Galling is a combination of friction and adhesive wear. It appears when two materials, which are pressed against one another by compressive force, are moved, causing friction. The materials are welded together by these compressive forces. Upon moving, particles of one material are removed and placed on the other. Galling is not to be confused with abrasive wear and cold welding, although it does display similarities with both.
How Galling Works
When two surfaces made of metal are compressed against each other, they can merge at the highest force points, the contact zone. If compressive forces become too much, friction is created, increasing pressure in the contact zone. The energy that is generated by this caused adhesion between the surfaced, welding the two materials together and plastically deforming them. If one of the surfaces has protruding points, they can penetrate the surface oxide layer of the opposing material, causing damage to metal beneath.
The strong adhesion turns into galling when sliding friction occurs. The amount of stress increases in the contact zone, enabling the two surfaces to break away from each other and dissolving the contact zone. This results in one material losing particles to the other.
Insufficient lubrication and poor metal type selection can further enable welding. For instance, ductile metal surfaces are more prone to galling because of their high yield point. As one of the most ductile metals on Earth, aluminium and its compounds are common subjects of galling.
How to Improve Galling Resistance
Many austenitic and precipitation hardening stainless steels do not possess sufficient galling resistance when unlubricated. Particularly when combined with harder steel grades, austenitic stainless steels weld onto the opposing material due to their high ductility and surface softness. Generally, the softer the material, the more prone to galling it is.
To improve galling resistance of already existing parts, surface treatments such as BoroCoat® play an important role. We achieve excellent wear, galling and corrosion resistance by diffusing hard borides into the surface of the workpiece. The smooth, unyielding surface layer protects the material from many types of damages: acids, salts, wear, corrosion and galling, to name a few. We can also apply our boronizing technique on small parts and cavities, protecting the material all around.
Another option to prevent galling is by using lubrication fluids such as oil. However, the lubricant has to be reapplied after a certain amount of time in a sufficient thickness. Depending on the design of the workpiece, this might not be possible, which is why the selection of the right metal type beforehand is so important.
Difference between Abrasive Wear, Cold Welding and Galling
Cold welding and galling can be seen as two sides of the same coin. While cold welding is the merging of two parts, galling is the friction damage that follows cold welding. When two parts are cold welded and then moved by compressive forces, galling damage occurs on the surface of both materials. Abrasive wear is a type of material damage that is caused when a hard surface moves against a softer material with consequential, compressive force. Unlike galling, abrasive wear does not include welding before friction damage occurs.