Steel A335 P91 · heat-treated state
Steel A335 P91 N&T
Properties of the N&T condition, compared with the other A335 P91 tempers.
The only code-acceptable delivery condition for A335 P91 pressure piping — tempered martensite with balanced creep strength, toughness and 190–250 HB hardness.
What is A335 P91 N&T?
A335 P91 N&T is A335 P91 heat treated to N&T — the only code-acceptable delivery condition for A335 P91 pressure piping — tempered martensite with balanced creep strength, toughness and 190–250 HB hardness. A335 P91 N&T has a yield strength of 490 MPa (71.1 ksi) and a tensile strength of 660 MPa (95.7 ksi) — stronger than 61% of steel grades. Elongation at break is 23% and the elastic modulus is 218 GPa (31.6 Msi). A335 P91 N&T 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). HR is the default condition FabDigit quotes; N&T 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 — 28/100, better than 93% of steel grades
- Tough — resists crack growth — 120 MPa·√m (109.2 ksi·√in), better than 80% of steel grades
Limitations
- Poor heat conductor — 26 W/m·K (15 BTU/hr·ft·°F), worse than 86% of steel grades
- High embodied carbon — 2.6 kg CO₂/kg, worse than 86% of steel grades
A335 P91 N&T properties
Typical room-temperature values for A335 P91 N&T — 16 properties are specific to this condition; the rest are grade-level values shared by every A335 P91 temper. Each bar shows where the value sits among the steel grades in FabDigit's library — further right is higher.
Physical3
A335 P91 N&T 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
A335 P91 N&T has a yield strength of 490 MPa (71.1 ksi) and a tensile strength of 660 MPa (95.7 ksi) — stronger than 61% of steel grades. Elongation at break is 23% and the elastic modulus is 218 GPa (31.6 Msi).
Thermal6
A335 P91 N&T is rated for continuous service to 600°C (1,112 °F). It conducts heat at 26 W/m·K (15 BTU/hr·ft·°F), worse than 86% of steel grades. Thermal expansion is 10.8 µm/m·K (6 µin/in·°F).
Electrical2
A335 P91 N&T has an electrical resistivity of 6×10⁻⁷ Ω·m.
Chemical & Environmental3
Corrosion resistance is poor (28/100), better than 93% of steel grades.
Sustainability4
Producing a kilogram of A335 P91 N&T takes about 34 MJ of energy and emits 2.6 kg of CO₂ — more than 82% of steel grades. Typical recycled content is 30%.
Manufacturability7
A335 P91 N&T's machinability is fair (45/100, worse than 65% of steel grades); weldability fair (40/100); formability fair (35/100).
Values are nominal handbook figures for design screening. Certified mill or lot data ships with every FabDigit order on request.
Other A335 P91 tempers and conditions
A335 P91 is also supplied in 4 other conditions. The full side-by-side table is on the A335 P91 overview.
- HRDefaultMill intermediate state only: air-cooled hot-worked P91 is partly martensitic with erratic hardness and is not acceptable for ASTM A335 delivery without normalizing and tempering.700 MPa yield · 31 HRC
- NormalizedIntermediate mill/shop condition only: as-normalized P91 is untempered martensite, hard and brittle, and must be tempered before service or pressure testing.1,000 MPa yield · 44 HRC
- Q&TUsed on heavy-wall pipe, forgings and valve bodies where liquid quenching is needed to get full martensite through-thickness; gives slightly higher strength than N&T at some cost in toughness.530 MPa yield · 22 HRC
- N&TThe only code-acceptable delivery condition for A335 P91 pressure piping — tempered martensite with balanced creep strength, toughness and 190–250 HB hardness.490 MPa yield · 20 HRC
- AnnealedSoftened, over-tempered condition supplied for deep machining or cold bending; must be re-normalized and tempered afterwards because creep strength is lost.330 MPa yield · 170 HB
Working with A335 P91 N&T
Is A335 P91 easy to machine?
Machines like a 215 HB Cr-Mo steel — rigid setups, coated carbide, moderate speeds and heavy coolant; avoid overheating that could locally temper or re-harden the surface.
Can A335 P91 be welded?
Use matching 9Cr-1Mo-V consumables with 200–300 °C preheat/interpass, cool to below ~100 °C before PWHT, then mandatory 750–770 °C temper; keep Ni+Mn low to protect the Ac1 and check hardness 190–250 HB after PWHT.
Can A335 P91 be formed, bent or molded?
Hot form above ~900 °C or cold bend only in the softened condition; any hot working or cold strain over ~5–20 % requires full re-normalize and temper to restore creep strength.
What surface finishes work on A335 P91?
Not anodizable; protect with high-temperature aluminium/silicone paint, aluminizing or chromizing, and pickle/blast weld scale before insulation to limit under-insulation corrosion.
A335 P91 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 | 8.8 |
| 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 |
| Nimax; Ni+Mn ≤ 1.0 recommended by EPRI | ≤ 0.4 | 0.2 |
| Al | ≤ 0.02 | 0.01 |
| Ti | ≤ 0.01 | — |
| Zr | ≤ 0.01 | — |
| Fe | balance | — |
A335 P91 N&T — frequently asked
What is A335 P91 N&T used for?
A335 P91 N&T is typically used for boilers, power plant headers and pipework. In short: creep-resistant power pipe.
What is the yield strength of A335 P91 N&T?
A335 P91 N&T has a typical yield strength of 490 MPa (71.1 ksi) and a tensile strength of 660 MPa (95.7 ksi) — stronger than 61% of steel grades. Strength varies by condition: see the 5 listed tempers.
Is A335 P91 N&T easy to machine?
Reasonably — machinability is rated fair (45/100, worse than 65% of steel grades). Machines like a 215 HB Cr-Mo steel — rigid setups, coated carbide, moderate speeds and heavy coolant; avoid overheating that could locally temper or re-harden the surface.
Can A335 P91 N&T be welded?
With care — weldability is rated fair (40/100). Use matching 9Cr-1Mo-V consumables with 200–300 °C preheat/interpass, cool to below ~100 °C before PWHT, then mandatory 750–770 °C temper; keep Ni+Mn low to protect the Ac1 and check hardness 190–250 HB after PWHT.
What surface finishes work on A335 P91 N&T?
Not anodizable; protect with high-temperature aluminium/silicone paint, aluminizing or chromizing, and pickle/blast weld scale before insulation to limit under-insulation corrosion.
Can A335 P91 N&T be formed, bent or molded?
Hot form above ~900 °C or cold bend only in the softened condition; any hot working or cold strain over ~5–20 % requires full re-normalize and temper to restore creep strength.
What is the maximum service temperature of A335 P91 N&T?
A335 P91 N&T 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.
Can FabDigit make parts in A335 P91 N&T?
Yes — A335 P91 N&T is available for CNC machining and sheet metal with instant online pricing. Upload a STEP file to get a price and a DFM check.
Sources
- ASTM A335/A335M — Seamless Ferritic Alloy-Steel Pipe for High-Temperature Service — ASTM International (2023)
- ASME BPVC Section II Part D — Properties (Metric) — ASME (2023)
- GB/T 5310 — Seamless steel tubes for high pressure boilers (10Cr9Mo1VNbN) — SAC (2017)
- ASM Handbook Vol.1: Properties and Selection — Irons, Steels, and High-Performance Alloys — ASM International (1990)
- EPRI Guidelines and Specifications for Grade 91 Steel (Creep-Strength-Enhanced Ferritic Steels) — EPRI (2015)
- Grade 91 / T91-P91 seamless tube datasheet — Vallourec (2020)
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.