Stainless Steel bolt Thread Galling Issue

Galling is a long-standing common problem in the stainless steel fastener industry. It mainly occurs due to the following three reasons:

Material-Related Causes:

Austenitic 300 series stainless steel is relatively soft, with good toughness and excellent ductility, making it easy to cold work. However, it has poor thermal conductivity. During tightening, the pressure and heat generated between the threads can damage the chromium oxide layer. This leads to direct metal-to-metal contact between threads, causing blockage or shearing, which results in adhesion and eventually galling. This is the primary cause of the issue.

Usage-Related Causes:

Some incorrect usage practices during tightening also contribute to galling:

• Uneven or angled force applied with a wrench during tightening, which leads to misalignment between the screw and nut. This creates excessive pressure on the contact area, which overheats, melts the chromium oxide layer, and causes adhesion and galling.
• Using a power wrenchfor tightening. The rapid action generates high heat, which melts the oxide layer and results in galling.
• Excessive tightening torque, beyond the safe torque specification of the product.

Product-Related Causes:

During transportation or storage, improper handling may lead to damaged threads, burrs, or foreign material between threads. These can also cause galling during tightening.

How to Prevent Galling in Stainless Steel Fasteners

Follow correct usage practices:

1. Select bolts of appropriate length — ideally, 1–2 threads should protrude beyond the nut after tightening.
2. Ensure the bolt’s tensile strength and the nut’s safe load capacity meet design requirements.
3. Apply water-based wax or lubricating oil to the surface.
4. Clean the threads before use; if threads are damaged, at minimum ensure the nut can rotate freely along the bolt.
5. During tightening, the wrench should apply force perpendicularto the bolt axis — do not tighten at an angle.
6. Use a torque wrenchand avoid power tools (which can generate excessive heat due to high speed, causing galling).

Apply torque evenly and within the safe torque range during tightening.

Recommended Safe Torque Values – (For Reference Only) – Unit: N·m