Tightening stainless steel fasteners 12 February 2021

Here, Bill Eccles from Bolt Science answers a common question regarding stainless steel fasteners - why do some stainless steel fasteners seize during tightening, as well as when they are attempted to be untightened?

Stainless steel and high alloy fasteners 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.

The torque will not be converted into bolt preload if galling is occurring because of high friction. This may be the cause of the problems. The change may be due to the surface roughness changing on the threads or other similar minor change.

If galling is found to be a problem suggestions include 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.

Lubricating the internal and/or external threads frequently can eliminate thread galling. The lubricants usually contain substantial amounts of molybdenum disulphide (moly). Some extreme pressure waxes can also be effective. Be careful however, if using 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.

A finish applied to a stainless steel fastener can separate the surfaces and so prevent galling and allow the full tightening torque to be applied. A PTFE coating is sometimes used for this purpose.

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

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