Galling is adhesive wear. Galling is caused by macroscopic transfer of material between metallic surfaces, during transverse motion (sliding). Galling occurs frequently whenever metal surfaces are in contact, sliding against each other, especially with poor Lubrication. Galling often occurs in high load, low speed applications, but also occurs in high-speed applications with very little load. Galling is a common problem in sheet metal forming, bearings and pistons in engines, hydraulic cylinders, air motors, and many other industrial operations. Galling is distinctive from gouging or scratching in that galling involves the visible transfer of material as it is adhesively pulled (mechanically spalled) from one surface, leaving it stuck to the other in the form of a raised lump. Unlike other forms of wear, galling is usually not a gradual process, but occurs quickly and spreads rapidly as the raised lumps induce more galling.
Galling can often occur in screws and bolts, causing the threads to seize and tear free from either the fastener or the hole. In extreme cases, the bolt may lock-up to the point where all turning force is used by the friction, which can lead to breakage of the fastener or the tool turning it. Threaded inserts of hardened steel are often used in metals like Aluminum or Stainless Steel that can gall easily.