In daily use of stainless steel bolts, a troublesome problem is often encountered—screw seizing (also known as galling or sticking). When this happens, the screw and nut get completely stuck together, unable to be further tightened or loosened, causing great trouble to work and production.
Best Practices for Using Stainless Steel Bolts
1. Why Are Stainless Steels Prone to Seizing?
Stainless steels, especially austenitic stainless steels (such as 304 and 316), are commonly used materials for fasteners due to their excellent corrosion resistance and mechanical properties. These stainless steels themselves have relatively low hardness. There is a thin chromium oxide protective film on the surface of stainless steel, which can prevent corrosion. However, when stainless steel fasteners are tightened, the pressure and heat generated between the thread crests will damage the chromium oxide layer, making the metal threads contact directly, resulting in blockage or shearing, and then adhesion. The heat generated by friction will also cause plastic deformation on the surface of metal threads, thereby aggravating this process. Seizing is more likely to occur especially under conditions of high-speed tightening, foreign objects in threads or inclined screwing.
Common Causes of Seizing
- Incorrect Material Matching: 304 bolts matched with 304 nuts are a typical no-no! The friction coefficient between the same stainless steel materials is extremely high, doubling the seizing risk.
- Irregular Operation:
a. High-speed tightening with an electric wrench (>30 rpm), leading to concentrated heat burst.
b. The bolt is not perpendicular to the locked surface, and inclination causes a sudden increase in local pressure. - Torque Out of Control: Tightening force exceeding 90% of the yield strength will cause plastic deformation of thread crests.
- Thread Contamination: Welding slag and iron filings stuck between threads accelerate the scratch of the oxide layer.
- High-Temperature Environment: In equipment such as steam turbines, thermal expansion will intensify thread engagement, and rust scale will block gaps at the same time.
- Not Using Washers: Neglecting the use of flat washers and spring washers cannot effectively prevent over-tightening during locking.
2. Effective Methods to Prevent Seizing
1） Select Products Correctly
Before use, confirm whether the mechanical properties of the product can meet the usage requirements, such as the tensile strength of the screw and the safe load of the nut. The length of the bolt should be selected appropriately, preferably exposing 1-2 thread pitches after tightening. Consider matching screws and nuts of different grades (such as 304 with 316).
2）Standardize Operation: Remember the Six-Word Principle “Clean, Align, Slow”
Clean threads before installation to remove burrs and impurities; first screw in 2-3 turns by hand to confirm alignment, then use a torque wrench for operation, control the tightening speed <20 rpm, and the torque shall not exceed 85% of the design value. For high-temperature operations, cool the bolts first to avoid forced tightening in a hot state.
3）Utilize Surface Treatment Technology
Adopt appropriate coating treatments, such as Polyseal, Xylan, Delta®-Seal or wax, which can also prevent seizing. Fluoropolymer coatings are mixtures of resins and fluoropolymer lubricants, which can achieve low friction, chemical resistance and corrosion resistance at temperatures up to 550°F. Nuts treated with Coating have an additional lubricating film between the nut and the screw.
Note: Ordinary grease is easy to dry and fail, which will increase the seizing risk!
4）Use Washers
The use of washers can effectively prevent over-tightening during locking.
- Emergency Handling Methods When Seizing Occurs
- Chemical Penetration Method: Spray thread loosener and let it stand for 30 minutes, then tap and vibrate with a rubber hammer, suitable for slight seizing.
- Thermal Expansion and Contraction Method: Quickly heat the nut with a welding torch (avoid burning the bolt), and disassemble it using the expansion gap, suitable for large-diameter bolts.
- Mechanical Demolition Method: Precisely split the nut with a hydraulic nut splitter, and control the tool bit feed to protect the bolt for reuse.
Prevention is More Important Than Maintenance. The seizing of stainless steel bolts is actually an inevitable result of the combined effect of material characteristics and operational errors. Remember the three cores: “heterogeneous matching, effective lubrication, and standardized operation”, and the seizing risk can be reduced by more than 90%.
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