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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.
Austenitic and precipitation hardening stainless steels exhibit low galling resistance. An AISI booklet shows the galling resistance of various combinations of stainless steels in terms of an unlubricated threshold galling stress. These data show that combinations of the softer austenitic stainless steels with harder martensitic grades or with the manganese/nitrogen/austenitic grades should help reduce the risk of galling and seizure.
It has been found that combinations of the work hardened austenitic fastener grades A4-80 and A2-80 are less prone to seizure than combinations of the ’70’ property class materials together or with other class ’80’ components. (The specification of these property classes is shown in BS EN ISO 3506.) This can be considered in selection of materials for fasteners, provided the corrosion resistance of the harder martensitic grades is consistent with proposed service conditions.
In contrast to the austenitic stainless steels, the hardenable martensitic stainless steels have better resistance to galling as a result of their hardnesses that can be in excess of 53 HRc (Rockwell ‘C’).
The galling characteristics of duplex stainless steels is claimed by one US manufacturer to be similar to austenitic stainless steels.