We have a problem when tightening stainless steel bolts - they tend to seize - what's happening?

Stainless steel can unpredictably sustain galling (cold welding). Stainless steel self-generates an oxide surface film for corrosion protection. During fastener tightening, as pressure builds between the contacting and sliding, thread surfaces, protective oxides are broken, possibly wiped off, and interface metal high points shear or lock together. This cumulative clogging-shearing-locking action causes increasing adhesion. In the extreme, galling leads to seizing - the actual freezing together of the threads. If tightening is continued, the fastener can be twisted off or its threads ripped out.

If galling is occurring because of high friction the torque will not be converted into bolt preload. This may be the cause of the problems that you are experiencing. The change may be due to the surface roughness changing on the threads or other similar minor change. To overcome the problem - suggestions are:


1. Slowing down the installation RPM speed may possibly solve or reduce the frequency of the problem. As the installation RPM increases, the heat generated during tightening increases. As the heat increases, so does the tendency for the occurrence of thread galling.

2. Lubricating the internal and/or external threads frequently can eliminate thread galling. The lubricants usually contain substantial amounts of molybdenum disulfide (moly). Some extreme pressure waxes can also be effective. Be careful however, if you use the stainless steel fasteners in food related applications some lubricants may be unacceptable. Lubricants can be applied at the point of assembly or pre-applied as a batch process similar to plating. Several chemical companies, such as Moly-Kote, offer anti-galling lubricants.

3. Different combinations of nut and bolt materials can assist in reducing or even eliminating galling. Some organizations specify a different material, such as aluminum bronze nuts. However this can introduce a corrosion problem since aluminum bronze is anodic to stainless steel.

If you have questions about stainless steel or other fasteners, please contact us at AFT Fasteners (877) 844-8595 or via email Contact Us.