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Galling, or cold welding as it is sometimes referred to, is a form of severe adhesive wear. Adhesive wear occurs between two metal surfaces that are in relative motion and under sufficient load to permit the transfer of material. This is a solid-phase welding process. The load must be sufficient, during relative motion, to disrupt the protective oxide layer covering surface asperities of the metal and permit metal to metal contact. Under high stress and poor lubrication conditions, stronger bonds may form over a larger surface area. Large fragments or surface protrusions may be formed and the result is galling of the surfaces. Severe galling can result in the seizure of metal components.
Materials which are highly ductile or which possess low work-hardening rates tend to be prone to galling. Austenitic stainless steels show a tendency to gall under certain conditions.
Lubrication is often applied to stainless steel fasteners as a method of preventing galling and seizure. This can take several forms, depending on service conditions, from the application of oils and greases to solid lubrication systems such as thermosetting or PTFE coatings. Work carried out at the Corus (British Steel) Technology Centre (see References) shows that PTFE provides the best galling resistant system, when compared with greases. It is also suggested that this system has the additional advantage that the coatings can be applied under ‘workshop’ conditions rather than ‘on-site’. This reduces the risk of contamination pick up, compared with the use of greases which should help maintain optimum corrosion resistance of the fastener system.
Improvements in both wear and galling resistance may be obtained by altering the surface characteristics of stainless steels by nitriding or chromium plating.
Austenitic stainless steels do not respond well to nitriding treatments and so the scope for nitriding is limited. Better results can be obtained on martensitic and precipitation-hardening stainless steels, particularly as the “substrate” core of the steel can be strengthened. This helps support the hardened surface layer when loads are applied to mating components.
There are limitations even then to the scope for nitriding, as the corrosion resistance can be reduced due to depletion of chromium at the surface as chromium nitrides are formed. Additionally there can be a risk of intergranular corrosion as the nitriding temperature can result in the precipitation of carbides at the steel grain boundaries.
Hard chromium deposits are intended primarily to improve wear/galling resistance and are usually applied directly to the substrate. The thickness of hard chromium electroplated coatings range from 0.003 to 0.5 mm, while decorative coatings seldom exceed 0.003 mm. Electro-deposited chromium is not recommended for high temperature or high-pressure applications, which tend to reduce the hardness of the coating and may lead to cracking/spalling.