A galling fastener can cause major problems, from a seized-up joint to broken bolt threads. Preventing galling requires reducing the friction and heat that causes it. Using an anti-galling lubricant is one way to reduce friction. Another is slowing down installation speed to eliminate the heat build-up that leads to galling.
What Causes Galling?

Typically, metal surfaces that are prone to galling are those that undergo high amounts of friction and deformation with one another. For instance, stainless steel fasteners that are threaded to aluminum hardware often show galling due to the substantial amount of friction they go through when torqued. Any type of metal machinery that experiences excessive friction will also be susceptible to this problem.

The most common industrial operation prone to galling is the tightening or disassembly of threaded components. The frictional force from these operations causes the complimentary metals to seize together, resulting in damage or even the destruction of the threaded features in said components. This issue is most prevalent in metals such as aluminum and austenitic stainless steel, whereas hardened materials like tool steels and martensitic stainless steel are less likely to experience galling. The reason is that these softer metals have a ductile crystal structure that promotes cohesive attraction between the surfaces they come into contact with.
Types of Metals Prone to Galling

Metal galling can occur at a large or microscopic level. The latter may not be visible to the naked eye but can still make a component unusable. Metals that are prone to galling include aluminum and austenitic grades of stainless steel, with hardened materials like tool steel and martensitic stainless steel less likely to exhibit the issue.

A combination of friction and adhesion causes galling between sliding surfaces, often when there is a compressive load. The surface of the galling material will appear gouged with balled-up or torn lumps of metal stuck to it.

To reduce the risk of galling, Bodycote offers specialty lubrication that can help reduce friction between two materials. Additionally, selecting a cobalt-based alloy that has high galling resistance can help reduce the chance of this issue occurring. The alloys Stellite 6B and ULTIMET 25 both have excellent galling resistance, as does the nickel-based material Nitronic 60. The use of proprietary materials adds cost and complexity to an application, however, so these measures should only be used as a last resort.
Anti-Galling Devices

The good news is that galling can be prevented with the proper lubrication. Using an anti-seize compound is one of the best ways to prevent galling as it helps reduce friction and will not cause the materials to weld together.

Another great way to reduce the risk of galling is by plating the fasteners with electroless nickel. This abrasion resistant coating has excellent lubricity and can withstand the high temperatures that can lead to galling.

Keeping the temperature down by storing fasteners in a cool environment will also help to reduce the likelihood of galling. In addition, never over-torque your components as this can damage the threads and increase the chance of galling. If a galling incident does occur it is important to be able to dismantle the rigging quickly and safely. Otherwise, the structure could be compromised and costly repairs could be needed. The use of premium quality stainless steel bolts and nuts will also reduce the possibility of galling as these alloys typically have very thin oxide surface films that protect against corrosion.
Prevention

Galling is a major problem that can threaten the integrity of mechanical systems. While some workers in design and engineering are familiar with this phenomenon, many others may only become aware of its effects when a critical machine or production line fails.

The good news is that galling can be prevented by using lubrication and selecting the right alloys for mating components. Harder metals are generally less prone to galling than soft materials. In addition, using coarse threaded fasteners can help reduce friction and the likelihood of galling.

Slowing down the installation process can also reduce the likelihood of galling by allowing heat to dissipate and minimizing the amount of force exerted on the contacting materials. Finally, using a special anti-seize additive can help reduce the chances of galling by keeping the thread surfaces clean and adding extra resistance to their movement. This is particularly important for applications involving ductile metals such as stainless steel or aluminum.