Plasma Overlay Welding of Stellite 6 Alloy High-Temperature Wear Resistance Study

Literature Overview and Industrial Significance

The research by Guan Meng, Yong Xingping, Tian Yun, Zou Youxu, Lin Xuejian, and Huang Hongjun from Shenyang Blower Works Group Nuclear Power Pump Co., Ltd. and Shenyang University of Technology investigates the high-temperature wear resistance of Stellite 6 alloy overlay layers produced by plasma arc welding on 304H stainless steel bearings. Published in Special Casting and Nonferrous Alloys in 2023 (Vol. 43, No. 6, pp. 815–820), this study addresses a critical need in the nuclear power and high-temperature industrial equipment sectors.

Stellite 6 is a cobalt-chromium-tungsten alloy that is renowned for its excellent wear resistance, corrosion resistance, and high-temperature strength. It is widely used in applications involving severe wear, corrosion, and high temperatures, including nuclear power plant components, gas turbine parts, and chemical processing equipment. The 304H stainless steel substrate is a high-temperature austenitic stainless steel commonly used for nuclear power plant components, and the combination of Stellite 6 overlay with 304H substrate is particularly relevant for nuclear pump bearings and other high-temperature wear-prone components.

Microstructural Characterization and Phase Analysis

The plasma arc welding process used in this study provides a high-energy-density, low-dilution welding method that is well-suited for the application of Stellite 6 overlay layers. The plasma arc offers precise control over the heat input and penetration depth, resulting in a narrow heat-affected zone and minimal dilution of the overlay with the substrate. This is critical for preserving the high-performance properties of the Stellite 6 alloy.

The microstructure of the Stellite 6 overlay layer is characterized by a columnar grain structure, with the fusion zone exhibiting a planar crystal region. The overlay layer is primarily composed of a gamma-Co solid solution and M7C3-type complex carbides formed by Cr, Ni, W, and C elements, along with WC particles. This microstructure is typical of Stellite 6 and is responsible for its excellent wear resistance and high-temperature strength.

The hardness profile from the substrate to the overlay shows a continuous increase, with the hardness stabilizing at values above 460 HV at a distance of 0.2 mm from the fusion line. This indicates that the overlay layer achieves its full hardness properties within a relatively thin zone, and the remaining thickness of the overlay provides additional wear resistance through the M7C3 carbide and WC reinforcement.

High-Temperature Wear Performance and Mechanism Analysis

The wear testing was conducted at temperatures of 250, 350, 450, and 500 degrees Celsius against Stellite 21 overlay layers. The results show that both the friction coefficient and the wear mass loss increase with increasing temperature, indicating that the wear resistance of the Stellite 6 overlay decreases at higher temperatures. The roughness of the friction surface ranges from 3.9 to 8.6 micrometers across the temperature range.

The wear mechanisms identified at high temperatures include abrasive wear, adhesive wear, and oxidative wear. The relative contribution of each mechanism changes with temperature, with oxidative wear becoming more significant at higher temperatures due to the formation of oxide films on the wear surface. However, the wear mechanism does not change significantly across the temperature range, indicating that the Stellite 6 overlay maintains its fundamental wear resistance characteristics even at elevated temperatures.