Valve Preventing Galling and Seizure: Thread Lubrication and Material Pairing

What Is Galling and Seizure?

Galling is a form of adhesive wear that occurs when two metal surfaces slide against each other under high contact pressure. The metal surfaces weld microscopically, then tear apart, creating rough, galled surfaces. Seizure is the complete locking of the threaded connection, making it impossible to open the valve.

Galling commonly occurs on valve stems (stem-to-bonnet contact), valve bonnet bolts, and valve unions (threaded connections). This article explains the causes of galling, material pairing guidelines, and lubrication best practices to prevent galling and seizure.

Causes of Galling: Why It Happens

Cause 1: Same material pairing. When two identical metals (such as 316 stainless steel stem and 316 stainless steel bonnet nut) slide against each other, they are prone to galling because of the lack of galvanic protection and the similarity of hardness.

Cause 2: High contact pressure. Fine threads (such as in valve stems) have high contact pressure. When the lubricant film breaks down, metal-to-metal contact occurs, leading to galling. Cause 3: High temperature. High temperature softens the metal and accelerates adhesive wear.

Material Pairing: How to Avoid Galling

Rule 1: Avoid same-material pairing for threaded connections. For stainless steel stems, use bronze or brass bonnet nuts. For carbon steel stems, use stainless steel nuts or use a different grade of carbon steel (such as A194 Grade 2H nuts with A193 Grade B7 studs).

Rule 2: Use galling-resistant material combinations. 316 stainless steel paired with bronze (C95400 aluminum bronze) is excellent. 17-4PH stainless steel paired with XM-19 (Nitronic 50) is also good. Avoid 316-to-316, 304-to-304, and A193-B7-to-A194-2H (this pairing is acceptable but may gall without proper lubrication).

Thread Lubrication: Selecting the Right Lubricant

Thread lubricant (anti-seize) is essential for preventing galling. For ambient temperature: Use nickel-based anti-seize (for stainless steel) or copper-based anti-seize (for carbon steel). For high temperature (above 200 degrees C): Use graphite-based or ceramic-based anti-seize.

For oxygen service: Use oxygen-compatible lubricant (such as Krytox or halogenated hydrocarbon grease). Never use hydrocarbon-based grease in oxygen service. For food and pharmaceutical service: Use FDA-approved white grease or PTFE-based anti-seize.

Lubrication Application: Best Practices

Practice 1: Clean threads before applying lubricant. Remove old lubricant, rust, and debris. Practice 2: Apply lubricant to both male and female threads. Cover all thread surfaces, not just the first few threads.

Practice 3: Use the correct amount. Too little lubricant causes metal-to-metal contact. Too much lubricant attracts dirt and may contaminate the process fluid. Practice 4: Reapply lubricant after each disassembly. The old lubricant is contaminated and may have lost its anti-galling properties.

Stem Galling: Prevention for Valve Stems

Valve stem galling occurs between the stem and the bonnet bushing (or stem nut). Prevention: Use a stem material that is different from the bonnet material (316 stem + bronze bushing). Apply stem lubricant (graphite-based for high temperature, nickel-based for ambient).

For high-cycle valves (such as control valves), consider using a stem coating (such as chromium plating, nitriding, or PTFE coating) to reduce friction and prevent galling. The coating must be compatible with the process fluid and temperature.

Bolt Galling: Prevention for Bonnet Bolts

Bonnet bolt galling makes disassembly impossible and may require cutting the bolts (which damages the flange faces). Prevention: Use different material for bolts and nuts (A193-B7 studs + A194-2H nuts is acceptable; A193-B8 (316) studs + A194-8 (316) nuts requires anti-seize).

Always use anti-seize on bonnet bolts, even for dissimilar materials. Tighten bolts in a cross pattern to the specified torque. Over-tightening increases contact pressure and promotes galling. Use spring washers or lock nuts to prevent bolt loosening from vibration.

Galling Inspection and Remediation

Inspect threaded connections for galling during maintenance. Signs of galling: rough, torn thread surfaces; difficulty turning the stem or nut; visible metal transfer between mating surfaces.

Remediation: For minor galling, clean the threads, reapply anti-seize, and reassemble. For severe galling, replace the galled component (stem, bonnet nut, bolt). Never reuse a galled threaded connection - it will seize again.

Conclusion: Prevent Galling with Material Selection and Lubrication

Galling and seizure are preventable. By selecting dissimilar materials for threaded connections, applying the correct anti-seize lubricant, and following proper assembly practices, you can eliminate galling and ensure reliable valve operation.

At Wenzhou Wofer Valve Co., Ltd., we use galling-resistant material pairings and apply anti-seize to all threaded connections. Contact us today for valves that are designed for easy maintenance and long service life.

Contact Us

Ted Wang

Wechat / WhatsApp: +86 18267833722

Email: sales@wofervalve.com

Website: www.wofervalve.com

Wenzhou Wofer Valve Co., Ltd.