Galling Resistance of Stainless Steel

Galling, or cold welding is a form of severe adhesive wear. Adhesive wear occurs between two metal surface 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 surface. Severe galling can result in the seizure of metal component.

Materials which are highly ductile or which possess low work-hardening rates tend to be prone to galling. Austenitic stainless steel show a tendency to gall under certain conditions.

Austenitic and precipitation hardening stainless steel exhibit low galling resistance. An AISI booklet shows the galling resistance of various combinations of stainless steel in terms of an unlubricated threshold galling stress. These data show that combinations of the softer austenitic stainless steel with harder martensitic grades or with the manganese/nitrogen/austenitic grades should help reduce the risk of galling and seizure.

Combinations of the work hardening 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 properties classes is shown in 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 steel, the hardenable martensitic stainless steel have better resistance to galling as a result of their hardness that can be in excess of 53 HRc (Rockwell 'C'). The galling characteristics of duplex stainless steel is claimed by one US manufacturer to be similar to austenitic stainless steel.

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