F91 is the forged product form of modified 9Cr-1Mo-V steel, specified in ASTM A182 as Grade F91. Equivalent designations are ASME SA-182 F91, UNS K90901, Werkstoff 1.4903 and EN X10CrMoVNb9-1. The grade contains 8.00 to 9.50 % chromium and 0.85 to 1.05 % molybdenum with controlled vanadium, niobium and nitrogen. It is supplied normalised and tempered, with a minimum tensile strength of 585 MPa (85 ksi) and a minimum yield strength of 415 MPa (60 ksi). At 600 °C the 100,000 hour creep rupture strength is 94 MPa.
Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China. We forge F91 bars, hollow bars, seamless rolled rings, flanges, discs, valve bodies, bonnets, seats, stems, tees and wyes to customer drawings. Phone +86 189 2135 9659. Email sales@steelforgepieces.com.
What is ASTM A182 F91?
ASTM A182/A182M covers forged or rolled alloy and stainless steel pipe flanges, forged fittings, and valves and parts for high-temperature service. Grade F91 within that specification is modified 9Cr-1Mo steel, a martensitic creep-strength enhanced ferritic (CSEF) steel developed at Oak Ridge National Laboratory in the 1970s.
The alloy differs from plain 9Cr-1Mo (Grade F9) by controlled additions of vanadium, niobium and nitrogen. These elements form fine MX carbonitride precipitates during tempering. The precipitates restrict dislocation movement at service temperature and resist coarsening, which raises the allowable stress at 600 °C well above that of 2¼Cr-1Mo steel. Designers use that margin to reduce wall thickness, support loading and start-up times in high-temperature steam systems.
The same alloy is supplied in other product forms under other specifications: P91 seamless pipe to ASTM A335, T91 tube to ASTM A213, Grade 91 plate to ASTM A387 and C12A castings to ASTM A217. Forged flanges, valve bodies, header stubs and rolled rings in this alloy are ordered as A182 F91.
F91 equivalent grades and cross-reference
The table lists equivalent designations for ASTM A182 F91 in other standards and product forms. Equivalents are close but not identical. Confirm the chemistry and mechanical requirements of the standard named on the order.
| Standard or system | Designation | Product form |
|---|---|---|
| ASTM A182 / ASME SA-182 | F91 | Forged flanges, fittings, valves and parts |
| UNS | K90901 | Unified numbering |
| EN 10222-2 / EN 10216-2 | X10CrMoVNb9-1 | Forgings and seamless tubes |
| Werkstoff (DIN) | 1.4903 | Material number |
| ASTM A335 | P91 | Seamless ferritic alloy steel pipe |
| ASTM A213 | T91 | Seamless boiler, superheater and heat-exchanger tube |
| ASTM A387 | Grade 91 | Pressure vessel plate |
| ASTM A217 | C12A | Castings for pressure-containing parts |
| GB/T (China) | 10Cr9Mo1VNbN | Forgings and pipe |
| JIS (Japan) | STPA 28 / SFVAF 91 | Pipe / forgings |
| Common trade names | Grade 91, Gr 91, F 91, Mod 9Cr-1Mo | — |
Cross-reference compiled by Jiangyin Jiangnan Metal Co., Ltd. Verify against the governing edition of each standard before ordering.
F91 chemical composition
F91 is a low-carbon, high-chromium alloy. The specification holds vanadium, niobium and nitrogen within narrow limits. Aluminium is restricted because it combines with nitrogen to form AlN, which reduces the nitrogen available for V(C,N) precipitation and lowers creep strength.
| Limit | C | Si | Mn | P | S | Al | Cr | Cu | Mo | N | Nb | Ni | V |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Min % | 0.08 | — | 0.30 | — | — | — | 8.00 | — | 0.85 | 0.030 | 0.060 | — | 0.18 |
| Max % | 0.12 | 0.50 | 0.60 | 0.020 | 0.005 | 0.040 | 9.50 | 0.30 | 1.05 | 0.070 | 0.100 | 0.30 | 0.25 |
| Balance iron. Values as published in Key to Steel (2010). Confirm against the governing edition of ASTM A182/A182M on the order. Sulphur, aluminium and nickel limits have been tightened in later revisions. | |||||||||||||
Heat chemistry, including nitrogen and aluminium, is reported on the mill test certificate supplied with each forging.
F91 mechanical properties at room temperature
Values apply to material in the normalised and tempered condition, tested at 20 °C (68 °F).
| Property | Value | Condition or basis |
|---|---|---|
| Tensile strength Rm | ≥ 585 MPa / 85 ksi | ASTM A182 F91 minimum |
| Yield strength Rp0.2 | ≥ 415 MPa / 60 ksi | ASTM A182 F91 minimum |
| Tensile strength Rm | 620–850 MPa | Normalised and tempered, typical range |
| Yield strength Rp0.2 | ≥ 450 MPa | Normalised and tempered, typical |
| Elongation A5, longitudinal | ≥ 20 % | Normalised and tempered |
| Impact energy, transverse | 40 J | Normalised and tempered |
| Hardness | ≤ 248 HBW | Typical as-supplied limit for N + T material |
Two tensile values are listed. 585 MPa is the specification minimum used for design. 620 to 850 MPa is the range normally obtained on normalised and tempered forgings and reported on the test certificate.
F91 yield strength at elevated temperature
The 0.2 % proof stress of F91 falls to roughly half its room-temperature value at 600 °C. The figures below are used as the starting point for design checks above room temperature.
| Temperature °C / °F | Rp0.2 MPa / ksi |
|---|---|
| 20 / 68 | 450 / 65.0 |
| 100 / 212 | 410 / 59.4 |
| 200 / 392 | 380 / 55.1 |
| 300 / 572 | 360 / 52.2 |
| 400 / 752 | 340 / 49.3 |
| 500 / 932 | 300 / 43.5 |
| 600 / 1112 | 215 / 31.1 |
| ksi values calculated from the metric figures. | |
F91 creep rupture strength
Creep rupture strength governs the design of F91 components in long-term high-temperature service. The 100,000 hour values are used for piping design life.
| Temperature °C | Rp0.2 MPa / ksi | Rupture, 10,000 h MPa / ksi | Rupture, 100,000 h MPa / ksi |
|---|---|---|---|
| 500 | 300 / 43.5 | 289 / 41.9 | 258 / 37.4 |
| 550 | 270 / 39.1 | 199 / 28.8 | 166 / 24.0 |
| 600 | 215 / 31.1 | 122 / 17.6 | 94 / 13.6 |
| 650 | — | 70 / 10.1 | 49 / 7.1 |
| Between 600 °C and 650 °C the 100,000 hour rupture strength falls from 94 MPa to 49 MPa, a reduction of about 48 %. For this reason 600 °C is normally taken as the practical limit for F91 pressure parts. | |||
F91 physical and thermal properties
| Property | Value |
|---|---|
| Density | 7.76 kg/dm³ |
| Modulus of elasticity | 218,000 MPa (218 kN/mm²) |
| Specific heat capacity | 460 J/kg·K |
| Thermal conductivity | 26 W/m·K |
| Magnetisable | Yes, F91 is ferromagnetic |
| Temperature °C / °F | Specific heat J/kg·K | Thermal conductivity W/m·K | Modulus of elasticity kN/mm² | Mean expansion from 20 °C 10⁻⁶/K |
|---|---|---|---|---|
| 20 / 68 | 460 | 26 | 218 | — |
| 200 / 392 | — | — | 207 | 11.3 |
| 400 / 752 | — | — | 190 | 12.0 |
| 500 / 932 | — | 30 | — | 12.3 |
| 600 / 1112 | — | — | — | 12.6 |
| 650 / 1202 | — | — | 162 | 12.7 |
| Values not given in the source data are shown as a dash. Contact us for figures at intermediate design temperatures. | ||||
Property data on this page published by Jiangyin Jiangnan Metal Co., Ltd., open-die forging factory, Jiangyin, Jiangsu, China. Last reviewed 14 September 2026.
F91 heat treatment: normalising and tempering
F91 is supplied normalised and tempered. The heat treatment produces the tempered martensite structure that carries the creep strength of the grade.
Normalising
ASTM A182 requires normalising from 1040 °C (1900 °F) minimum. The soak dissolves the vanadium and niobium carbonitrides so that they re-precipitate in fine form during tempering. Insufficient soaking leaves them undissolved and reduces creep strength permanently. Excessive soaking coarsens the prior austenite grain and reduces toughness. Cooling is in still or forced air.
Tempering
Tempering follows at 730 °C (1350 °F) minimum, commonly between 750 °C and 780 °C, then air cooling. The martensite finish temperature is approximately 100 °C. Forgings are cooled below that point before tempering starts so that the transformation is complete through the section.
Post-weld heat treatment
PWHT on F91 weldments is normally carried out between 750 °C and 770 °C. The temperature must stay below the lower critical temperature Ac1, which lies between approximately 800 °C and 830 °C depending on chemistry. Exceeding Ac1 re-austenitises the weld and produces untempered martensite.
Heat treatment is carried out in-house. Time and temperature charts, furnace records and post-temper hardness results are issued with the mill test certificate, so the 1040 °C and 730 °C minima can be verified on receipt.
F91 forgings we manufacture
We are an open-die forging factory. Each F91 item below is forged to the customer drawing or size list, heat treated, and machined to the agreed allowance.
Forged bars
Round bars and rods, square, rectangular and flat bars, hex bars. Rough turned, peeled or black as required.
Hollow bars and sleeves
Bored hollow bars, sleeves, bushes and bushings, forged pipe and tube sections, barrels, casings, shells, cylinders, hubs and housings.
Rings
Seamless rolled rings and forged rings, plain or profiled, for headers, flanged connections and valve internals.
Flanges and discs
Forged flanges, blocks, discs and disks, plates and blanks for machining.
Valve components
Valve bodies and bonnets, valve seats and seat rings, valve stems and gate discs for high-temperature steam service.
Fittings
Equal, eccentric and lateral tees, T-pieces, Y-pieces, wye fittings and piggable wyes.
Size capability depends on the product form. Send the drawing, finished dimensions, required forging ratio and quantity, and we will confirm feasibility and lead time. Where no drawing is available, an OD × ID × length or OD × thickness sketch with the part weight is sufficient to start a quotation.
Testing, inspection and certification
F91 is a code material. Test scope is set on the order rather than assumed. The following are available.
- Chemical analysis. Full heat analysis including nitrogen and aluminium, with product analysis on request.
- Mechanical testing. Tensile, yield, elongation and reduction of area, with Charpy V-notch impact testing at the specified temperature.
- Hardness. Brinell survey after tempering, reported per piece or per heat-treatment lot.
- Ultrasonic examination to ASTM A388 or the customer acceptance class.
- Surface NDT. Magnetic particle or dye penetrant inspection.
- Positive material identification (PMI) on finished parts to confirm grade.
- Certification. EN 10204 3.1 mill test certificate as standard, 3.2 with a third-party inspector such as TÜV, BV, SGS, Lloyd's or DNV when the order requires it.
Applications of F91 forgings
F91 is specified for components that operate in the creep range, where long-term rupture strength rather than room-temperature yield governs the design.
- Supercritical and ultra-supercritical power plant. Main steam and hot reheat piping, header stubs, tees and wyes, forged flanges and valve bodies.
- Boilers. Superheater and reheater headers, outlet connections and desuperheater components.
- Petrochemical and refining. Hydroprocessing and catalytic reformer service at elevated temperature, transfer line components.
- High-temperature valves. Gate, globe and check valve bodies, bonnets, seats and stems rated for steam service.
- Waste-to-energy and biomass plant. Steam-side components operating in the creep range.
Welding and machining F91
Matching consumables are used for F91: AWS E9015-B9 covered electrodes or ER90S-B9 solid wire, both depositing 9Cr-1Mo-V weld metal.
Preheat is typically 200 °C to 250 °C. Interpass temperature is held below approximately 300 °C. After welding, the joint is cooled to 80 °C to 100 °C so that the martensite transformation completes, then given PWHT at 750 °C to 770 °C without exceeding Ac1.
Type IV cracking is the characteristic long-term failure mode of Grade 91 weldments. Creep damage concentrates in the fine-grained heat-affected zone outside the weld and can develop after extended service in joints that met every acceptance criterion at fabrication. Joint location away from high-stress positions and close control of PWHT are the normal countermeasures.
For machining, F91 in the normalised and tempered condition at approximately 200 to 248 HBW behaves in a similar way to quenched and tempered 4140. Rigid setups, sharp carbide tooling and moderate cutting speeds with generous feed are used. Forgings can be supplied rough machined with an agreed allowance to reduce finishing work.
How to order F91 forgings
The following information is required for a quotation. Sending it together avoids a round trip and allows same-day pricing in most cases.
- Drawing or dimensions, including OD, ID, length or thickness with tolerances.
- Specification and edition, for example ASTM A182 F91 latest edition, or a company specification.
- Quantity and delivery schedule.
- Supply condition: black forged, rough machined or finish machined.
- Testing scope, including impact test temperature, UT class, PMI and hardness reporting.
- Certification level, EN 10204 3.1 or 3.2, and the inspection agency if third-party witnessing is required.
Frequently asked questions about F91
What is the difference between F91 and P91?
F91 and P91 are the same modified 9Cr-1Mo-V alloy in different product forms. F91 is the forging grade under ASTM A182, used for flanges, fittings, valve bodies and forged components. P91 is the seamless pipe grade under ASTM A335. Chemistry and heat treatment are essentially identical. The specifications differ because the manufacturing route and test requirements differ.
Is F91 the same as Grade 91?
Yes. Grade 91 is the general industry name for modified 9Cr-1Mo-V steel. F91 is the designation for the forged product form of that grade under ASTM A182, in the same way that T91 is tube and P91 is pipe.
What is the maximum service temperature of F91?
F91 is normally applied up to about 600 °C (1112 °F) for pressure-containing parts. It can be used to 650 °C, but the 100,000 hour creep rupture strength falls from 94 MPa at 600 °C to 49 MPa at 650 °C, so allowable stresses become uneconomical and oxidation resistance also becomes a limiting factor.
What heat treatment does F91 require?
Normalising from 1040 °C minimum followed by air cooling, then tempering at 730 °C minimum, commonly 750 °C to 780 °C. The forging is cooled below about 100 °C between the two operations so that the martensite transformation completes before tempering.
Is F91 magnetic?
Yes. F91 has a tempered martensitic structure and is ferromagnetic, which also allows inspection by magnetic particle examination.
What is the chemical composition of F91?
F91 contains 0.08 to 0.12 % C, up to 0.50 % Si, 0.30 to 0.60 % Mn, 8.00 to 9.50 % Cr, 0.85 to 1.05 % Mo, 0.18 to 0.25 % V, 0.060 to 0.100 % Nb and 0.030 to 0.070 % N, with phosphorus limited to 0.020 % and sulphur to 0.005 %, balance iron.
What is the tensile strength of F91?
ASTM A182 F91 requires a minimum tensile strength of 585 MPa (85 ksi) and a minimum yield strength of 415 MPa (60 ksi). Normalised and tempered forgings typically test in the 620 to 850 MPa range with yield above 450 MPa.
Why does F91 fail by Type IV cracking?
Welding creates a narrow fine-grained zone in the heat-affected zone where the strengthening precipitates are partly dissolved. Under long-term creep loading this band is weaker than both the parent metal and the weld metal, so damage concentrates there. It is a long-term service phenomenon rather than a fabrication defect, and it is managed through joint design, PWHT control and inspection planning.
Can you supply F91 forgings to a custom drawing?
Yes. Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory and works to customer drawings for bars, hollow bars, seamless rolled rings, flanges, discs, valve bodies, bonnets, seats, stems, tees and wyes in F91. Send the drawing to sales@steelforgepieces.com for a quotation.
Who supplies F91 open-die forgings in China?
Jiangyin Jiangnan Metal Co., Ltd., at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China, manufactures ASTM A182 F91 open-die forgings including bars, rings, flanges, discs and valve components, supplied normalised and tempered with EN 10204 3.1 or 3.2 certification. Phone +86 189 2135 9659 or email sales@steelforgepieces.com.
About the manufacturer
Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory in Jiangyin, Jiangsu Province, China. We forge carbon steel, alloy steel, tool steel, stainless steel and nickel alloys into bars, hollow bars, seamless rolled rings, flanges, discs, shafts and valve components, then heat treat, machine and certify them for export.
F91 is part of our alloy steel range, which also covers AISI 4140, 42CrMo4, 34CrMo4 and AISI 4130. For service temperatures above the range of F91 we forge nickel alloys such as Inconel 601 and Incoloy 800.
Request an F91 quotation
Jiangyin Jiangnan Metal Co., Ltd., open-die forging factory
- Address
- No.1 Chengxiqiao Road, Zhouzhuang Town
- Jiangyin City, Jiangsu Province, China
- Phone, WhatsApp, WeChat
- +86 189 2135 9659
Related grades
Other alloy steel and high-temperature grades we forge: