- NOT a material defect!
- NOT caused by corrosion!
- NOT a common phenomenon, but it can happen!
One characteristic property of stainless and acid-resistant steel is the “galling” of the threads during use. This is not to be confused with the galling of conventional screws, where the phenomenon is a consequence of rusting and develops over a relatively long period. With screws of austenitic material the phenomenon is extremely fast, since it can happen already during the first tightening that the thread seizes as if it had been welded together! We are not far from the truth, because cold flow can occur from the friction, and the contacting surfaces can flow into one another. This can occur even in the case of a still-loose joint, before we have fully tightened the fastening. A single point where the friction between the two contacting surfaces is stronger is enough, and the fusing takes place. After that it can only be undone in the most drastic way — by cutting or tearing the screw apart.
Methods for preventing galling
One of the most tried-and-tested methods is to create a joint of differing material grades, e.g. to use an A4 grade nut with an A2 grade screw. The molybdenum content of the A4 grade material (which is not present in A2) prevents galling. The effectiveness of the method is not 100%, but it is widespread and good.
Friction has a decisive role in the development of galling, therefore every method that reduces the friction arising during the tightening of the screw also reduces the chance of galling. This is precisely why it is advisable to tighten stainless screws more slowly than usual. Wherever possible, tighten stainless screws by hand, and if you feel binding, let the thread cool for a short time, then carefully unscrew the nut and try another one and/or use anti-seize paste! (You can read about anti-seize pastes below.)
During storage and transport, take care that the threads of the screws are not damaged! With large, heavy screws it is common for the threads of the screws to knock together during transport and be damaged by this. A damaged thread is far more likely to gall.
If the thread is contaminated, that likewise increases the friction and thereby heightens the risk of galling. From this point of view, sand, dust, metal filings, etc. can be dangerous contaminants. Clean a contaminated thread before screwing it together!
The prevention method mentioned above is the use of pastes and sprays of various makes. With these it is always important to observe the manufacturers’ instructions for use, because alongside preventing galling these materials also provide lubrication!
Guaranteed freedom from galling does exist
This is especially important if we use stainless or acid-resistant fasteners outdoors, under heavy loads or in critical structures.
There is a fastener distributor that undertakes a guarantee against galling on the stainless fasteners it sells.
Leave a Reply