C&N HUITONG limited are leading steel pipe,fitting and flange Supplier since 1997.
Phone: +86-0317-6287569     E-mail: alloypipefitting@gmail.com

PTFE Coated Studs

PTFE Coated Studs: A Little Background

PTFE is short for Polytetrafluoroethylene, a chemical applied to common bolting materials (such as B7 stud bolts) to provide corrosion and chemical resistance.

PTFE Coating Specifications
Tensile Strength (ASTM D1708)3000-5000 psi
Elongation (ASTM D1457)300-500%
Impact Strength (ASTM D256)3.5 ft-lb/in
Hardness (ASTM D2240)50-65 HB (shore D)
Abrasion Resistance (Tabor)12 mg
Coefficient of Friction (ASTM D1894)0.12 -0.15 static, 0.05 -0.10 dynamic
Dielectric Strength (ASTM D149)450 volts per mil
Use Temperature600°F max
Melting Point625°F
Thermal Conductivity1.7 BTU-in/h-ft 2-°F
Chemical Resistance (ASTM D543)Excellent
Salt Spray Resistance (ASTM B117)Fair
Water Absorption (ASTM D570)0.01%
Thickness20 – 40 Microns

Some other common coatings for PTFE coated fasteners are Teflon® and Xylan®. In this article, we’ll refer to all of them as PTFE.

PTFE coated studs have been used for many years within the bolting industry, especially in any application that requires corrosion resistance or in offshore applications (salt spray is hard on Grade B7 material). PTFE is also useful if you’ve had “galling” and need a lower breakout torque for safe removal.

PTFE should not be used in high-temperature applications. Fluoropolymer coatings manufacturers use temperatures in the 400F-500F as the maximum temperatures, so you typically do not see alloy steel bolts (such as B8) with PTFE coatings.

While PTFE coated studs (including Xylan® coated studs) have corrosion-resistant properties, which are functionally understood by assemblers, the technical aspects of fluoropolymer coatings are often misunderstood.

A Brief History of PTFE

Before PTFE coated bolts came along, the petrochemical industry used other methods of making corrosion-resistant bolting components, such as hot dip, galvanized, cadmium or zinc-plated fasteners.

Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. The most well-known brand name of PTFE-based formulas is Teflon by Chemours. Chemours, a spin-off from DuPont, originally discovered the compound in 1938.

PTFE (polytetrafluoroethylene) coatings are considered non-stick coatings. Therefore you need a process to apply the “non-stick” coating to the “B7 stud material”. Therefore, the typical manufacturing process is a three-step process.

  1. Apply a corrosion resistant base coating
  2. Apply an adhesion coating
  3. Apply a polytetrafluoroethylene nonstick topcoat

It’s unclear exactly when the bolting industry started using PTFE coated studs, but they have been around for a while. They tend to be used in highly corrosive environments.

In this article we’ll provide practical advice, and address some of the most common questions we hear from craft assemblers today, including:

  1. Are all PTFE coated studs the same?
  2. Do I lubricate PTFE coated studs?
  3. Can I re-use PTFE coated studs?
  4. Why do PTFE studs have low friction and lower break out torque than regular studs? (Grade B7 with 2H hex nuts for example)
  5. Does the PTFE coating help with corrosion resistance?

Click any of those questions above to jump directly to the answer you want.

1. Are all PTFE coated studs the same?

Absolutely not.

While PTFE is the same as Teflon® on a chemical level, they are not applied the same way by each manufacturer. There also are several different types of base coats.

Therefore, the thickness of the coating on the fastener is not a standard — and quite frankly, it’s not controlled.

As a result, you must choose one manufacturer and test your k-factor for their product, and recognize that those values do not transfer to other PTFE coated stud manufacturers.

2. Do I lubricate PTFE coated Studs?

Yes!

At 30,000psi bolt load (not to be confused with 30% tensile strength), you start stripping your coating. Also, the coating will start binding. Therefore, you’ll have better accuracy and less bolt scatter (or differences in bolt loads on each bolt) by using lubricating them.

However, you will have to test your lubricant and manufacturer combination in order to correctly determine your K-Factor. Note: Don’t confuse K-Factor with low coefficient of friction, which is not used for this calculation.

(Learn proper lubrication techniques in our Level 1 course — now available free.)

3. Can I re-use PTFE coated studs?

You shouldn’t.

If you were to re-use PTFE coated studs, the corrosion-resistant coating on the threads will most likely be at least somewhat degraded or damaged, meaning your K-Factor will change again.

Physically the bolt might still hold up to corrosion. So visually, they would still look good, which would lead someone to think that the life expectancy of the stud would be longer. But appearances can be deceiving, and you shouldn’t reuse PTFE coated studs.

4. Why do PTFE studs have low friction and lower break out torque than regular studs?

What would you say if I told you that, in order to fit the nut on the stud with the PTFE coating, you must drill a bigger threaded hole (tap) through a 2H nut? Yes, it’s true. And there is are no technical specifications on this.

I couldn’t believe it either, but you will be effectively taking 30%-50% of the contact surface away. Therefore, it is not the PTFE that makes it easier to disassemble (and you have a low friction between stud and nut on assembly). This phenomenon is due to having considerably less contact area!

5. Does the PTFE coating help with corrosion resistance?

Yes…to an extent.

Let’s say you have a B7 stud (ASTM A193) and 2H nuts. The application process and proprietary materials of the PTFE coating is intended to help with corrosion resistance. However, since the coating is proprietary to the manufacturer, it is hard to say how well it helps with corrosion resistance.

One way to test this is with a salt spray test with ASTM B117, which sprays the stud bolts up to 3,000 hrs while not freezing the nuts.

Prev:

Next:

Leave a Reply

Leave a message