F91 and F92 are the two creep-strength-enhanced ferritic steels you will meet on modern high-temperature steam valves. They exist because the older chrome-moly grades run out of strength before modern steam conditions do.
Neither is difficult to specify. Both are easy to ruin.
The grades
F91 - nominally 9Cr-1Mo-V, ASTM A182 Grade F91. Modified 9Cr with vanadium and niobium additions that form fine carbonitride precipitates. Those precipitates pin dislocations and give the creep strength. Usable to roughly 600 °C in valve service, with the actual limit read from the ASME B16.34 rating table for the grade.
F92 - nominally 9Cr-2W, ASTM A182 Grade F92. Similar base chemistry with tungsten substituted for part of the molybdenum and a boron addition. Higher creep rupture strength than F91 at the same temperature, which either buys margin or allows thinner sections.
F22 - 2.25Cr-1Mo, the grade both replaced. Perfectly good to around 540 °C. Above that its allowable stress falls away quickly, and the wall thickness needed to compensate becomes impractical.
Choosing between them
The honest answer is that on most projects you do not choose. The piping specification chooses, and the valve follows the pipe. Matching the valve body grade to the connecting pipe simplifies the weld and the PWHT, and mismatched creep grades across a weld is a problem nobody needs.
Where a choice does exist:
- F91 covers the great majority of main steam and hot reheat duty on supercritical units. It is more widely available, more suppliers have qualified procedures for it, and there is far more service experience.
- F92 is worth it where the design condition puts F91 at its limit, or where wall thickness has to be reduced - thinner walls mean lower thermal stress during transients, which on a cycling plant is a real benefit rather than a weight saving.
F92 is not a general upgrade. It is more expensive, less widely available, and its welding and heat treatment windows are, if anything, tighter than F91’s.
The part that actually matters: heat treatment
Both grades get their properties from a normalise-and-temper cycle that produces tempered martensite with the right precipitate distribution. Get the cycle wrong and the material fails to perform - not immediately, but years later, by creep.
Over-tempering is the dangerous one. Tempering too hot or too long coarsens the precipitates. Creep strength drops and cannot be recovered by re-tempering - the damage is to the precipitate structure, and only a full re-normalise would fix it. An over-tempered F91 valve passes its hydrostatic test, passes visual inspection, looks completely normal, and has lost a large part of the creep life you paid for.
Under-tempering leaves the material hard and low in toughness. It is easier to detect, because hardness testing finds it.
Missing or incorrect PWHT after welding produces untempered martensite in the heat-affected zone. Hard, brittle, and a crack initiation site.
This is why hardness testing after PWHT is not a formality on these grades. It is the only practical production check that the heat treatment did what it was supposed to.
What to demand on the order
For any F91 or F92 item:
- Mill certificate with full chemistry, traceable to the heat. Check the vanadium, niobium and nitrogen on F91, and the tungsten and boron on F92 - off-specification material in these elements does not develop the intended structure.
- Heat treatment charts for the forging: actual temperatures, hold times and cooling, with thermocouple positions identified. Not a certificate stating the cycle was performed - the chart.
- PWHT charts for every pressure-boundary weld, on the same basis.
- Hardness survey after PWHT, with the acceptance range stated and the actual results recorded per location.
- PMI on every pressure-containing part. F91 and F22 are visually indistinguishable and mix-ups happen.
- WPS and PQR for the welding procedures used, qualified for the grade.
If a supplier can produce items 1 and 6 but not 2, 3 and 4, that is worth a conversation before the order rather than during expediting.
Common failure modes in service
From the published failure literature and industry experience, the recurring themes are:
- Type IV cracking in the fine-grained heat-affected zone of welds, appearing after extended service. Weld design and PWHT control are the defences.
- Over-tempered material delivered as compliant, discovered only when creep damage appears.
- Mixed grades - an F22 component fitted into an F91 system, or the reverse, because PMI was not performed and the paperwork was taken on trust.
- Cycling damage on plants that now two-shift, where thermal fatigue interacts with creep in ways the original design did not consider.
Three of those four are procurement and quality control issues, not metallurgy. That is the practical point: with these grades, the documentation is the product quality, because you cannot see the problem in the finished valve.
A note on what we will and will not claim
We supply F91 and F92 bodies with the records listed above. We do not publish creep life figures or service life claims for them - those depend on the actual operating profile of your plant, which we do not have. What we can evidence is the material chemistry, the heat treatment, the hardness after PWHT and the NDE results. Ask for a sample document package before you place the order.