Regarding galling/seizing of stainless steel screws

A Frustrating Reality: What is Thread Galling?

If you have ever been in the middle of a critical assembly only to have a stainless steel bolt and nut suddenly seize up—refusing to move either forward or backward—you have experienced Galling (also known as “cold welding” or “locking”). For a first-time user, it is a baffling and frustrating experience. At Ever Power, even with our decades of experience in the fastener industry, we still find this phenomenon to be one of the most common yet avoidable technical hurdles.

Technically, galling is a form of severe adhesive wear. When an assembly is forcibly disassembled or cut after galling, the internal threads often appear “ulcerated” or covered in metal “nodules.” Once the threads have locked, the molecular bond between the bolt and nut is often stronger than the parent metal itself.

The “Danger Zones”: When is Galling Most Likely?

In our experience, galling isn’t random. It is highly correlated with specific installation practices and environments:

1. High-Speed Power Tool Installation

Speed is the enemy of stainless steel threads. When using electric or pneumatic tools to quickly install untreated SUS304 or SUS316 fasteners, galling becomes a near-certainty. The heat generated by high-speed friction cannot dissipate fast enough, leading to instantaneous welding. This risk reaches 100% when using power tools on locking nuts (nylon insert or all-metal types).

2. Imbalanced Flange Mounting

Flanges are the most common victims of thread seizing. Workers often tighten bolts to their limit without a sequence. This “brute force” approach creates uneven pressure across the flange face, causing bolts to tilt or become non-concentric. When a bolt is under high torque while being slightly misaligned, the pressure on the thread flanks spikes, triggering a lock.

The Science of Seizing: Stickiness vs. Heat

Why does stainless steel gall while carbon steel simply snaps or strips? The answer lies in two critical physical properties: Ductility and Thermal Conductivity.

The “Stickiness” Factor (Ductility Comparison)

Stainless steel is inherently “sticky” due to its high ductility. Think of it like dough versus stone. In a direct comparison between SUS304 Stainless Steel and 10B21 Carbon Steel, the data is eye-opening:

• Elongation: SUS304 boasts a massive 62% elongation, which is over 230% higher than the 27% elongation found in 10B21 carbon steel.
• Reduction of Area: SUS304 measures at 78%, compared to 60% for carbon steel—a 30% difference.

This high ductility means the metal deforms plastically rather than breaking. During machining, this is why stainless steel produces long, spring-like chips that are difficult to clear, whereas carbon steel produces brittle flakes.

The Thermal Bottleneck

Thermal conductivity is where stainless steel fails to protect itself. Compare the following thermal conductivity coefficients ($W/(m·K)$):

• Copper: 383 (Excellent heat dissipation)
• Carbon Steel: 45 (Moderate)
• Stainless Steel: 16.2 (Poor)

Because stainless steel is a poor conductor, the heat generated by thread friction remains trapped at the contact points. This localized heat destroys the protective chromium oxide layer, allowing raw metal-to-metal contact.

The Anatomy of a Lock: Step-by-Step

The entire galling process happens in a fraction of a second:

1. Pressure & Heat: Tightening creates pressure that generates heat.
2. Oxide Depletion: The chromium oxide layer is wiped away at friction points.
3. Shearing: Raw metal contact causes microscopic “clogging” and shearing.
4. Adhesion: The “sticky” nature of the metal causes it to adhere.
5. Seizing: This adhesion spreads across the thread flank, forming a complete weld within a single turn.

Ever Power Expert Solutions: How to Stop the Seize

Preventing galling requires a collaborative effort between the supplier and the end-user.

For Suppliers (Manufacturing Controls)

At Ever Power, we implement thermal management and lubrication during the secondary processing phase. By applying specialized dry-film lubricants or wax coatings to the threads, we significantly reduce the friction coefficient and increase heat dissipation capacity.

For Users (Installation Best Practices)

• Slow Down: If using power tools, reduce the RPM. Manual installation is always the safest route for stainless steel.
• Torque Management: Never “tighten to death.” Use a calibrated torque wrench to ensure you stay within the bolt’s elastic limit.
• Anti-Seize Paste: For high-torque applications like flange connections, applying an anti-seize lubricant (Molybdenum or Nickel-based) is the only 100% effective cure.

What if it’s already locked?

If the bolt has only just started to seize at the mouth of the thread, you might be able to back it out with extreme force. However, if it is already deep in the thread—save yourself the frustration. Saw it off and start fresh. It’s a sad reality, but once cold-welded, the metal is permanently transformed.

Need help choosing the right anti-galling coating for your project? Contact Ever Power today.