Steel A335 P91
ASTM A335 P91
P91 is a modified 9Cr-1Mo creep-strength-enhanced ferritic (CSEF) steel micro-alloyed with V, Nb and N, supplied as seamless pipe/tube per ASTM A335 in the normalized-and-tempered condition where it develops a tempered-martensite structure.
Values for the HR condition
A335 P91 tempers and conditions
A335 P91 is supplied in 5 conditions, soft to hard. HR is the default FabDigit quotes; the values on this page are for it.
Compare all 5 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
full/subcritical anneal (~800–900 °C, slow cool) for machining or intermediate forming | Softened, over-tempered condition supplied for deep machining or cold bending; must be re-normalized and tempered afterwards because creep strength is lost. | 330 | 560 | 28 | 170 HB | — |
Normalized and tempered | The only code-acceptable delivery condition for A335 P91 pressure piping — tempered martensite with balanced creep strength, toughness and 190–250 HB hardness. | 490 | 660 | 23 | 20 HRC | 26 W/m·K |
Quenched and tempered | Used 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 | 700 | 20 | 22 HRC | — |
HRDefault Hot rolled | Mill 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 | 950 | 14 | 31 HRC | — |
air cooled from 1040–1080 °C | Intermediate mill/shop condition only: as-normalized P91 is untempered martensite, hard and brittle, and must be tempered before service or pressure testing. | 1,000 | 1,350 | 10 | 44 HRC | — |
What is A335 P91?
P91 is a modified 9Cr-1Mo creep-strength-enhanced ferritic (CSEF) steel micro-alloyed with V, Nb and N, supplied as seamless pipe/tube per ASTM A335 in the normalized-and-tempered condition where it develops a tempered-martensite structure. It is chosen for main steam lines, headers, superheater/reheater piping and heavy forgings in supercritical boilers because its creep strength at 550–620 °C is roughly double that of P22 while thermal expansion is lower and conductivity higher than austenitic stainless. Its limits are strict fabrication discipline (preheat, controlled interpass temperature, mandatory 750–770 °C PWHT, hardness/ferrite control), Type IV creep cracking in the HAZ of weldments, and poor steam-side oxidation resistance above about 620 °C. A335 P91 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). A335 P91 has a yield strength of 700 MPa (102 ksi) and a tensile strength of 950 MPa (138 ksi) — stronger than 81% of steel grades. Elongation at break is 14% and the elastic modulus is 218 GPa (31.6 Msi).
What is A335 P91 used for?
- Boilers
- Power plant headers
- Pipework
Advantages
- Good corrosion resistance — 28/100, better than 94% of steel grades
- High strength — 700 MPa (102 ksi), better than 81% of steel grades
- Tough — resists crack growth — 120 MPa·√m (109.2 ksi·√in), better than 81% of steel grades
Limitations
- Limited service temperature — 200°C (392 °F), worse than 93% of steel grades
- Difficult to machine — 25/100, worse than 93% of steel grades
- Poor heat conductor — 26 W/m·K (15 BTU/hr·ft·°F), worse than 87% of steel grades
- Limited formability — 15/100, worse than 87% of steel grades
- High embodied carbon — 2.6 kg CO₂/kg, worse than 87% of steel grades
A335 P91 properties
Typical room-temperature values for A335 P91-HR; bars are the position among steel grades.
Physical3
A335 P91 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 has a yield strength of 700 MPa (102 ksi) and a tensile strength of 950 MPa (138 ksi) — stronger than 81% of steel grades. Elongation at break is 14% and the elastic modulus is 218 GPa (31.6 Msi).
Thermal6
A335 P91 is rated for continuous service to 200°C (392 °F). It conducts heat at 26 W/m·K (15 BTU/hr·ft·°F), worse than 87% of steel grades. Thermal expansion is 10.8 µm/m·K (6 µin/in·°F).
Electrical2
A335 P91 has an electrical resistivity of 6×10⁻⁷ Ω·m.
Chemical & Environmental3
Corrosion resistance is poor (28/100), better than 94% of steel grades.
Sustainability4
Producing a kilogram of A335 P91 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's machinability is poor (25/100, worse than 93% of steel grades); weldability poor (20/100); formability poor (15/100).
Common Calculations5
Values are nominal handbook figures for design screening. Certified mill or lot data ships with every FabDigit order on request.
Strength versus weight in the steel family
Yield strength against density for every steel grade FabDigit runs. A335 P91 is highlighted; hover a dot for its name, or open the interactive family chart.
Working with A335 P91
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.
Manufacturability
The process limits FabDigit applies to A335 P91. Every A335 P91 part is priced by an engineer; design inside these and the quote comes back without a redesign loop.
CNC machining
| Machinability | Poor25 / 100 |
|---|---|
| ISO material group | P.2 |
| Cutting speed (rough / finish) | 180 / 210 m/min |
| Minimum wall | 0.8 mm |
| Minimum internal radius | 1 mm |
| Drill depth limit | 5× diameter |
| Best surface (machined / polished) | Ra 1.6 / — |
| Hardenable after machining | No |
Sheet metal
| Formability | Fair15 / 100 |
|---|---|
| Weldability | Better20 / 100 |
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 |
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.
Surface treatments FabDigit runs on steel parts — pick them in the quote configurator or call them out on the drawing. Each links to its datasheet in the surface-finish library (thickness, specs, call-out, material verdicts).
Plating
CVD Coating
Black / Phosphate
Thermal Spray
How much does A335 P91 cost?
A335 P91 is a standard-cost steel ($$), cheaper than about 71% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
A335 P91 — frequently asked
What is A335 P91 used for?
A335 P91 is typically used for boilers, power plant headers and pipework. In short: creep-resistant power pipe.
What is the yield strength of A335 P91?
A335 P91 has a typical yield strength of 700 MPa (102 ksi) and a tensile strength of 950 MPa (138 ksi) — stronger than 81% of steel grades. Strength varies by condition: see the 5 listed tempers.
Is A335 P91 easy to machine?
Not especially — machinability is rated poor (25/100, worse than 93% of steel grades). Machines like a 215 HB Cr-Mo steel — rigid setups, coated carbide, moderate speeds and heavy coolant; avoid overheating that could locally temper or re-harden the surface.
Can A335 P91 be welded?
Not readily — weldability is rated poor (20/100). Use matching 9Cr-1Mo-V consumables with 200–300 °C preheat/interpass, cool to below ~100 °C before PWHT, then mandatory 750–770 °C temper; keep Ni+Mn low to protect the Ac1 and check hardness 190–250 HB after PWHT.
What surface finishes work on A335 P91?
Not anodizable; protect with high-temperature aluminium/silicone paint, aluminizing or chromizing, and pickle/blast weld scale before insulation to limit under-insulation corrosion.
Can A335 P91 be formed, bent or molded?
Hot form above ~900 °C or cold bend only in the softened condition; any hot working or cold strain over ~5–20 % requires full re-normalize and temper to restore creep strength.
What is the maximum service temperature of A335 P91?
A335 P91 is rated for continuous use to about 200°C (392 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
How much does A335 P91 cost?
A335 P91 is a standard-cost steel ($$), cheaper than about 71% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
Can FabDigit make parts in A335 P91?
Yes — A335 P91 is available for CNC machining and sheet metal; every part is priced by an engineer rather than instantly. 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; hardness values on non-Brinell scales are converted approximately for charting only. Manufacturability limits are FabDigit quote-engine defaults and may be relaxed by engineering review. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.