X10CrMoVNb9-1 (1.4903) Open-Die Forgings

Grade 91 creep-resistant 9Cr-1Mo-V steel. Chemical composition, EN and ASTM mechanical properties, heat treatment, welding data and forged product forms.

EN designation

X10CrMoVNb9-1

EN material number

1.4903

UNS number

K90901

Common name

Grade 91 / P91 / F91 / T91

Material class

Martensitic creep-resisting alloy steel

Density

7.7 g/cm³

Delivery condition

+NT (normalised and tempered)

Long-term service temp.

550 to 600 °C

What is X10CrMoVNb9-1?

X10CrMoVNb9-1 is a high-alloy ferritic-martensitic steel used where creep strength at elevated temperature is required. The designation follows EN 10027-1 and can be read directly. X marks a high-alloy steel. 10 gives a nominal carbon content of 0.10 %, that is, carbon multiplied by 100. CrMoVNb lists the alloying elements in descending order of content. 9-1 gives the nominal chromium and molybdenum contents of 9 % and 1 %.

The steel differs from earlier 9Cr-1Mo grades in its controlled additions of vanadium, niobium and nitrogen. During tempering these form fine MX-type carbonitrides that remain stable at temperature and restrict dislocation movement within the tempered martensite matrix. Creep rupture strength at 600 °C over 100,000 hours is roughly double that of conventional 9Cr-1Mo. This allowed X10CrMoVNb9-1 to replace austenitic stainless steels in many supercritical boiler applications, at lower cost and with a lower coefficient of thermal expansion.

Forgings compared with castings and plate weldments

Open-die forging refines the as-cast dendritic structure, closes internal porosity and aligns grain flow with the principal stress direction. On pressure parts qualified under ASME B31.1 or PED, a forged X10CrMoVNb9-1 body usually returns more consistent Charpy impact values and cleaner ultrasonic results than a casting of the same shape. Forging also removes the longitudinal weld seam. On a fabricated component that seam is normally the feature that limits creep life.

Equivalent grades and cross-reference

X10CrMoVNb9-1 belongs to the international Grade 91 family. The table lists the designation each standard uses for the same base alloy.

These are nearest-equivalent designations rather than guarantees of interchangeability. EN and ASTM Grade 91 specifications differ in tensile acceptance limits, elongation basis and impact testing requirements. Where a design code names one specification, order to that specification. We can supply X10CrMoVNb9-1 forgings certified dual to EN 10222-2 and ASTM A182 F91 where both sets of acceptance criteria are met.

Chemical composition of X10CrMoVNb9-1 (1.4903)

The chemical composition is given below in mass percent, with ASTM A182 Grade F91 alongside for comparison. Nitrogen, niobium and vanadium control creep strength and should be verified on every heat. Aluminium is held low because it combines with nitrogen to form AlN, which takes nitrogen out of the strengthening MX precipitates.

EN 10302:2008 lists slightly wider residual limits for the same steel name: P 0.025 % max, S 0.015 % max, Al 0.030 % max. Recent editions of ASTM A182 divide F91 into Type 1 and Type 2 with different Nb, V, N and Ni limits, so the specification edition should be confirmed at enquiry stage. Balance in all cases is iron plus incidental residuals.

Mechanical properties of X10CrMoVNb9-1

The values below apply to the normalised and tempered (+NT) condition at room temperature. EN 10216-2 and EN 10222-2 specify a tensile strength of 630 to 830 MPa and a minimum 0.2 % proof strength of 450 MPa.

Hardness is an acceptance criterion for Grade 91, not a by-product of the heat treatment. A reading below 190 HBW indicates over-tempering or an excursion into the intercritical range. Creep strength is then already reduced even though tensile results may still pass, so material is rejected on hardness alone.

Elevated-temperature strength

Proof strength falls as temperature rises. Indicative minimum 0.2 % proof strength values for design screening are about 400 MPa at 300 °C, 370 MPa at 400 °C, 330 MPa at 500 °C and 270 MPa at 600 °C. Design allowable stresses must be taken from the governing code table, ASME BPVC Section II Part D or EN 13480 and EN 12952, and not from this page.

Physical properties of X10CrMoVNb9-1

Density is 7.7 g/cm³. Elastic modulus is 218 GPa at 20 °C and 167 GPa at 600 °C. The coefficient of thermal expansion is about 30 % lower than that of austenitic stainless steel, which is the main reason the steel is preferred for thick-section parts subject to thermal cycling.

Thermal expansion values are mean coefficients between 20 °C and the stated temperature. Maximum recommended long-term service temperature is 550 to 600 °C depending on stress and design life. Short excursions to 650 °C are permitted under some code cases.

Heat treatment of X10CrMoVNb9-1

X10CrMoVNb9-1 is supplied normalised and tempered. The cycle is normalising at 1040 to 1080 °C followed by air cooling, then tempering at 730 to 800 °C followed by air cooling. Both stages are required. Normalising dissolves the carbides and produces a fully martensitic structure. Tempering precipitates the fine MX and M₂₃C₆ particles that carry the creep strength.

Control points

• Cool fully before tempering. The martensite finish temperature is around 100 °C. A part tempered while still warm retains untransformed austenite, which converts to fresh untempered martensite on the final cool.
• Stay below the intercritical range. Ac₁ for this alloy is near 800 to 830 °C. Tempering or PWHT above Ac₁ re-austenitises part of the structure and reduces creep life permanently. Tempering and PWHT ceilings are therefore set below it.
• Control nickel and manganese. Mn plus Ni is commonly capped at 1.0 to 1.5 % by purchaser specification, since both elements lower Ac₁ and narrow the safe PWHT window.
• Calculate soak time on the ruling section. Attach thermocouples to the forging itself; furnace temperature alone is not reliable on heavy sections.
• Verify with hardness. Each heat-treatment lot should be hardness tested. A result of 190 to 250 HBW confirms the cycle was correct.

Open-die forging practice for X10CrMoVNb9-1

X10CrMoVNb9-1 has a narrower hot-working window than carbon or low-alloy steel and work-hardens quickly as it cools. The practice we apply is set out below.

• Billet preparation. Electric-arc furnace with ladle refining and vacuum degassing. Hydrogen content is controlled to reduce flake sensitivity in heavy sections.
• Heating. Soak at 1120 to 1180 °C. Holding above 1200 °C coarsens the grain and risks incipient melting at segregated regions.
• Forging. Finish forging above 900 °C. Deformation below about 850 °C causes surface and internal cracking.
• Reduction ratio. Minimum 3:1 from the cast section for general work, 4:1 or higher for critical pressure parts, so that the as-cast structure is fully broken down.
• Post-forge cooling. Controlled or furnace cooling rather than still air for heavy sections, followed where required by a dehydrogenation or intermediate anneal before the final normalise and temper.
• Final condition. Every X10CrMoVNb9-1 forging leaves the works normalised and tempered. As-forged material has no valid mechanical properties and must not be placed in service.

Welding and post-weld heat treatment

X10CrMoVNb9-1 can be welded by all common arc processes, but the procedure has to be followed closely. Post-weld heat treatment is mandatory on this grade.

Follow the qualified WPS and the governing construction code: ASME B31.1, ASME BPVC Section I or VIII, or EN 13480. Filler metals must be stored and baked to control hydrogen, since Grade 91 weld metal is susceptible to hydrogen-assisted cold cracking before PWHT.

X10CrMoVNb9-1 forged product forms

We produce the following X10CrMoVNb9-1 and F91 open-die forgings to customer drawing or to standard dimensions.

Applications of X10CrMoVNb9-1

X10CrMoVNb9-1 is specified where steam or process fluid is held above about 540 °C and a ferritic steel is preferred to an austenitic one on cost, thermal expansion or thermal fatigue grounds.

• Power generation. Supercritical and ultra-supercritical boiler headers, main steam and hot reheat piping, superheater and reheater components, HRSG units.
• Steam turbines. Casings, inner casings, nozzle boxes, valve chests and rotor components.
• Valves and fittings. High-temperature valve bodies, bonnets, seats, stems, tees and wyes for main steam service.
• Petrochemical and refining. Hydroprocessing reactors, FCC internals, hot-wall piping and hydrogen-service components.
• Nuclear balance of plant. Secondary-circuit piping and support components where code allows.
• Waste-to-energy and biomass plants. High-temperature headers and connecting pipework.

Testing, inspection and certification

Every X10CrMoVNb9-1 forging is supplied with an EN 10204 type 3.1 mill test certificate as standard. Type 3.2 certification witnessed by TÜV, SGS, BV, Lloyd's Register or the customer's own inspector is available on request.

How to order X10CrMoVNb9-1 forgings

Send a drawing or the details below to sales@steelforgepieces.com. We normally return a quotation within one working day.

1. Governing specification and edition: EN 10222-2, ASTM A182 F91, ASTM A336 F91, or dual certification.
2. Product form and finished dimensions, or a drawing with machining allowances.
3. Quantity and required delivery date.
4. Delivery condition and any hardness range narrower than the standard.
5. NDT requirements and acceptance class.
6. Certification type, EN 10204 3.1 or 3.2, and any third-party inspection.
7. Additional purchaser requirements such as Al/N ratio, Mn plus Ni cap, elevated-temperature tensile, delta-ferrite limit, PED or AD 2000-W0 compliance.
8. Marking, preservation and packing requirements.

Frequently asked questions about X10CrMoVNb9-1

Standards referenced

The property values on this sheet are compiled from the published requirements of the standards below, together with our own production and inspection records. The standards themselves are not reproduced here. Work from the current edition of the governing specification.

• EN 10222-2, Steel forgings for pressure purposes: ferritic and martensitic steels with specified elevated-temperature properties
• EN 10216-2, Seamless steel tubes for pressure purposes with specified elevated-temperature properties
• EN 10028-2, Flat products made of steels for pressure purposes
• EN 10302, Creep-resisting steels, nickel and cobalt alloys
• EN 10027-1 and EN 10027-2, Designation systems for steels
• ASTM A182/A182M and ASTM A336/A336M, Forged or rolled alloy-steel pipe flanges, fittings, valves and parts for high-temperature service
• ASTM A335/A335M and ASTM A213/A213M, Seamless ferritic alloy-steel pipe and tube for high-temperature service
• ASME BPVC Section II Part D, Properties (Metric), allowable stress tables
• ASME B31.1, Power Piping, PWHT requirements for P-No. 15E materials