A thread lubricant’s temperature rating is often the limit of one ingredient, not of the whole compound. ASME PCC-1-2026 points out the same thing. Here is why it matters when you choose a lubricant for hot service, and how the Moly-B and Nickel 328 ratings are stated.
One number, many ingredients
A thread compound is a carrier oil, a soap thickener, solid lubricants such as moly, graphite or nickel flake, and additives. Each one reaches its limit at a different temperature. Many data sheets give only the limit of the ingredient that lasts longest, usually the metal flake (Rocol). A higher number does not make a product better at normal flange temperatures, where the oil and thickener still control friction.
What PCC-1-2026 says
Appendix K of ASME PCC-1-2026 makes the same point (paraphrased): a lubricant maker’s maximum temperature has not proven to be a reliable sign that a joint will come apart more easily after hot service. The figure is usually the melting or degradation point of the highest-temperature solid. PCC-1 advises getting test results for materials and conditions like your own.
Where the ingredients give out
- Carrier oil and thickener: a few hundred °F. After that, only the solids are left on the threads.
- Moly: starts to oxidize in air at about 750°F (Parker). It holds to about 1,706°F only when there is no air.
- Graphite: oxidation starts at roughly 570–840°F (SGL Carbon).
- Nickel: melts at 2,651°F (RSC).
How our ratings are stated
| Moly-B (PB-LD-301) | Nickel 328 (PB-LD-302) | |
|---|---|---|
| Published rating | 750°F, with prorated results to 1,500°F | Cryogenic to 2,500°F |
| What sets the rating | Moly starts to oxidize in air at about 750°F. The rating sits there, not at the 1,706°F no-air figure. | The nickel solids. The datasheet shows how much of the solids reaches its limit at each temperature. |
| Breakdown shown on the datasheet | 67% of the solids decompose at 750°F and 33% at 1,500°F. Most of the 67% is replaced by a byproduct that is also a lubricant and stays intact to 1,200°F. | Melting point of solids: 2% at 350°F, 3% at 1,900–2,000°F, 75% at 2,500°F. Carrier boiling point above 300°F. |
| After the carrier burns off | No ash or silica | No ash or silica. No rust inhibitors, because they turn to ash at the rated temperatures. |
| Nut factor (K) at assembly | 0.100 | 0.172 / 0.144 / 0.120, depending on bolt load |
| Solids content | 74% | 77%, mostly nickel (a blend of three solids) |
Moly-B is rated at the point where moly oxidizes in air, not at the no-air figure. Both datasheets show what happens to the compound as it gets hotter.
Full article and printable PDF: Thread Lubricant Temperature Ratings Explained (PB-TD-226), with the ingredient-by-ingredient breakdown, PCC-1 notes and a checklist for choosing a lubricant for hot service.
