Thread galling, sometimes known as cold-welding, is a common problem with fasteners made of stainless steel (SS), aluminum, titanium, and other alloys which self-generate oxide film for corrosion protection. Galling can occur when the surfaces of male and female threads are installed under heavy pressure, which may cause protective oxide to break and increase friction between nut and bolt that generates enough heat to fuse them together.
During minor galling, the fastener can still be removed, but in severe cases of galling, a strong bond between the bolt and nut can prevent removal of fastners. Unfortunately, little is known on how to control it, but there are a few ways to minimize this effect:
- Decreasing installation RPM speed will cause less friction and decrease heat generation.
- Lubrication used prior to assembly can reduce or eliminate galling. Recommended lubricants should contain higher amounts of molybdenum disulfide, such as graphite which is very commonly used as a solid lubricant or special anti-galling lubricants sold by chemical companies.
- Using varying grades of SS will also reduce galling due to differing hardness.
- Smoother surface roughness will also reduce galling. Polishing them before installation will help, especially with nuts because these threads are always cut so that they have a rougher finish.