How to Prevent Thread Galling on Stainless Steel Bolts and Screws

Stainless steel bolts and screws are notoriously prone to thread galling. When galling occurs, microscopic particles are released that cause the mating fasteners to “stick” together and seize or block up. In severe cases, the two fasteners can become so seized that they are impossible to separate. The good news is that thread galling can be prevented with the right care and maintenance.

Thread galling occurs when the contacting surfaces of mating fasteners have been eroded or stripped by friction and pressure. The most common causes of thread galling are improper tightening or installation procedures, use of a wrench that is too small for the job, debris in the threads, and reusing a fastener that has already been overtightened.

When stainless steel and aluminum fasteners are threaded together, they form thin protective oxide films on their exposed surfaces that prevent corrosion and reduce the amount of direct metal to metal contact during assembly. When these protective oxide layers are rubbed away or stripped, the relatively soft metals are exposed to much higher levels of friction and heat during fastening. These high levels of friction and heat cause the metals to quickly shear, forming microscopic particles that stick to the mated fasteners. This causes the mated fasteners to “stick” together or “gall.”

The good news is that there are many ways to help prevent galling, including the use of a good quality lubricant like Never-Seez and slowing down installation speed. The lubricant creates a barrier between the mated fasteners and reduces the “shoveling” effect of high points and asperities on the threaded surfaces. It also helps to dissipate the heat caused by friction and prevents the formation of a harder, more resistant oxide layer.

Other good methods for preventing galling include choosing coarser threads or cold rolled threads as they are less prone to galling, keeping the assembly area clean from debris and dirt, and utilizing electropolishing to smooth out rough surfaces that tend to gall more than others. Another option is to specify fasteners that are plated with an antigalling coating. These are often plated with molybdenum disulfide, which provides a barrier between the mated threads and prevents galling by reducing the amount of metallic contact.

The best way to prevent galling is to always use the recommended lubrication on all threaded connections and to slowly tighten or install fasteners. If you notice that a screw or nut begins to bind before you’ve finished tightening it, stop and wait for a minute or two to allow the heat to dissipate and then back off the fastener. If you’ve allowed the galling to advance too far, it may be necessary to remove and replace the fastener. For more information on thread galling and other helpful tips to protect your fasteners, contact an engineer at MarshFasteners today. We have an experienced team of engineers ready to answer your questions and provide technical support for your next project.

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