High-Temperature Friction and Wear Characteristics of Plasma Clad Stellite Alloy

Literature Overview and Research Background

This 2016 study published in the Journal of Tribology (Chinese and English editions) examines the high-temperature tribological behavior of Stellite alloy deposited by plasma transferred arc (PTA) cladding. Conducted by researchers from South China University of Technology and Huaiji Dengyun Auto Parts Co., Ltd., the work addresses a critical need in the automotive and power generation industries where components operate under sustained elevated temperatures with severe frictional contact.

Stellite alloys, particularly Stellite 6 (Co-Cr-W with approximately 21% Cr, 5% W, 5% Mo, 4% Ni), are renowned for their exceptional hot hardness, oxidation resistance, and abrasive wear performance. The PTA cladding process offers a cost-effective alternative to solid Stellite components by depositing a thin overlay on more economical substrates such as carbon steel or low-alloy steel, combining the structural strength of the base material with the tribological superiority of the overlay.

Core Technical Findings

High-Temperature Wear Mechanisms

The friction and wear tests were conducted at temperatures ranging from room temperature to 800°C using a pin-on-disc or ring-on-ring apparatus. The results reveal distinct transition behaviors in the wear mechanisms as temperature increases:

• Room temperature to 400°C: The wear mechanism is dominated by microploughing and oxidative wear, with a protective Cr2O3 layer forming on the surface. The wear rate remains relatively low and stable.
• 400°C to 600°C: Adhesive wear becomes increasingly significant as the yield strength of the matrix decreases. The wear rate increases by approximately 3 to 5 times compared to room temperature values.
• 600°C to 800°C: Severe oxidative wear dominates, with rapid formation of thick oxide scales that spall and expose fresh material. The wear rate can increase by 10 to 20 times relative to ambient conditions.

Microstructural Stability at Elevated Temperatures

The PTA-clad Stellite layer exhibits excellent microstructural stability up to 600°C. The W2C and Cr7C3 carbides remain coherent within the FCC cobalt-chromium matrix, providing effective strengthening through solid solution and precipitation hardening. Above 600°C, partial dissolution of carbides and formation of equilibrium phases (such as L12-type Ni3(Fe,Cr) and BCC sigma phase) begins to reduce the strengthening effect.

The dilution rate in PTA cladding of Stellite alloys is typically controlled below 15%, which is essential for maintaining the intended alloy chemistry. Excessive dilution with the steel substrate introduces excessive iron content, promoting sigma phase formation and reducing hot hardness. The carbon content of the as-deposited layer (typically 0.5-1.5% C for Stellite 6) is critical for carbide formation, and any loss of carbon during the cladding process must be compensated through powder composition adjustment.

Process Optimization and Performance Enhancement

The PTA process parameters significantly influence the as-deposited microstructure and subsequent high-temperature performance. Lower heat input and higher cooling rates promote finer carbide distributions and reduced grain size, which delay the onset of microstructural degradation at elevated temperatures. The typical PTA parameters for Stellite cladding include:

• Arc current: 150-250 A
• Powder feed rate: 100-200 g/min
• Travel speed: 100-200 mm/min
• Torch oscillation: 8-12 mm width
• Layer thickness per pass: 0.5-1.5 mm

Post-weld solution treatment at 1120°C followed by air cooling can homogenize the composition and reduce residual stresses, but must be carefully controlled to avoid excessive grain growth. For automotive applications such as exhaust valve guides and turbocharger components, the clad layer thickness of 0.8-2.0 mm is typically sufficient.

Engineering Practice Integration

In the automotive industry, Stellite PTA cladding is applied to components such as turbocharger compressor wheels, exhaust valve stems, and turbocharger bearing journals. The qualification process requires compliance with OEM specifications and often involves accelerated wear testing under simulated engine operating conditions. The bond strength between the Stellite overlay and the steel substrate must exceed 40 MPa to withstand the thermal cycling and centrifugal loading in service.

A critical engineering consideration is the compatibility between the Stellite overlay and the mating surface material. When Stellite-clad components slide against softer materials (such as cast iron or aluminum alloys), the tribological couple may exhibit severe adhesive wear on the mating surface. Surface texturing or the use of dissimilar hardfacing alloys on opposing surfaces can mitigate this issue.

Study Insights and Reflections

The research confirms that PTA-clad Stellite alloys maintain superior tribological performance well beyond the capabilities of conventional steel substrates, even at temperatures approaching 600°C. The key advantage of PTA over other cladding methods (such as electroslag welding or flame spraying) is the ability to achieve low dilution and fine microstructures that resist thermal degradation. However, the economic feasibility of PTA cladding for high-volume automotive production remains a challenge, as the process is inherently slower than thermal spray methods. The research underscores the importance of understanding temperature-dependent wear mechanisms when selecting cladding alloys for specific service conditions, as the optimal alloy composition and process parameters differ significantly between ambient and high-temperature applications.