By Dale Norman and Angelica Pajkovic

 
 

Bolted flange joints remain one of the most common connection points in industrial facilities, and among the most failure-prone. Achieving reliable sealing typically depends on a predictable relationship between torque and tension, which compresses the gaskets. However, variability in fastener performance and environmental degradation have long undermined this predictability, leading to leaks, maintenance challenges, and unplanned downtime.

Recent research and developments in fastener coating technology are addressing these shortcomings. By improving torque-to-tension consistency and providing enhanced corrosion resistance, coated B7 studs are emerging as a critical step forward in bolted joint reliability.

 

The Bolt–Gasket Relationship

The gasket in a bolted joint does not seal on its own; it relies on the clamp load generated by the fasteners. When torque is applied to the nut, tension is created in the stud, compressing the flange and gasket. This tension translates into compressive stress across the gasket surface, which makes the seal.

Reliable gasket performance requires three conditions:

• Uniform load distribution: Unevenly tensioned bolts create irregular compression, leading to potential leak paths.
• Sufficient seating stress: Gaskets need a threshold level of compression to conform to flange irregularities.
• Sustained stress over time: Relaxation of bolt load due to creep, corrosion, or vibration reduces gasket stress and eventually causes leakage.

As bolt torque does not perfectly correspond with gasket compression, reliability in torque-to-tension transfer is essential. Variability in fastener behavior directly affects whether the gasket achieves and maintains its required sealing stress.

 

The Challenges: Variability and Corrosion

1. Torque-to-tension variabilityStuds display wide variability in torque-to-tension performance, including the most often used ASTM A193-B7 studs, even if they are PTFE coated. Past studies have documented scatters of up to ±44% across manufacturers. Causes include over-tapped nuts, irregular coatings, and differences in thread friction. This scatter results in non-uniform gasket compression, which increases leakage risk or overstresses parts of the joint.¹
2. Corrosion and gallingUncoated or minimally coated studs are more vulnerable to oxidation, galling, and seizure. In aggressive environments, this shortens service life and disrupts uniform gasket loadingwhen bolts cannot be consistently tightened or retightened.²

 

Engineered Coatings: Industry Approaches vs. PROTINN® Results

The performance of bolted joints largely depends on the fasteners’ ability to translate applied torque into consistent clamp load. Variability in frictional behavior, often represented by the nut factor (K-factor), has been a key obstacle to reliable gasket compression. Engineered fluoropolymer coatings, such as PTFE or thin-film fluoropolymer topcoats over phosphate-treated substrates, are widely used to address this issue, as they simultaneously reduce friction and provide corrosion protection.

Fluoropolymer coatings have been applied to B7 studs due to their inherently low coefficient of friction and their resistance to environmental degradation, including salt spray and chemical exposure. Testing of over 700 coated studs across six manufacturers revealed that while low friction improves torque-to-tension efficiency, inconsistencies in manufacturing, such as thread geometry adjustments to accommodate coating thickness, can produce significant variability in dry K-factors. This variability can sometimes range from 0.11 to 0.18, with bolt load scatter reaching 38–44% at a 95% confidence interval.^1,6

Lubrication can narrow this range, bringing K-factors toward 0.10–0.12, but many chemical process environments, such as chlorine service, prohibit the use of lubricants, leaving operators exposed to high variability in dry conditions.

Controlled studies of PROTINN® coated studs demonstrate that process-controlled coating applications can stabilize K-factors at 0.12 under both dry and lubricated conditions, with bolt load scatter significantly reduced.⁴ Flange validation testing using ultrasonic bolt elongation on a 10” Class 150 flange confirmed that these torque-to-tension values translate into predictable, real-world bolted joint behavior.

This type of engineered fluoropolymer coating also provides robust corrosion protection. For example, zinc phosphate underlayers combined with thin-film fluoropolymer topcoats (~25 µm thickness) have demonstrated resistance exceeding 1,500 hours in ASTM B117 salt fog tests and strong performance in DIN 50018 sulfur dioxide (SO₂) Kesternich cycling, with minimal red rust formation.⁵ These properties extend fastener service life and help maintain thread integrity under harsh offshore, coastal, or chemical plant conditions.

By combining controlled friction characteristics with environmental durability, engineered fluoropolymer coatings address both the mechanical and chemical challenges of bolted joints. Targeted process control in coating applications transforms these widely used materials into precision-engineered solutions that ensure reliable gasket sealing and consistent clamp load performance.

 

System-Level Perspective

While coatings can be integral to the integrity of the bolt joint, how the fastener interacts with every other component in the assembly is of equal importance. Bolted joint reliability is not determined by the fastener alone. It emerges from the interaction of studs, nuts, lubricants, gasket materials, and assembly practices. Within this system, B7 studs with controlled, engineered fluoropolymer contribute by:

1. Enabling precise and repeatable clamp loads, giving gaskets the compression they need to seal.
2. Preserving long-term serviceability of threads, ensuring bolts can be retorqued during maintenance.
3. Reducing variability between bolts, leading to balanced gasket loading and fewer weak points.

When bolts and gaskets function together as designed, leakage risk is minimized, joint life is extended, and overall piping system integrity is strengthened.

 

Final Thoughts

Reliable sealing starts long before the flange is tightened. It begins with engineered components that limit guesswork from the assembly process. The evolution of fastener coatings illustrates how material engineering can solve persistent reliability challenges.

For industries facing stringent emissions targets and the high costs of unplanned downtime, advances in coated stud technology, like the engineered fluoropolymer topcoat PROTINN®, represent a practical, engineering-driven step toward more predictable and durable bolted connections. In bolted flange joints, improved torque-to-tension consistency and corrosion resistance offer a measurable impact on sealing reliability, uptime, and safety.

 

Why PROTINN® Through TEADIT®

 TEADIT® is proud to make this solution available to its customers because it believes every detail matters when building a leak-free future.

Choosing PROTINN® through TEADIT® provides more than just access to high-performance studs.

Customers also benefit from:

• Technical support and application expertise: Our engineering teams can advise on torque tables, assembly practices, and gasket compatibility.
• Consolidated supply chain: Simplify sourcing by pairing gaskets and studs from a single, trusted supplier.Proven track record: With decades of experience in sealing systems, TEADIT® ensures products are not only tested but field-proven.
• To learn more about PROTINN® studs or to request a trial for your facility, contact TEADIT® today.

 

References

1. Hamilton, S., Meigs, B., Province, J., & Tiney, B. (n.d.). Additional Research into PTFE Coated Studs.
2. Hex Technology. How to Prevent Galling. 2021. Retrieved from https://www.hextechnology.com/articles/how-to-prevent-galling/
3. Whitford. Xylan® coatings for fasteners: Friction control and corrosion protection. Product literature. 2016. https://www.whitfordww.com
4. PROTINN®. PTFE-coated studs: Torque-tension performance and real-world flange testing. 2023. [Internal Technical Data Sheet]
5. Whitford Xylan® 1424. High-performance fluoropolymer coating for fasteners. Product datasheet. 2023. https://www.whitfordww.com/xylan-1424
6. Hex Technology. K-Factor: The relationship between torque and tension. 2021. https://www.hextechnology.com/articles/k-factor
   

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