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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.
The main factors affecting wear and galling are:
Design tolerances should provide sufficient clearance. The contact load on sliding components should be kept to a minimum, while the contact area should be maximised. In this situation, lubrication plays an important role and the design must ensure that adequate lubrication can be effectively delivered to the components in relative motion.
Surface finish of components is important as highly polished (< 0.25 mm Ra) or very rough (> 1.5 mm Ra) surfaces increase the tendency for wear and galling. Smooth surfaces result in more contact. The small ‘valleys’ and ‘asperities’ on the smooth surface means that lubricant cannot be held in place between the surfaces and dislodged material is retained in close contact with the surfaces resulting in wear. Rough surfaces result in interlocking of asperities, which promote severe tearing and galling. Surface finishes between these extremes are therefore preferable.
The hardness and microstructure of the material play an important role in adhesive wear and galling. A high hardness (obtained by work hardening) austenitic stainless steel and a stable oxide film can provide resistance to galling. Surface hardening treatments can also have the same effect. Companies who carry out surface hardening treatments can be found here.
The microstructure can also reduce adhesive wear and cold welding, where there are two or more phases (austenite with ferrite or austenite with martensite). A similar effect can result if the steel structure has dispersions of “particles” such as carbides, nitrides, sulphides etc embedded in it. Additions such as sulphur made to improve machinability in grades such as 303 (1.4305) and 416 (1.4005) can affect the behaviour of the steel. The inclusions formed (mainly manganese rich sulphides) act as solid lubricants in sliding contact and so these steels exhibit better galling resistance than the non treated 304 (1.4301) and 420 (1.4021) grades from which they are derived.
The inducement of compressive stresses by shot peening has also been found to be beneficial. Companies involved in this process can be found here.