Screw fasteners made of stainless steel (A2, AISI 304) and acid-resistant steel (A4, AISI 316) have a tendency to gall. This means that when installing a screw or nut, the fastener may seize and become stuck in place, making it difficult or even impossible to remove without special tools.
Galling occurs for several reasons, the most significant being the mechanical properties of the steel material. A2 and A4 steels are mechanically “rough,” meaning their machined surface remains rougher. This is due to the physical properties of the material, which are enhanced by adding alloying elements to improve the product’s strength and durability. However, these additives can also increase the tendency to gall.
Galling occurs especially when the fastener is tightened with a power screwdriver or impact wrench, which produce a faster driving speed. This creates friction and heat buildup on the thread contact surfaces, which affects the steel’s structure and causes galling. Even if the fastener cools down, the thread will no longer loosen. With manual tightening, the friction is the same, but the driving speed is lower, so heat-related changes do not occur as easily.
Another factor is dirt, which can accumulate in the threads of screw fasteners. A2 and A4 grade fasteners should always be clean, as sand and grinding dust can increase the risk of galling.
There is no single clear-cut solution for preventing galling. One way is to use a lubricant, such as ceramic paste or copper paste, which reduces friction. However, these products can make the surrounding area messy, so an alternative is to wax the products before use, for example with a floor wax solution. Wax provides the same effect as lubricating pastes, but it must be reapplied after each use. It should also be noted that in the food industry, the use of friction-reducing substances is not permitted unless they are approved for food use.