Galling occurs when two metals come into contact with each other, creating excessive friction which generates excess heat. It is a serious problem in metalworking which can lead to worn edges and tools, seized fasteners and overall poor performance. It also imposes unnecessary costs and downtime on equipment and production. There are several methods which can be used to prevent galling, these include lubrication and proper material selection.
Metals which are prone to galling are often those that have a high degree of ductility, metal on metal contact and/or low compressive loads. Aluminium compounds, aluminium alloys and some austenitic stainless steel grades are a common example of such materials. Stainless steels which have been precipitation hardened or hardened through cold working are generally less prone to galling than those that have been annealed. The ductility of the metals is related to their atomic structure, for instance softer metals such as aluminum tend to gall more easily than fully hardened carbon and stainless steels.
The nature of the contact between the metals is also important, for instance a protruding surface such as an uncut bolt thread can be much more likely to gall than a smooth, cylindrical bore. Additionally, the speed at which the two surfaces are contacted is important; galling will occur at higher stress levels than normal abrasive wear and can be exacerbated by rapid changes in torque.
In order to determine the potential for galling, a number of tests can be performed. One such test is the button and block galling test which uses a machined button specimen and a block specimen with parallel contacting surfaces to perform an unlubricated stress test. The button and block are then rotated against each other until the point of galling is observed. The test can be used to predict the maximum amount of galling that a given material pair will be capable of withstanding.
Using the results of these tests, it is possible to find an appropriate material pair for any application or operation. Stainless steels and other corrosion resistant metals tend to gall less than non-corrosive metals such as copper or brass. The use of a good anti-seize product, either pre-applied or applied during assembly will greatly reduce the chance of galling. In addition, reducing the torque load on the bolt will prevent galling by reducing the rate at which friction is generated during installation.
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