If you’ve ever worked in mechanical system assembly, operation or maintenance you may have encountered the problem of galling. This is when the threads of a bolt or nut weld to one another and are no longer removable. This causes a great deal of disruption to the process and can result in huge repair costs and lost production time. It’s important to understand the causes of galling and to take countermeasures.
The main cause of galling is friction between the metal surfaces that are touching. This friction creates heat and causes deformation of the materials. The friction between the fasteners also leads to a lubricant break down and the onset of adhesive wear. This type of wear is different than normal abrasive wear where the material is actually removed by compression and translation.
Metallurgical factors such as the atomic structure of the metals and the nature of their surface structures also affect the likelihood of galling. For example, ductile metals such as aluminum or stainless steel are much more likely to gall than martensitic or tool steels. In addition, annealed (soft) stainless steel is less resistant to galling than hardened or fully hardened steel. Some metals that are corrosion resistant have a natural passivated oxide layer that protects the exposed surfaces, however this layer can be punctured by friction and wear leaving the metal more reactive to galling.
Other factors that can affect galling include the use of similar materials and the thread smoothness and cleanliness. Duplex Stainless Steel Fasteners are more susceptible to galling than ferritic stainless steel or aluminum hardware due to the different crystalline structures and microscopic surface roughness of the two alloys. Using a harder material such as a Martensitic Stainless Steel Nut can help reduce galling.
The key to preventing galling is proper lubrication. Insufficient lubrication increases the amount of friction between the surfaces which in turn increases the temperature and deformation. A lubricant can help to reduce the friction between the fasteners and prevent damage. In addition to lubrication, the contacting surfaces must be clean and free of debris to avoid galling. Careful installation of fasteners is also important. Since a stainless steel’s passive chromium oxide layer is sensitive to high temperatures, tightening bolts more slowly can reduce the amount of heat and friction that can lead to galling. This may involve avoiding the use of power tools that can generate excessive friction and heat.