Steel A369 FP91 SUBCRIT-ANN
Heat-treated state
ASTM A369 FP91
Interim condition for forgings/fittings that will be re-normalized and tempered after machining; not code-acceptable for service.
Values for the SUBCRIT-ANN condition
Other A369 FP91 tempers and conditions
A369 FP91 is listed in 5 conditions; see the grade overview at A369 FP91.
- SUBCRIT-ANN
- N+T-HIGH-TEMPER
- N+T
- AF
- N
Compare all 5 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
SUBCRIT-ANNThis page ~730–760 °C soften anneal (machining stock only) | Interim condition for forgings/fittings that will be re-normalized and tempered after machining; not code-acceptable for service. | 400 | 570 | 28 | 10 HRC | 3 % IACS |
N+T | Pick when hardness must sit at the low end (≤200 HB) for heavy machining or cold bending, accepting slightly lower room-temperature strength. | 450 | 615 | 26 | 190 HB | — |
N+TDefault Normalized and tempered | The only condition in which FP91 pipe is supplied and code-qualified for creep service; fully tempered martensite. | 500 | 660 | 23 | 18 HRC | — |
As-forged / as-bored, prior to final heat treatment | Mill intermediate form of the forged-and-bored pipe; properties are non-uniform and it must receive the full N+T cycle. | 550 | 800 | 16 | 25 HRC | 2.7 % IACS |
Normalized | Intermediate state after normalizing before tempering — hard, brittle and never shipped for service; must be tempered. | 900 | 1,250 | 10 | 43 HRC | — |
What is A369 FP91 SUBCRIT-ANN?
A369 FP91 SUBCRIT-ANN is A369 FP91 heat treated to SUBCRIT-ANN — interim condition for forgings/fittings that will be re-normalized and tempered after machining; not code-acceptable for service. A369 FP91 SUBCRIT-ANN has a yield strength of 400 MPa (58 ksi) and a tensile strength of 570 MPa (82.7 ksi) — weaker than 60% of steel grades. Elongation at break is 28% and the elastic modulus is 218 GPa (31.6 Msi). A369 FP91 SUBCRIT-ANN has a density of 7.77 g/cm³ (0.281 lb/in³), lighter than 92% of steel grades. It melts at 1,450°C (2,642 °F). N+T is the default condition FabDigit quotes; SUBCRIT-ANN is available on request or by drawing note.
Advantages
- High service temperature — 600°C (1,112 °F), better than 98% of steel grades
- Good corrosion resistance — 30/100, better than 95% of steel grades
- Ductile — tolerates forming and impact — 28%, better than 89% of steel grades
- Tough — resists crack growth — 130 MPa·√m (118.3 ksi·√in), better than 89% of steel grades
Limitations
- Low electrical conductivity — 3 % IACS, worse than 91% of steel grades
- Poor heat conductor — 27 W/m·K (15.6 BTU/hr·ft·°F), worse than 86% of steel grades
- High embodied carbon — 2.5 kg CO₂/kg, worse than 82% of steel grades
A369 FP91 SUBCRIT-ANN properties
Typical room-temperature values for A369 FP91 SUBCRIT-ANN; 22 are specific to this condition. Other listed values are A369 FP91 reference data and may not represent this condition. Bars are the position among steel grades.
Physical3
A369 FP91 SUBCRIT-ANN has a density of 7.77 g/cm³ (0.281 lb/in³), lighter than 92% of steel grades. It melts at 1,450°C (2,642 °F).
Mechanical16
A369 FP91 SUBCRIT-ANN has a yield strength of 400 MPa (58 ksi) and a tensile strength of 570 MPa (82.7 ksi) — weaker than 60% of steel grades. Elongation at break is 28% and the elastic modulus is 218 GPa (31.6 Msi).
Thermal6
A369 FP91 SUBCRIT-ANN is rated for continuous service to 600°C (1,112 °F). It conducts heat at 27 W/m·K (15.6 BTU/hr·ft·°F), worse than 86% of steel grades. Thermal expansion is 10.8 µm/m·K (6 µin/in·°F).
Electrical3
A369 FP91 SUBCRIT-ANN conducts electricity at 3 % IACS, worse than 91% of steel grades.
Chemical & Environmental3
Corrosion resistance is poor (30/100), better than 95% of steel grades.
Sustainability4
Producing a kilogram of A369 FP91 SUBCRIT-ANN takes about 34 MJ of energy and emits 2.5 kg of CO₂ — more than 82% of steel grades. Typical recycled content is 25%.
Manufacturability8
A369 FP91 SUBCRIT-ANN's machinability is good (55/100, better than 62% of steel grades); weldability fair (45/100); formability fair (42/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with A369 FP91 SUBCRIT-ANN
Is A369 FP91 easy to machine?
Machines like a 200–250 HB tempered alloy steel: rigid setups, carbide inserts, moderate speed and generous feed; expect ~45 % of AISI 1212 tool life and avoid work-hardening dwell.
Can A369 FP91 be welded?
GTAW root + SMAW/FCAW fill with matching 9Cr-1Mo-V consumables, 200–250 °C preheat, interpass ≤300 °C, cool to ~100 °C to transform, then PWHT 750–770 °C — Ni+Mn in the weld metal must be limited to keep Ac1 high.
Can A369 FP91 be formed, bent or molded?
Hot bend/form at 900–1050 °C followed by a full re-normalize and temper; cold bending is limited and any cold work above ~5 % strain calls for re-heat-treatment.
What surface finishes work on A369 FP91?
Not stainless — protect with high-temperature aluminium/silicone paint, phosphating or aluminising for storage and outdoor runs; shot-blast before coating and never leave chloride-bearing residues on the surface.
A369 FP91 chemical composition (wt %)
Limits by weight percent from the governing specification, written the way the spec states them — a single maximum for impurities, a range for alloying elements, and the base element as balance. Nominal is the typical mid-range value.
| Element | Spec limit (wt %) | Nominal |
|---|---|---|
| C | 0.08 – 0.12 | 0.1 |
| Mn | 0.3 – 0.6 | 0.45 |
| P | ≤ 0.02 | 0.01 |
| S | ≤ 0.01 | 0 |
| Si | 0.2 – 0.5 | 0.35 |
| Cr | 8 – 9.5 | 8.75 |
| Mo | 0.85 – 1.05 | 0.95 |
| V | 0.18 – 0.25 | 0.21 |
| Nbcolumbium | 0.06 – 0.1 | 0.08 |
| N | 0.03 – 0.07 | 0.05 |
| Ni | ≤ 0.4 | 0.2 |
| Almax, total | ≤ 0.02 | 0.01 |
| Ti | ≤ 0.01 | 0.01 |
| Zr | ≤ 0.01 | 0.01 |
| Fe | balance | — |
A369 FP91 SUBCRIT-ANN — frequently asked
What is A369 FP91 SUBCRIT-ANN used for?
A369 FP91 SUBCRIT-ANN is typically used for main steam piping for ultra-supercritical units, hot reheat piping, boiler outlet headers, superheater and reheater headers, forged pipe fittings and elbows and HRSG high-temperature pipework. In short: 600 °C steam-line standard; creep-strength-enhanced ferritic grade; strict PWHT requirements.
What is the yield strength of A369 FP91 SUBCRIT-ANN?
A369 FP91 SUBCRIT-ANN has a typical yield strength of 400 MPa (58 ksi) and a tensile strength of 570 MPa (82.7 ksi) — weaker than 60% of steel grades. Strength varies by condition: see the 5 listed tempers.
Is A369 FP91 SUBCRIT-ANN easy to machine?
Reasonably — machinability is rated good (55/100, better than 62% of steel grades). Machines like a 200–250 HB tempered alloy steel: rigid setups, carbide inserts, moderate speed and generous feed; expect ~45 % of AISI 1212 tool life and avoid work-hardening dwell.
Can A369 FP91 SUBCRIT-ANN be welded?
With care — weldability is rated fair (45/100). GTAW root + SMAW/FCAW fill with matching 9Cr-1Mo-V consumables, 200–250 °C preheat, interpass ≤300 °C, cool to ~100 °C to transform, then PWHT 750–770 °C — Ni+Mn in the weld metal must be limited to keep Ac1 high.
What surface finishes work on A369 FP91 SUBCRIT-ANN?
Not stainless — protect with high-temperature aluminium/silicone paint, phosphating or aluminising for storage and outdoor runs; shot-blast before coating and never leave chloride-bearing residues on the surface.
Can A369 FP91 SUBCRIT-ANN be formed, bent or molded?
Hot bend/form at 900–1050 °C followed by a full re-normalize and temper; cold bending is limited and any cold work above ~5 % strain calls for re-heat-treatment.
What is the maximum service temperature of A369 FP91 SUBCRIT-ANN?
A369 FP91 SUBCRIT-ANN is rated for continuous use to about 600°C (1,112 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between A369 FP91 N+T and A369 FP91-N?
N+T is the default condition — the only condition in which FP91 pipe is supplied and code-qualified for creep service; fully tempered martensite. N: intermediate state after normalizing before tempering — hard, brittle and never shipped for service; must be tempered. Yield strength is 500 MPa in N+T versus 900 MPa in N.
Sources
- ASTM A369/A369M — Carbon and Ferritic Alloy Steel Forged and Bored Pipe for High-Temperature Service — ASTM International (2018)
- ASTM A335/A335M — Seamless Ferritic Alloy-Steel Pipe for High-Temperature Service (Grade P91) — ASTM International (2022)
- EN 10216-2 — Seamless steel tubes for pressure purposes, X10CrMoVNb9-1 (1.4903) — CEN (2020)
- ASME BPVC Section II Part D — Allowable stresses and physical properties, SA-335 P91 — ASME (2023)
- ASM Handbook Vol. 1 — Properties and Selection: Irons, Steels and High-Performance Alloys — ASM International (1990)
- Grade 91 seamless pipe technical data — Vallourec / Tenaris (2021)
- GB 5310 — (10Cr9Mo1VNb) — SAC (2017)
Property data is compiled from published supplier and standards handbooks and normalised for comparison. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.