Thread galling, also known as cold welding, is a severe form of adhesive wear that occurs during the installation of fasteners. It is particularly common in metals that generate a self-protecting oxide surface film, such as stainless steel, titanium, and aluminum.
What is Thread Galling?
During bolt tightening, pressure and friction build up between the mating threads. If the protective oxide coating is scraped off, the high points of the metal threads rub directly against each other. Under extreme pressure, the threads shear and weld together, locking the fastener in place. Once galled, the bolt usually cannot be loosened or tightened without breaking.
How to Prevent Galling
- Lube the Threads: Applying an anti-seize lubricant (like nickel- or copper-based compounds) is the most effective prevention. It minimizes friction and acts as a barrier.
- Slow Down Installation: High installation speeds generate heat, which increases the likelihood of galling. Avoid using impact wrenches; install stainless fasteners manually or at slow RPMs.
- Avoid Pulling Joints: Do not use the bolt to pull components together. Align parts first, then tighten.
- Use Different Alloy Grades: Mating a Grade 304 nut with a Grade 316 bolt can reduce galling because different alloys have slightly different hardness and crystalline structures.
Summary
While stainless steel fasteners offer excellent corrosion resistance, they require careful handling. By applying lubricants and slowing down assembly speeds, industrial operations can prevent costly downtime caused by galled fasteners.
