Galling is the term used when two surfaces in contact seize up as a result of cold welding. The problem (also known as 'adhesive wear') is most commonly seen when tightening bolts made from materials such as stainless steel, aluminium or titanium. These materials gain their corrosion resistance from a passive oxide layer over their surface. Under high contact-force sliding this oxide can be deformed, broken and removed from parts of the component, exposing bare reactive metal. When the two such surfaces are the same materials, these exposed surfaces can easily fuse together, causing seizure or galling. In threaded bolts the galling leads to freezing of the threads, and applying further tightening force may simply shear off the head of the bolt, or strip the treads.
There are three suggested methods for avoiding the problem:
- Firstly, lubricate the bolt threads and/or inner threads of the nut, either pre-applied by the supplier or applied during assembly. Many anti-galling lubricants contain molybdenum compounds, but if the application is for the food industry there may be limitations on what chemical lubricants are acceptable.
- Secondly, reduce the tightening speed; a stainless steel bolt usually needs to be tightened more slowly than an equivalent size carbon steel bolt in order to reduce the friction heating and risk of galling.
- Thirdly, use different materials for the nut and bolt, such as different grades of stainless steel. Different materials have different hardnesses and hence unequal levels of damage to their oxide layers under friction, which avoids having the bare mating surfaces that can fuse. Care needs to be taken to avoid any potential corrosion issues when using completely different alloys for the bolt and nut.