Abstract— Cobalt-based alloys are known for their excellent wear resistance, particularly under high temperature and corrosive environments. However the cast cobalt-based alloys have relatively high brittleness, and low toughness, due to their coarse carbide structure. This paper aims to comprehend if carbide refinement, caused by changing the processing route from sand casting to powder consolidated Hot Isostatic Pressing (HIPing), can improve the tribo-mechanical properties of cobalt-based alloys. The alloy selected for this investigation had a nominal wt.% composition of Co-30Cr-14W-1C, which is similar to the composition of the commercially available Stellite®4 alloy. The Hot Isostatic Pressed (HIPed) alloy had a much finer microstructure than the cast alloy, which showed a typical hypoeutectic dendritic microstructure. Both alloys had similar hardness. Although the cast alloy showed slightly better abrasive and sliding wear resistance than the HIPed alloy due to their coarser eutectic carbides, the HIPed alloy had a significant advantage on the impact toughness and contact fatigue performance. The results of this comparative investigation indicated that the HIPed alloy had an attractive combination of tribo-mechanical properties, i.e. improved impact and fatigue resistance, whilst preserving the high hardness and good wear resistance associated with the cast alloy, making it suitable for relatively higher stress applications. Index Terms— Fatigue; HIPing; Stellite 4; Wear I. INTRODUCTION The cobalt-based alloys, which are generally known as Stellite 1 alloys, are widely used in industry due to their excellent wear resistance, and the ability to retain high strength at elevated temperatures. The properties of cobalt-based alloys are derived from the sluggish phase transformation (F.C.C. to H.C.P.) of Co, solid solution strengthening by W/Mo, and the formation of hard carbides [1]-[5]. The alloy selected for this Manuscript submitted March 20, 2007. H. Yu is with School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK. *R. Ahmed is with School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK (corresponding author, e-mail: R.Ahmed@hw.ac.uk ; phone: +44(0)1314514722). H. de Villiers Lovelock is with Deloro Stellite, Cheney Manor Industrial Estate, Swindon, SN2 2PW, UK. S. Davies is with Bodycote HIP Ltd, Sheffield Road, Sheepbridge, Chesterfield, S41 9ED, UK. 1 Stellite is a registered trade name of Deloro Stellite Company Inc. investigation had a nominal wt.% composition of Co-30Cr-14W-1C, which was similar to the commercially available Stellite®4 alloy. Applications of this alloy include dies for hot pressing and extrusion. The relatively high tungsten content and low carbon content of this alloy ensures that sufficient tungsten is retained in the solid solution, instead of being depleted to form carbides. Therefore this alloy can maintain good hardness and strength at elevated temperatures. Cobalt-based alloys can be used as castings, powder metallurgy (P/M) parts, Hot Isostatic Pressed (HIPed) consolidated parts, weld hard-facings, laser hard-facings and thermal spray coatings [1], [2], [6]. Although the cast alloys have had extensive usage since their introduction in 1910’s, the relatively coarser carbide structure and the presence of defects such as porosity within the castings often result in high brittleness and low impact and fatigue resistance. Carbide refinement by varying the processing route from casting to Hot Isostatic Pressing (HIPing) is one way to improve the combined properties of cobalt-based alloys. The HIPing process involves the simultaneous application of high temperature (up to 2000ºC), and pressure (up to 200 MPa) in a HIPing vessel. Although the structure-property relationships of cobalt-based alloys have been a topic of research for a number of investigations [4]-[9], comparative studies between the alloys produced via different processing routes (especially HIPing) are limited. Hence this paper aims to conduct a comparative investigation of the structure-property relationships between the alloys produced via sand casting and powder consolidated HIPing. Results of this investigation are discussed on the basis of the microstructural examination via Scanning Electron Microscopy (SEM), and tribo-mechanical evaluations including hardness, impact toughness, abrasive wear, sliding wear, and contact fatigue resistance. II. EXPERIMENTAL TEST PROCEDURE A. Materials and Microstructure The cast alloys under the current investigation were sand castings. The HIPed alloys were produced from the gas-atomised powders and consolidated in the HIPing vessel at a temperature and pressure of 1200˚C and 100 MPa, respectively, for four hours. The sieve analysis (+250 µm: 0.0wt.%, +180 µm: 0.2wt.%, +125 µm: 2.3wt.%, +45 µm: Tribo-Mechanical Evaluations of Cobalt-Based (Stellite 4) Alloys Manufactured via HIPing and Casting H. Yu, R. Ahmed*, H. de Villiers Lovelock, and S. Davies Proceedings of the World Congress on Engineering 2007 Vol II WCE 2007, July 2 - 4, 2007, London, U.K. ISBN:978-988-98671-2-6 WCE 2007