Effects of Temperature on the Hardness and Wear Resistance of High-Tungsten Stellite Alloys
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NTRODUCTION
STELLITE alloys are a range of cobalt-chromium (Co-Cr) alloys designed mainly for wear resistance application. It may also contain tungsten (W) or/and molybdenum (Mo) and a certain amount (0.25 to 3 wt pct) of carbon (C). The chief difference among the individual Stellite wear-resistant alloys is C content and, thus, carbide volume fraction in the materials. Stellite alloys can conventionally be categorized as highC alloys designed for wear service; low-C alloys for high-temperature service; and low-C alloys to combat corrosion or simultaneous corrosion and wear. In Stellite alloys, the Cr content is generally in a range between 20 and 30 wt pct; it has a dual function in Stellite alloys. It is both the predominant carbide former, that is, most of the carbides are Cr-rich, and the most important alloying element in the matrix, where it provides added strength, as a solute, and resistance to corrosion and oxidation. Tungsten, normally between 4.5 and 15 wt pct, and molybdenum up to 18 wt pct, in Stellite alloys, serve to provide additional strength to the matrix. They do so by virtue of their large atomic size, that is, they impede dislocation flow when present as solute atoms. They also improve general corrosion resistance of the alloys.[1] When present in large quantities, for example, in Stellite 1 RONG LIU, Professor, and SAMIT KAPOOR, Master Student, are with the Department of Mechanical and Aerospace Engineering, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada. Contact e-mail: [email protected] X.J. WU, Senior Research Officer, is with the Institute for Aerospace Research, National Research Council of Canada, 1200 Montreal Road, Ottawa, ON K1A 0R6, Canada. M.X. YAO and RACHEL COLLIER, Engineers, are with the Kennametal Stellite Inc., P.O. Box 5300, Belleville, ON K8N 5C4, Canada. Manuscript submitted April 12, 2012. Article published online November 18, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
and Stellite 3, tungsten also participates in formation of W-rich carbides during solidification. Other elements may also be added to Stellite alloys to create specific features. For example, addition of boron (B) can provide strengthening of grain boundaries, in particular, at high temperatures can retard creep and reduce aggregation of carbides at grain boundaries. On the other hand, the presence of B can lower the liquidus temperature of the alloys, thus reduce the required temperature for the furnace during alloy processing and enhance the mobility of the alloys in hardfacing welding process.[2] Previous studies using differential scanning calorimetry (DSC) on the Stellite alloys containing B content (1.8 to 3.5 wt pct) observed the occurrence of phase transformation at lower temperatures between 1223 K and 1273 K (950 °C and 1000 °C), which represented the formation of borides, according to the microstructures of the alloys.[3,4] Because of the wide application in various industries, Stellite alloys have been studied extensively, in particular, in their tribological properties.
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