ASTM A182 F91 Type 1 – UNS K90901
Modified 9Cr-1Mo-V-Nb ("P91") steel for creep
Martensitic 9Cr-1Mo steel modified with V, Nb and N, forged to ASTM A182. The fine carbonitrides give creep resistance far superior to F22 and F9, allowing thinner walls in supercritical steam. It requires rigorous control of heat treatment and welding.
Designations and equivalents
| System | Designation |
|---|---|
| UNS | K90901 |
| W.Nr./EN | X10CrMoVNb9-1 – 1.4903 (EN 10222-2) |
| DIN | X10CrMoVNb9-1 |
| JIS | SFVAF28 (JIS G3203) |
| Trade names | Grade 91, P91, T91, mod 9Cr-1Mo |
| Others | Pipe A335 P91; plate A387 Gr 91; fittings A234 WP91; casting A217 C12A |
Standards by product form
| Form | Standard |
|---|---|
| Forging (flanges, fittings, valves) | ASTM A182/A182M F91 Type 1 / Type 2; ASME SA-182 |
| Larger forgings | ASTM A336 F91 |
| Pipe/tube | ASTM A335 P91 / A213 T91 |
| Plate | ASTM A387 Gr 91 |
| Butt-welding fittings | ASTM A234 WP91 |
| Casting | ASTM A217 C12A |
Tolerances and HS codes
- Forged bars (round and square): machining allowance from 2 mm (Ø 6–16 mm) to 24 mm (Ø 400–500 mm) on the diameter. E.g. Ø 100–125 mm finished: +7 mm, tolerance +6 / −0 (length up to 3,500 mm).
- Forged discs: allowance of 4 to 9 mm on the diameter and 3 to 11 mm on the height, plus-only tolerance.
- Cutting and machining: band saw −0 / +5 to +15 mm on length; waterjet +10 % of thickness on the diameter (min. 2 mm); machined parts −0 / +2 to +5 mm on the diameter. See the full tables (INF 002 and INF 003, the same for every alloy).
| Form | NCM (Brazil, 8 digits) | HS (international, 6 digits) | HTS (USA, 10 digits)* | CN (European Union, 8 digits)* |
|---|---|---|---|---|
| Forged bar | 7228.40.00 | 7228.40 | 7228.40.0000 | 7228 40 90 |
| Plate (> 10 mm, width ≥ 600 mm) | 7225.40.90 | 7225.40 | 7225.40.3050 | 7225 40 40 |
| Seamless pipe | 7304.19.00 | 7304.19 | 7304.19.5050 | 7304 19 10 |
| Welded pipe | 7306.19.00 | 7306.19 | 7306.19.5150 | 7306 19 00 |
| Flange and fitting | 7307.91.00 | 7307.91 | 7307.91.5030 | 7307 91 00 |
Tariff class: alloy steel (by chemical composition, under the Chapter 72 Notes and Section XV Note 5).
Pipe: 7304.19 and 7306.19 are line pipe for oil and gas pipelines; boiler, heat exchanger or process tubes go under 7304.39/7304.59 (seamless) and 7306.30/7306.50 (welded).
*: US and EU codes for reference. NCM per current legislation (General Rules of Interpretation, Notes to Chapters 72, 74 and 75 and Section XV Note 5). Confirm the final code with your customs broker according to the item size and finish. HS = first 6 digits of the NCM.
Chemical composition (%)
ASTM A182/A182M – F91 Type 1
| Element | Range (%) |
|---|---|
| C | 0.08 – 0.12 |
| Mn | 0.30 – 0.60 |
| P | ≤ 0.020 |
| S | ≤ 0.010 |
| Si | 0.20 – 0.50 |
| Ni | ≤ 0.40 |
| Cr | 8.0 – 9.5 |
| Mo | 0.85 – 1.05 |
| V | 0.18 – 0.25 |
| Nb | 0.06 – 0.10 |
| N | 0.030 – 0.070 |
| Al | ≤ 0.02 |
| Ti / Zr | ≤ 0.01 each |
Mechanical properties
Normalized + tempered – ASTM A182 F91 (Types 1 and 2)
| Property | Value |
|---|---|
| Yield strength (0.2%) | ≥ 415 MPa |
| Tensile strength | ≥ 585 MPa |
| Elongation | ≥ 20% |
| Reduction of area | ≥ 40% |
| Hardness | 190 – 248 HBW |
Typical physical properties
| Property | Value |
|---|---|
| Density | 7.77 g/cm³ |
| Modulus of elasticity | ≈ 218 GPa |
| Thermal conductivity (20 °C) | ≈ 26 W/m·K |
| Thermal expansion (20–100 °C) | ≈ 10.9 µm/m·K |
Heat treatment, welding and machining
| Item | Usual practice |
|---|---|
| Service temperature | Up to ≈ 600–625 °C in creep service (ASME allows up to 649 °C). |
| Heat treatment | Normalizing 1,040–1,080 °C + tempering 730–800 °C (Type 1). Outside the window or with inadequate cooling it forms ferrite/untempered martensite and loses creep resistance. |
| Welding | Preheat 200–250 °C, interpass ≤ 300–350 °C, cool below ≈ 100 °C before PWHT; consumable AWS E9015-B91 / E9018-B91 (A5.5), ER90S-B91 (A5.28), E91T1-B91; PWHT mandatory 745–775 °C. Control Ni+Mn of the weld metal (< 1.2%). |
| Machining | Fair; harder than F22, carbide tooling. |
Filler metal for welding
The wire composition is usually not identical to the material: each page explains why.
Typical applications
- Main and reheat steam lines of supercritical power plants
- Boiler and HRSG headers and fittings
- High-pressure, high-temperature valves
- High-temperature refinery furnaces and exchangers
- Vessel nozzles and components for creep
Alternatives: F22 for lower temperature/pressure and simpler welding; F92 (with W) for even higher creep resistance; F9 when the focus is sulfidation, not creep; 347H stainless above ≈ 625 °C.
ASTM A182 F91 Type 1: complete guide
What ASTM A182 F91 steel is
F91 is the forged version of Grade 91, the same alloy as P91 and T91 tubes. It belongs to the family of Cr-Mo alloy steels for high temperature, alongside F11, F22, F5 and F9. Unlike stainless steels, F91 is not chosen for aqueous corrosion resistance but for creep resistance, that is, for the ability to withstand stress for tens of thousands of hours between 500 and 650 °C without excessive deformation.
Chemical composition and role of the elements
The 9% chromium and 1% molybdenum give resistance to oxidation by steam and solid-solution strengthening. What sets F91 apart from F9 are the controlled additions of vanadium (0.18 – 0.25%), niobium (0.06 – 0.10%) and nitrogen (0.030 – 0.070%): these elements form fine, stable carbonitrides that pin the microstructure during service and raise creep resistance over conventional 9Cr-1Mo. Aluminum is limited to 0.02% so as not to consume the nitrogen.
Heat treatment
The standard cycle is normalizing at 1,040 – 1,080 °C followed by tempering at 730 – 800 °C. Dustre performs this cycle in its own furnaces, with temperature records, as part of its heat treatment service. Parts that undergo forging or significant cold deformation must go through the full cycle again.
Welding
Welding F91 is critical and requires a qualified procedure: consumables of the same class (9Cr-1Mo-V), preheating to 200 – 250 °C, interpass control and post-weld heat treatment at 745 – 775 °C. Without PWHT the heat-affected zone remains martensitic, hard and brittle, and the joint loses creep resistance. This is the main point of attention when buying and fabricating with this material.
Applications
Flanges, forged fittings, valves, headers, heads and steam piping parts in boilers and turbines; refinery and petrochemical equipment operating above 500 °C; furnace and reactor components. At temperatures below 450 °C F91 is usually replaced to economic advantage by F22 or F11.
Supply forms and Dustre services
Round bars and blocks forged in open die, cut to size on a band saw, with rough or finish machining and hardness, tensile and impact testing. All material is supplied with a 3.1 inspection certificate and heat traceability.
Frequently asked questions
What is the chemical composition of ASTM A182 F91 Type 1?
Per ASTM A182/A182M – F91 Type 1: C 0.08 – 0.12; Mn 0.30 – 0.60; P ≤ 0.020; S ≤ 0.010; Si 0.20 – 0.50; Ni ≤ 0.40.
Which standards cover ASTM A182 F91 Type 1?
Forging (flanges, fittings, valves): ASTM A182/A182M F91 Type 1 / Type 2; ASME SA-182. Larger forgings: ASTM A336 F91. Pipe/tube: ASTM A335 P91 / A213 T91. Plate: ASTM A387 Gr 91. Butt-welding fittings: ASTM A234 WP91. Casting: ASTM A217 C12A.
Up to what temperature can ASTM A182 F91 Type 1 be used?
Up to ≈ 600–625 °C in creep service (ASME allows up to 649 °C).
Which materials can replace ASTM A182 F91 Type 1?
F22 for lower temperature/pressure and simpler welding; F92 (with W) for even higher creep resistance; F9 when the focus is sulfidation, not creep; 347H stainless above ≈ 625 °C.
