Effect of water contamination in bearing greases on wear and corrosion

Abstract

Bearings used in steel rolling mills are usually exposed to high loads and temperatures and require sufficient cooling. This is achieved by constant water flushingof the rolls. A major drawback of this established approach is the inevitable mixing of water, wear debrisand lubricant causing significant changes in the lubricant’s properties over time. This procedure causes challenging conditions for both the lubricant and the bearing material. The aim of this work is the investigation of the interaction between water infused grease and the surrounding materials.The focus of the studies lies onthe consequences of water absorption for the grease’scorrosion protection and tribological performance. The first part of this work focuses on the characterization of the present system parameters based on field samples taken from different positions of the mills. The findings of the grease sample investigations lead to different model mixtures of grease and water for standardized tribological tests. Two different tribometers areused forthe determination ofwear properties of the grease mixtures. Ona four-ball-test machine,the extreme pressure properties of the mixtures areinvestigated according to DIN 51350-4as wellas the wear protective properties according to ASTM D2266. Similarmechanicaltests areperformed on a linear-oscillation test machine (SRV®) according to ASTM D5706andD5707.More insight on the effects on tapered roller bearings is gathered from a specific test setup designed to assessfilm thicknessin said bearings. Further,the effect of absorbed and free water on corrosive attack is studiedfor the previously mentioned model mixturesby assessing the corrosion prevention properties of the grease using modified standardized testsand in house methods. More specific system parameters, such as temperature dependent water separation and standstill,are consideredwith the design of a test methodfor corrosion inhibition properties.The informationand findingsgathered from both field samples and laboratory experiments should be utilized in the development of a measuring device or test method capable of detecting the end of usability of the grease which is subsequently used to find greasesbest suitable for the use in hot and cold rolling mills.