Technically reviewed by Vincent, Founder at HEXVARO
Last reviewed
What the coating is
A typical system has a corrosion-resistant base coat, an adhesion coat and a fluoropolymer topcoat. Published descriptions vary, and some systems use a zinc phosphate underlayer. Colour alone does not identify the system.
No ASTM standard governs how PTFE coating is applied, and thickness is not standardised. Write the coating system, thickness and test requirements into your order so that offers can be compared fairly.
Key figures to compare
These are published industry figures, not a guarantee for any coating. Confirm the values with the data sheet for the system you buy.
| Property | Published figure | Why it matters |
|---|---|---|
| Coating thickness | About 20–50 µm (0.0008–0.002 in) | Thick coating affects thread fit |
| Maximum temperature | Typically 204–260 °C (400–500 °F) | Check against design temperature, not just normal running temperature |
| Nut factor K, dry | 0.11–0.18 across six manufacturers | The same torque can give a different bolt load |
| Nut factor K, lubricated | About 0.10–0.12 | Lubrication narrowed the range in that test set |
| Bolt load scatter | 38–44 % at 95 % confidence | Plan tightening and verification around it |
Why torque needs extra care
Torque controls bolt load only indirectly. K-factor results do not transfer between coating makers, or between lubricants, so use values tested for your combination.
Testing of more than 700 coated studs from six manufacturers found a dry K range of 0.11 to 0.18, narrowing to about 0.10 to 0.12 with lubrication. That study was co-authored by a coated-stud supplier, so treat it as useful data and confirm it for your own system.
Some process services do not allow lubricants, including chlorine service. In those joints the dry value applies, so ask for test data on the dry condition.
Over-tapped nuts and thread engagement
Nuts are often over-tapped so they fit over the coating. This reduces thread engagement, and it is one reason a coated stud can feel easier to loosen. Ask how the nut is sized for the coated thread, and whether the nut is coated too.
Damage and reuse
- High assembly loads can thin or scrape the coating and expose the base metal.
- Damage changes the K-factor even when the stud still looks acceptable.
- One published recommendation is not to reuse coated studs. Follow your site's bolting procedure.
- Protect the threads during handling, and use packing that stops studs rubbing together.
PTFE compared with zinc coatings
| Point | PTFE system | Zinc plated or HDG |
|---|---|---|
| Thickness | About 20–50 µm | Plated 3–12 µm; hot-dip about 50 µm |
| Temperature limit | Similar to or slightly above zinc; often rated to 204–260 °C | About 199 °C (390 °F) |
| Friction | Low, but varies with coating quality | HDG thickness variation can affect nut factor |
| Stainless flanges | Confirm with your materials engineer | Zinc corrodes preferentially on stainless in the presence of an electrolyte |
When PTFE may not be the right choice
- The design temperature is above the coating's rated limit.
- The joint needs tight control of bolt load, and no K-factor data exists for your exact stud, nut and lubricant.
- The specification calls for a different coating, or forbids coatings.
- The extra cost is not justified. One practitioner review warns that PTFE studs are often over-prescribed.
What to include in your RFQ
HEXVARO can supply A193 B7 studs with PTFE coating. Send the points above and we will confirm the coating system and documents with your quotation.
- Stud grade, size and length, for example ASTM A193 B7, M24 × 200 mm.
- Mating nut grade, for example A194 2H, and whether the nut is coated.
- Coating system, colour, thickness range and whether it is one or two coats.
- Maximum design temperature and the service fluid.
- Corrosion test requirement, such as ASTM B117 hours, if your project sets one.
- Lubricant to be used, and any K-factor test requirement.
- Mill test certificates and coating inspection documents required.
Frequently asked questions
What temperature can PTFE coated studs handle?
Fluoropolymer coatings are typically rated to about 204–260 °C (400–500 °F). Use the data sheet for the exact coating, and check it against the design temperature.
Does PTFE coating change the bolt strength?
The strength grade comes from the base stud, such as A193 B7. The coating changes friction, so the torque needed to reach a given load can change.
Do I need to reduce torque on PTFE coated studs?
Do not reuse a torque value from uncoated studs. Calculate it from the K-factor tested for your stud, nut, coating and lubricant, and follow your bolting procedure.
Is blue PTFE coating better than other colours?
Colour does not define performance. Compare the coating system, thickness and test data.
Can PTFE coated studs be reused?
It is generally not recommended, because coating damage changes the K-factor even if the stud looks fine. Follow your site's procedure.
Is there a standard for PTFE coating on bolts?
No ASTM standard governs the application process, so the order should state the coating system, thickness and test requirements.
Products to this standard