These two carry the majority of chrome-moly flange work, and the choice between them comes up on nearly every refinery enquiry. F11 is 1.25Cr-0.5Mo. F22 is 2.25Cr-1Mo. F22 has roughly twice the chromium and roughly twice the molybdenum, and it is the stronger material at temperature.
The temptation is to treat F22 as the safe default. On heavy wall hydroprocessing work that is often correct. Elsewhere it buys capability you are not using and restrictions you did not need.
What F22 gives you
- More hydrogen envelope. The API 941 Nelson curve for 2.25Cr-1Mo sits substantially above the one for 1.25Cr-0.5Mo, so it tolerates higher temperature at a given hydrogen partial pressure.
- Better creep strength, so higher allowable stress at temperature and the option of a thinner section.
- Better sulfidation resistance, though if sulfidation is the dominant mechanism you are usually looking at 5Cr rather than either of these.
- Heavy wall capability. F22 is the established grade for thick section hydroprocessing reactors and their connections, with a large body of industry practice behind it.
What F22 costs you
Beyond the material price, which is the obvious part.
- Temper embrittlement management. This is the big one. 2.25Cr-1Mo is the grade that loses toughness after long exposure in roughly the 650 to 1000 degree F range, and hydroprocessing specifications respond by restricting chemistry: a J-factor cap on base metal, an X-bar cap on weld metal, and often a step cooling test. F11 does not normally attract those requirements.
- Higher preheat and more demanding PWHT, because more chromium means more strongly air hardening.
- Narrower material availability once restricted chemistry is imposed, which lengthens lead time.
- More documentation, and more that can go wrong in it.
Restricted chemistry is not automatic. Ordinary A182 F22 to the bare ASTM specification satisfies the alloy chemistry and imposes no J-factor limit. If your service requires restricted chemistry, it has to be stated on the enquiry. Material that is dimensionally perfect and fails the J-factor is scrap for your purpose.
Side by side
| Factor | F11, 1.25Cr-0.5Mo | F22, 2.25Cr-1Mo |
|---|---|---|
| Chromium | ~1.00 to 1.50% | ~2.00 to 2.50% |
| Molybdenum | ~0.44 to 0.65% | ~0.87 to 1.13% |
| Forging classes | Class 1, 2, 3 | Class 1, 3 |
| Hydrogen envelope | Good | Substantially better |
| Creep strength | Good | Better |
| Temper embrittlement concern | Not normally restricted | Restricted on hydroprocessing service |
| Preheat and PWHT demand | Moderate | Higher |
| Cost and lead time | Lower | Higher, more so with restricted chemistry |
| Typical use | Refinery process, moderate hydrogen, steam piping | Hydroprocessing, heavy wall, higher temperature |
How to decide
- Get the hydrogen partial pressure and the design temperature. Not "hydrogen service", the actual numbers. Without them the Nelson curve cannot be read and nobody can confirm a grade.
- Plot the operating point against the F11 curve. Inside it with sensible margin, F11 is the correct engineering answer.
- Outside it, move to F22. And then check whether the specification imposes restricted chemistry, because that changes the order.
- Check the creep range separately. If the temperature puts you in creep, the design code allowables may decide the grade before the hydrogen curve does.
- Check sulfidation separately too. If sulfur is the dominant mechanism, neither of these may be the answer, and 5Cr or 9Cr is.
The grades either side
F12, 1Cr-0.5Mo, sits just below F11 and shows up on moderate temperature steam and process work where the hydrogen duty is light.
F21, 3Cr-1Mo, sits just above F22 and is less common in North American practice, though it appears where a little more envelope is wanted without going to 5Cr.
F1, C-0.5Mo, is the grade to be careful with. It is no longer recommended for new hydrogen construction, after field experience showed attack inside what the historic Nelson curve treated as safe. If there is hydrogen in the service, start at F11.
See chrome-moly grades for the full range, and temper embrittlement, J-factor and step cooling for the restriction detail.