engineering Application Cases

The study reported field trials on steam turbine control valves at a 300 MW power station. The original carbon steel valve seats required replacement every 8,000 operating hours. After cladding with the optimized Co-Cr-W alloy using the recommended multi-pass procedure, the service life extended to over 40,000 hours, representing a fivefold improvement. The overlay layer maintained a hardness of 340-360 HV even after 20,000 hours of service at 565°C steam temperature.

Key Questions and Reflections

The 1996 study raises several questions that remain relevant to modern practice:

1. Long-term oxidation resistance: While the cobalt-based alloys showed excellent short-term performance, the study did not extend testing beyond 1,000 hours at 650°C. In modern practice, it is known that cobalt-based alloys can suffer from internal oxidation at temperatures above 700°C, where chromium diffuses inward to form internal carbides, depleting the surface of the protective Cr₂O₃ layer.
2. Cost-benefit analysis: The study did not address the economic aspect. Cobalt-based alloys are significantly more expensive than nickel-based alternatives. In modern applications, the selection must balance performance with cost, especially for large numbers of valves in a power station.
3. Alternative processes: The 1996 study focused on GTAW and SAW. Modern practices increasingly use plasma transferred arc (PTA) powder cladding, which offers lower dilution rates (typically 5-15%), higher deposition rates, and better surface finish. The hardness retention at high temperatures would likely be improved with PTA due to the more homogeneous microstructure.
4. Microstructural evolution: The study provided limited metallographic analysis of the overlay after high-temperature exposure. Modern understanding recognizes that carbide precipitation, grain boundary segregation of sulfur and phosphorus, and phase transformations (such as the formation of brittle sigma phase in high-chromium alloys) can significantly affect long-term wear resistance.

Study Insights and Implications

This 1996 study represents an important contribution to the understanding of high-temperature wear resistance in valve cladding applications. The systematic comparison of different alloy systems, combined with field validation, provides a reliable foundation for material selection. The emphasis on dilution control and multi-pass overlay strategies remains a core principle in modern cladding practice.

The study's principal limitation is the relatively narrow process window examined. The exclusive focus on GTAW and SAW, while appropriate for the era, does not capture the advances in PTA, laser cladding, and hot-wire TIG that have since become standard in high-performance cladding applications. Nevertheless, the fundamental materials science principles established in this work—particularly the role of carbide stability, oxidation resistance, and dilution control—remain directly applicable to contemporary engineering practice.

For modern engineers working on valve cladding projects, this study serves as a valuable historical reference that validates the selection of cobalt-based alloys for high-temperature sealing surfaces. The recommended multi-pass approach with a transition layer is now considered best practice and is codified in standards such as ASME B31.3 and API 6D for high-pressure valve applications.