PREN Calculator for Stainless Steel
PREN equals chromium plus 3.3 times molybdenum plus 16 times nitrogen. Useful on its own, more useful next to what that corrosion resistance costs you in the machine.
The same number across the grades
PREN typical against machinability, from the same dataset as the grade reference. Read down and the pattern is hard to miss: the grades that cut easily are the ones with the least chromium and molybdenum doing useful work.
| Grade | PREN typical | Machinability | Usually specified for |
|---|---|---|---|
| 2507 super duplex | 43 | 20 | Hot seawater, desalination, offshore, aggressive chloride service |
| 904L | 35 | 33 | Sulphuric and phosphoric acid service, seawater |
| 2205 duplex | 35 | 30 | Seawater, brine, chemical process, pressure parts needing strength |
| 316 | 24 | 36 | Chloride service where the part is not welded |
| 316L | 24 | 36 | Food, dairy, pharma, medical instruments, marine, chemical process |
| 316Ti | 24 | 36 | Sustained service above 400 C where the low carbon trick is not enough |
| 304 | 19 | 45 | General purpose, indoor and mild outdoor, food equipment away from chloride |
| 304L | 19 | 45 | Welded 304 parts and assemblies |
| Nitronic 60 | 19 | 35 | Valve stems, sliding fits, threads that cycle, wear parts that cannot be lubricated |
| 303 | 18 | 78 | Dry indoor parts with threads and cross holes, fittings, bushings, shafts |
| 321 | 18 | 36 | Exhaust parts, furnace hardware, aerospace fittings |
| 430 | 17 | 55 | Trim, appliance panels, indoor decorative parts |
| 440C | 17 | 40 | Bearings, knife edges, wear parts, nozzles |
| 17-4 PH | 16 | 50 | Shafts, valve components, aerospace fittings, instrument parts needing 30 to 44 HRC |
| 15-5 PH | 15 | 48 | The same jobs as 17-4 PH where transverse toughness matters |
| 416 | 13 | 85 | Dry indoor shafts, studs, screw machine parts |
| 420 | 13 | 45 | Cutlery, moulds, surgical instruments that are dried between uses |
| 410 | 12 | 54 | Valve parts, fasteners, pump shafts that stay dry |
What PREN does not cover
PREN is a pitting index derived from composition. It is silent on the four things that actually kill most stainless parts in service.
Crevice corrosion. Under a gasket, in a thread or in a lap joint, attack starts at a lower chloride level than open surface pitting. Geometry beats composition here.
Stress corrosion cracking. Austenitic grades crack in warm chloride under tensile stress even when they are not visibly pitting. This is the failure that surprises people two years in, and duplex resists it far better than 316L at a similar PREN gap.
Surface condition. Embedded free iron from tooling rusts regardless of PREN, which is why every corrosion resistant part needs passivation. A rough or smeared surface pits sooner than a smooth one of the same alloy.
Galling. Nothing to do with corrosion, and it seizes more stainless assemblies than pitting ever does. See galling and Nitronic 60.