What is Galling?
Metal galling is a form of adhesive wear that happens when the threads of fasteners or bolts come into contact with one another. This causes a strong adhesion between the two materials which creates localized frictional heat. This heat is concentrated into a small area and forms a solid lump of material that can be difficult to remove. This can lead to expensive repairs and downtime in industrial machinery and equipment. This type of galling is distinct from abrasive wear which occurs through the force of compression and translation between the surfaces of the metal. This is an important distinction because metal galling is generally much more destructive than abrasive wear.
Galling occurs in metal surfaces that are sliding against each other when there is inadequate lubrication. Metals such as aluminum and stainless steel are particularly prone to galling when they are not properly lubricated. Stainless steels and aluminum both have natural passivation layers which help protect them from corrosion and wear but these coatings are broken down during high compressive loads when they are being fastened together. When these coatings break down the chances of galling are increased significantly.
To cause metal galling, the contacting metals must have three characteristics: friction, ductility and cohesive attraction. For a galling event to occur, the contact points must also be in very close proximity. The occurrence of a galling event is also highly dependent on the amount of pressure that is exerted during the contact process. Metals such as aluminum and austenitic stainless steels are prone to galling, whereas harder metals such as tool steels and martensitic stainless steels are less susceptible to galling.
A common application of galling is when threaded fasteners are being torqued together. This can be very damaging to both the threaded fastener and the nut. In addition, the nut can break loose and leave the fastener exposed to moisture, which can cause rust and corrosion. It is possible to reduce the risk of galling in threaded fastener applications by using a lubricant, selecting a softer grade of metal or by increasing the coarseness of the threaded fastener.
While galling can be a serious problem for manufacturers, it is also often an overlooked issue because it is difficult to detect. Many people only become aware of a galling issue when an expensive piece of equipment breaks down. Therefore, it is important for manufacturers to understand what galling is and how to avoid it.
The most common way to prevent galling is to use a lubricant. A good lubricant will reduce the amount of friction between metals during the fastening process and it will also help to disperse any heat generated. It is also important to make sure that the threaded surfaces of the nuts and bolts are clean, as debris can increase the friction between them. Finally, the speed at which the fastener is tightened can greatly impact the likelihood of galling. It is best to slow down the installation speed to prevent the formation of a galling layer between the nut and the threaded fastener. what is galling