Galling

• Galling occurs when the oxidized surface layer of a product like stainless steel is rubbed off through friction during solid-surface contact. This could happen, for example, when a fastener is misaligned during assembly. If the two components are subsequently subjected to strong forces -- as can very easily happen with fasteners -- the two metals can adhere to each other, and material can potentially be transferred from one surface to the other, eventually creating tears on the metal surface.
  
  

Hardness

• Hardness is an important factor; hardened steels are less likely to gall. Work-hardening a steel by straining it to impart more strength and hardness helps to make it more gall-resistant. Surface hardening or adding a layer of alloy to the surface also helps to make it less likely the surface layer will rub off and permit galling to occur. Metals that are more ductile -- that deform more readily, in other words -- are more likely to succumb to galling.
  
  

Surface

• If the surface of the metal is too smooth, lubricant is excluded and any dislodged material remains stuck between the surfaces, so excessive smoothness promotes galling. An excessively rough surface allows irregularities on both surfaces to interlock and thereby promotes galling as well. A more intermediate finish tends to promote galling resistance by avoiding these two undesirable extremes. Certain kinds of coatings like titanium carbide can also create a hard surface on the metal that is more gall-resistant and less prone to wear.
  
  

Other Effects

• Tiny carbide, nitride or sulfide particles embedded within the surface of the steel help to reduce galling. Adding sulfur to certain stainless steels can form a layer that acts as a kind of solid lubricant, thereby providing similar benefits. Certain stainless steel alloys that contain small quantities of silicon seem to exhibit superior gall resistance, although too much silicon is undesirable because it will damage the metal's other properties. It's also important to realize that gall-resistant steels can still gall -- they simply exhibit a far lower propensity to do so.