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Research Papers: Friction & Wear

Influence of Manufacturing Process and Alloying Element Content on the Tribomechanical Properties of Cobalt-Based Alloys

[+] Author and Article Information
H. Yu

School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK

R. Ahmed1

School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UKr.ahmed@hw.ac.uk

H. de Villiers Lovelock

 Deloro Stellite, Cheney Manor Industrial Estate, Swindon, SN2 2PW, UK

S. Davies

 Bodycote HIP Ltd., Sheffield Road, Sheepbridge, Chesterfield, S41 9ED, UK

Stellite is a registered trade name of Deloro Stellite Company Inc.

1

Corresponding author.

J. Tribol 131(1), 011601 (Dec 04, 2008) (12 pages) doi:10.1115/1.2991122 History: Received February 24, 2008; Revised August 07, 2008; Published December 04, 2008

Manufacturing process routes of materials can be adapted to manipulate their microstructure and hence their tribological performance. As industrial demands push the applications of tribological materials to harsher environments of higher stress, starved lubrication, and improved life performance, manufacturing processes can be tailored to optimize their use in particular engineering applications. The aim of this paper was therefore to comprehend the structure-property relationships of a wear resistant cobalt-based alloy (Stellite 6) produced from two different processing routes of powder consolidated hot isostatic pressing (HIPing) and casting. This alloy had a nominal wt% composition of Co–28Cr–4.5W–1C, which is commonly used in wear related applications in harsh tribological environments. However, the coarse carbide structure of the cast alloy results in higher brittleness and lower toughness. Hence this research was conducted to comprehend if carbide refinement, caused by changing the processing route to HIPing, could improve the tribomechanical performance of this alloy. Microstructural and tribomechanical evaluations, which involved hardness, impact toughness, abrasive wear, sliding wear, and contact fatigue performance tests, indicated that despite the similar abrasive and sliding wear resistance of both alloys, the HIPed alloy exhibited an improved contact fatigue and impact toughness performance in comparison to the cast counterpart. This difference in behavior is discussed in terms of the structure-property relationships. Results of this research indicated that the HIPing process could provide additional impact and fatigue resistance to this alloy without compromising the hardness and the abrasive/sliding wear resistance, which makes the HIPed alloy suitable for relatively higher stress applications. Results are also compared with a previously reported investigation of the Stellite 20 alloy, which had a much higher carbide content in comparison to the Stellite 6 alloy, caused by the variation in the content of alloying elements. These results indicated that the fatigue resistance did not follow the expected trend of the improvement in impact toughness. In terms of the design process, the combination of hardness, toughness, and carbide content show a complex interdependency, where a 40% reduction in the average hardness and 60% reduction in carbide content had a more dominating effect on the contact fatigue resistance when compared with an order of magnitude improvement in the impact toughness of the HIPed Stellite 6 alloy.

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Figures

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Figure 1

Schematic illustration of the cup assembly for the rolling contact fatigue tests

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Figure 2

The images showing (a) the morphology of the alloy powder, the microstructure of (b) and (c) cast Stellite 6 alloy, and (d) HIPed Stellite 6 alloy

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Figure 3

The XRD pattern of the (a) alloy powder, (b) HIPed Stellite 6 alloy, and (c) cast Stellite 6 alloy

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Figure 4

The fractographs after the un-notched Charpy impact tests on (a) the cast Stellite 6 alloy and (b) the HIPed Stellite 6 alloy

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Figure 5

The average volume loss of cast and HIPed Stellite 6 alloys after dry sand rubber wheel tests

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Figure 6

The wear scar after dry sand rubber wheel tests of the (a) cast Stellite 6 alloy and the (b) HIPed Stellite 6 alloy

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Figure 7

The wear loss of the cast and HIPed Stellite 6 alloys after the pin-on-disk and ball-on-flat tests

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Figure 8

The SEM images showing the wear scars after pin-on-disk tests of the (a) cast alloy pin, (b) cast alloy disk, (c) HIPed alloy pin, and (d) HIPed alloy disk, and the wear scars after ball-on-flat tests of (e) cast alloy disc and (f) HIPed alloy disc

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Figure 9

Stress cycles to failure of cast and HIPed alloys after the rolling contact fatigue tests: (a) Stellite 6 alloys and (b) Stellite 20 alloys

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Figure 10

The failure areas after rolling contact fatigue tests of the (a) cast Stellite 6 alloy, 2.24 GPa, and (b) HIPed Stellite 6 alloy, 2.24 GPa

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Figure 11

Schematic of the Hertzian stress distribution during the RCF test

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