What Is ASTM A182 F91 Steel?
ASTM A182 F91 is a 9% chromium, 1% molybdenum, vanadium-niobium modified (9Cr-1Mo-V-Nb) martensitic alloy steel, standardised under ASTM A182/A182M for forged and rolled alloy-steel pipe flanges, fittings, valves, and pressure-containing components for high-temperature, high-pressure service. Jiangsu Liangyi Co., Limited manufactures a full range of ASTM A182 F91 forging parts — bars, rings, valve bodies and custom shapes — from its ISO 9001:2015 certified facility in Jiangyin, China.
Developed by Oak Ridge National Laboratory and Combustion Engineering in the 1980s, F91 was designed as a major step forward from the older F22 (2.25Cr-1Mo) grade. The addition of vanadium (V), niobium (Nb), and nitrogen (N) to the classic 9Cr-1Mo base produces a fine dispersion of MX-type carbonitride precipitates within the tempered martensitic matrix — the microstructural feature directly responsible for F91’s exceptional long-term creep strength at elevated temperatures.
Why Engineers Specify F91
F91 delivers 40–60% higher allowable design stress compared to F22 at temperatures between 550°C and 650°C. This means thinner walls, lighter components, and lower fabrication costs in supercritical power plant design — without sacrificing required safety margins.
F91 is manufactured to several parallel standards depending on product form:
| Standard | Grade | Product Form |
|---|---|---|
| ASTM A182 | F91 | Forgings — flanges, fittings, valves |
| ASME SA182 | F91 | ASME Boiler & Pressure Vessel Code equivalent |
| ASME SA336 | F91 | Large open-die pressure vessel forgings |
| ASTM A335 | P91 | Seamless ferritic alloy steel pipe |
| ASTM A182 | F911 | Modified variant — added tungsten and boron |
| EN 10302:2008 | X10CrMoVNb9-1 / 1.4903 | European equivalent for forgings |
Chemical Composition of ASTM A182 F91
The chemistry of F91 is tightly controlled. Small deviations — particularly in aluminium, nitrogen, and the vanadium-to-niobium ratio — can dramatically degrade long-term creep performance. Permitted ladle analysis ranges per ASTM A182/A182M:
| Element | Symbol | Min % | Max % | Function in F91 |
|---|---|---|---|---|
| Carbon | C | 0.08 | 0.12 | Martensite formation; M₂₃C₆ carbide former |
| Chromium | Cr | 8.00 | 9.50 | Oxidation resistance; Cr₂O₃ scale formation |
| Molybdenum | Mo | 0.85 | 1.05 | Solid-solution creep strengthening |
| Vanadium | V | 0.18 | 0.25 | MX precipitate former; primary creep strengthening |
| Niobium | Nb | 0.06 | 0.10 | Grain refinement; secondary MX precipitates |
| Nitrogen | N | 0.030 | 0.070 | MX carbonitride stabiliser; solid-solution strengthening |
| Aluminium | Al | — | 0.040 max | Must be limited — ties up N, weakens MX precipitates |
| Manganese | Mn | 0.30 | 0.60 | Deoxidiser; hardenability |
| Silicon | Si | — | 0.50 max | Deoxidiser |
| Nickel | Ni | — | 0.40 max | Toughness; limited to control delta ferrite |
| Phosphorus | P | — | 0.025 max | Low — grain boundary embrittlement risk |
| Sulphur | S | — | 0.015 max | Low — hot shortness risk in forging |
⚠ Critical: Aluminium Content Control
Aluminium above 0.04% forms AlN instead of the required V(C,N) and Nb(C,N) MX precipitates, eliminating the creep-strengthening carbonitrides and causing premature rupture in service. Always request confirmed OES aluminium test data on every heat before accepting F91 material from any supplier.
Mechanical Properties
Minimum mechanical requirements for F91 forgings in the normalised-and-tempered (N+T) condition per ASTM A182/A182M:
| Property | Requirement | Notes |
|---|---|---|
| Tensile Strength (UTS) | 620 – 850 MPa | Both min and max specified — upper limit prevents over-hardening |
| 0.2% Proof Strength (Rp0.2) | ≥ 415 MPa | Minimum value; typical 480–580 MPa achieved |
| Elongation (A5) | ≥ 20% | 5× gauge-length specimen |
| Reduction of Area | ≥ 45% | Fracture toughness indicator |
| Hardness | ≤ 265 HBW | Maximum — prevents brittle HAZ in welds |
| Charpy Impact (V-notch) | ≥ 41 J at 20°C | Typical project minimum — verify project specification |
Creep and Rupture Strength
The decisive engineering advantage of F91 is its long-term creep rupture strength. At 600°C, the 100,000-hour creep rupture strength of F91 is approximately 78 MPa — more than double that of F22 (approximately 35 MPa) at the same temperature. ASME Section I and EN 12952 assign substantially higher allowable stresses to F91, reducing required wall thickness by 30–40% compared to F22 in supercritical steam components.
Heat Treatment of F91 Forgings
Correct heat treatment is not optional for F91. It is the mechanism by which the alloy achieves its required martensitic microstructure and mechanical properties. F91 forgings require a mandatory two-stage thermal cycle:
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Normalising — Austenitising and Rapid Cooling
Heat to 1040–1080°C and hold for 1 minute per mm section thickness (minimum 30 minutes). Oil quench for sections over 80 mm; controlled-atmosphere forced-air cooling for thinner sections. Martensite transformation occurs on cooling below the Ms temperature (approximately 400°C).
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Tempering — Precipitation and Stress Relief
When the forging cools to 100–150°C, immediately reheat to 760–780°C. Hold minimum 60 minutes or 2 min/mm section thickness, whichever is greater. Cool in still air. This precipitates MX carbonitrides and M₂₃C₆ carbides, relieves quench stresses, and achieves the required creep strength and toughness combination.
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Post-Weld Heat Treatment (PWHT) — Mandatory After Welding
After any welding, PWHT is mandatory at 740–770°C. Hold minimum 2 hours or 1 hour per 25 mm weld throat thickness. Cool at ≤55°C/hr to 300°C, then free-air cool. Inadequate or skipped PWHT is the leading root cause of premature creep failure and Type IV cracking in F91 weldments in service.
⚠ Warning — Temper Embrittlement Zone
Never hold or slow-cool F91 in the range 500–700°C during fabrication or PWHT ramp-up/cool-down. Prolonged time in this range causes temper embrittlement through phosphorus segregation to grain boundaries, reducing impact toughness by 50% or more. Ramp quickly through this zone in both directions.
Welding ASTM A182 F91 — Critical Parameters
F91 is weldable by SMAW, GTAW, SAW, and FCAW. A qualified Welding Procedure Specification (WPS) per ASME Section IX or EN ISO 15614-1 is mandatory. Critical control parameters:
| Parameter | Requirement |
|---|---|
| Preheat Temperature | 200°C min (wall ≤50 mm) · 250°C min (wall >50 mm) |
| Max Interpass Temperature | 300°C — must not be exceeded during welding |
| Post-weld Cooling Before PWHT | Cool to 100–150°C before interrupting or transferring to furnace |
| PWHT Temperature | 740–770°C · minimum 2 hr, or 1 hr/25 mm weld throat |
| PWHT Cooling Rate | ≤55°C/hr from 760°C to 300°C; free air below 300°C |
| Filler Metal (SMAW) | AWS A5.5 E9015-B91 |
| Filler Metal (GTAW/TIG) | AWS A5.28 ER90S-B91 |
| Consumable Storage | Bake at 300°C × 2 hr before use; store in heated quiver |
Industrial Applications of F91 Forgings
Power Generation — Primary Application
F91 was developed specifically for supercritical and ultra-supercritical (USC) thermal power plants operating above 565°C and 24 MPa. It is now the global standard material for the hottest sections of 600 MW and 1,000 MW power units. Key F91 forged components supplied from China include:
- High-pressure main steam gate valves, globe valves, and check valves
- Steam turbine stop valves, control valves, and bypass valve bodies
- Boiler drum nozzles, header flanges, and steam chest covers
- Steam turbine inner and outer casing ring forgings
- HRSG header nozzle forgings in combined-cycle plants
Oil, Gas and Petrochemical
In refineries and petrochemical complexes, F91 forgings are specified for high-temperature process piping flanges and fittings above 550°C; catalytic reformer and hydrocracker reactor nozzle forgings; heat exchanger shell and tubesheet forgings in high-temperature hydrogen service; and pressure vessel nozzle reinforcing forgings.
Nuclear Power
F91 alloy steel meets the material composition and mechanical property requirements referenced in ASME Section III for nuclear pressure boundary applications. Jiangsu Liangyi supplies F91 forgings with full material traceability documentation to support customers’ own nuclear qualification and code compliance processes. Formal nuclear code qualification status (N-stamp or equivalent) must be confirmed and held by the equipment manufacturer or plant owner — not the forging material supplier.
F91 vs Other Cr-Mo Grades — How to Choose
F91 sits within a well-established family of chromium-molybdenum alloy steels. Understanding each grade helps engineers select the right material for each application and temperature range:
1.25Cr-0.5Mo
Max service ~550°C. Lowest cost and simplest to weld. NACE MR0175 compliant. Best when temperature is below 540°C and cost is critical.
2.25Cr-1Mo
Max service ~565°C. Global benchmark for oil and gas and sub-critical power. Excellent H₂ resistance per API 941 Nelson Curves.
9Cr-1Mo-V-Nb
Max service ~650°C. 40–60% higher creep strength than F22 above 550°C. Standard choice for supercritical and USC power plant components worldwide.
9Cr-2W-Mo-V-Nb
Max service ~620°C continuous. ~21% higher creep rupture strength than F91 at 600°C. For advanced USC plants targeting 600–620°C. Higher cost.
For a detailed technical comparison, see our guide on ASTM A182 F92 forged steel components.
Sourcing Quality F91 Forged Parts from China
China is the world’s largest producer of F91 open-die forgings and seamless rolled rings. Quality between suppliers varies significantly. Apply the following due-diligence checklist when evaluating any F91 forging source:
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Current ISO 9001:2015 Certification Covering Forging
Verify the certificate is current, covers open-die forging (not only machining), and names the specific production facility — not a head office elsewhere. Cross-check the certificate number with the issuing body.
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In-House Heat Treatment with Calibrated Furnaces and Thermocouple Records
Normalise-and-temper and PWHT must be performed on-site with calibrated furnaces traceable to national standards. Request thermocouple records for every production charge. Subcontracted heat treatment introduces uncontrolled variables that are difficult to audit.
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Full Chemical Traceability — EN 10204 Type 3.1 MTCs as Standard
Every heat must have a ladle analysis traceable to the original VD/AOD/VOD melt. EN 10204 Type 3.1 mill test certificates should be provided as standard with every shipment. Type 3.2 (countersigned by a customer-nominated third-party inspector) is available when customers arrange their own TPI witness.
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In-House NDE Capability
Ultrasonic testing (UT), magnetic particle inspection (MT), and liquid penetrant inspection (PT) should be performed on-site. Request NDE personnel qualification records and equipment calibration certificates. Third-party inspection bodies (Bureau Veritas, SGS, TÜV, Intertek, CCIC) are welcome at any production stage.
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Confirmed Aluminium ≤0.04% Reported on Every Heat MTC
Excess aluminium is the most common quality failure in Chinese F91 material. Always verify Al is explicitly reported on the MTC and is at or below 0.04% on every heat — some certificates omit Al if not specifically required.
Quality & Certification Statement — Jiangsu Liangyi Co., Limited
Company Certification Held: ISO 9001:2015 Quality Management System — covering the full manufacturing scope from raw material procurement, melting, open-die forging, heat treatment, machining, inspection, and dispatch.
Documentation Provided: EN 10204 Type 3.1 mill test certificates are supplied as standard with every shipment. EN 10204 Type 3.2 certificates (countersigned by an independent inspection body) are available when customers nominate and arrange a third-party inspector (Bureau Veritas, SGS, TÜV, Intertek, CCIC, or customer-nominated body) to witness testing at our Jiangyin facility.
Product Compliance: Our F91 forgings are produced and tested to meet the requirements of ASTM A182, ASME SA182, ASME SA336, and EN 10302 standards. Products are manufactured to meet API 6A material composition and mechanical property requirements where specified. Third-party inspector coordination is provided at no extra charge.
Jiangsu Liangyi Co., Limited is an ISO 9001:2015 certified open-die forging manufacturer in Jiangyin, Jiangsu, China, with over 25 years of experience supplying F91 forged components to customers in 50+ countries. In-house capabilities: electric-furnace melting with AOD/VOD refining, a 6,300-ton hydraulic press, normalise-and-temper furnaces with calibrated thermocouples, CNC machining, and a full mechanical and chemical testing laboratory. To discuss your project requirements or request a custom F91 forging quote, contact our engineering team — responses within 24 hours.
Frequently Asked Questions about ASTM A182 F91
Answers to the most common technical and procurement questions about F91 steel, as received by the Jiangsu Liangyi engineering team from customers worldwide.
Is ASTM A182 F91 the same as ASME SA182 F91?
For all practical engineering and procurement purposes, yes. ASME SA182 is the ASME Boiler and Pressure Vessel Code adoption of ASTM A182 with identical chemical, mechanical, and dimensional requirements. Material certified to ASTM A182 F91 is accepted under ASME Code when the MTC references both designations. Jiangsu Liangyi routinely supplies dual-designated MTCs referencing both ASTM A182 F91 and ASME SA182 F91 on request.
What is the European equivalent of ASTM A182 F91?
The nearest European equivalent is X10CrMoVNb9-1, Material Number 1.4903, covered by EN 10302:2008. EN 10302 adds a minimum nitrogen of 0.030% and aluminium maximum of 0.040%. For PED 2014/68/EU-compliant pressure equipment, both designations should appear on the purchase order. Jiangsu Liangyi can supply dual-certified material meeting both standards simultaneously — specify this at enquiry stage.
What is the maximum service temperature of ASTM A182 F91 steel?
ASTM A182 F91 is rated for continuous service up to approximately 650°C (1202°F). Above this, carbide coarsening and Z-phase formation destroy its creep resistance. For long-term continuous applications above 600°C, consider ASTM A182 F92 (rated to 620°C), or austenitic stainless and nickel-base alloys for temperatures above 620°C.
What is the difference between F91 and P91 steel?
F91 and P91 share the same 9Cr-1Mo-V-Nb chemistry in different product forms. F91 (ASTM A182) covers forged components — flanges, valves, fittings, and rolled rings. P91 (ASTM A335) covers seamless pipe. Both use the same preheat and PWHT requirements when welded together, with matching ER90S-B91 filler metal.
What is the creep rupture strength of ASTM A182 F91 at 600°C?
The 100,000-hour creep rupture strength of ASTM A182 F91 at 600°C is approximately 78 MPa — more than double that of F22 (approximately 35 MPa) at the same temperature. This higher creep strength allows 30–40% thinner wall designs in supercritical power plant pressure components.
What is the minimum preheat temperature for welding F91 steel?
Minimum preheat for welding ASTM A182 F91 is 200°C for wall thickness ≤50 mm, and 250°C for wall >50 mm. Maximum interpass temperature must not exceed 300°C. Post-weld heat treatment (PWHT) at 740–770°C is mandatory for all F91 weldments and must never be omitted.
How long does F91 forging production take from order to delivery?
Standard lead time at Jiangsu Liangyi for F91 open-die forgings and seamless rolled rings is 15–30 days for pieces up to 5,000 kg. Larger pieces (10,000–30,000 kg) require 35–60 days including heat treatment and NDE. Air freight is available for urgent project requirements.