What is galling?

29th May 2020

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Galling is when two metal surfaces join unintentionally, caused by micro-welding across their surfaces at localised points of contact. This phenomenon is influenced by the surface condition (roughness), type of metal, environment, temperature and load exerted on the surface. Galling typically refers to the damage that occurs when breaking the items apart, with a tearing of the surface. Not only can this phenomenon repeat over time, but the debris generated can exacerbate the damage during subsequent movements.

Limiting the Occurrence of Galling

There are a number of approaches to limit the occurrence of galling.

i) Use coatings or lubricants that will keep the bare metal surfaces apart and reduce the coefficient of friction between the moving surfaces.

ii) Reduce the surface roughness of the components. A ground or polished surface will have fewer, and less pronounced peaks when seen at the microscopic levels. The presence of peaks encourages galling, due to the high load and small contact area at these point contacts.

iii) Using dissimilar metals. The design of bearings promotes the use of relatively hard and relatively soft metal in combination. The softer metal will quickly conform to the shape and surface of the harder metal, in effect removing any large peaks.

iv) Selecting alloys with particularly good resistance to galling.

For components with a degree of repeated movement or sliding, or requiring occasional disassembly, galling would be highly problematic. Langley Alloys stocks many different alloys, but a number are clearly identifiable as possessing excellent resistance to galling.

a) Hiduron® 130 (2.1504, DTD900/4805) and Hiduron® 191 (NES835, DEFSTAN 02-835) are high-strength copper nickel alloys, invented by Langley Alloys. They offer excellent resistance to galling, particularly in combination with super duplex stainless steels. In addition to their high strength and resistance to galling, they are also non-magnetic, possess excellent resistance to corrosion in seawater, resist marine fouling and provide very good toughness down to low temperatures. Therefore, they are well suited to long-term exposure in seawater. Common applications include subsea connectors. Variations to the original Langley Alloys Hiduron 130 are produced by other manufacturers, such as Nibron®

b) Nitronic® 60 (UNS S21800, Fermonic® 60) is an austenitic stainless steel with a unique chemical composition. It contains significant additions of both manganese and silicon. Whilst manganese is used in other alloys to improve the strength, here it greatly improves the surface properties in combination with silicon. Its galling resistance is extremely good, in a self-couple or with a dissimilar metal. This is combined with very good wear resistance for a corrosion-resistant alloy, which makes it well suited to the production of specialist couplings. Nitronic 60 is also used selectively for valve seats, trim and stems, fasteners, couplings and break-away connectors, pins and bushes.

A more detailed exploration of the galling phenomenon is provided in a technical paper presented by Langley Alloys at Duplex World, which can be found here.

 

Hiduron and Fermonic are registered trademarks of Langley Alloys Ltd. Nibron Special is a registered trademark of Columbia Metals Ltd. Nitronic is a registered trademark of AK Steel Corporation.