1.4911 steel (also known as X8CrCoNiMo10-6 or Z10CKD10) is a martensitic, cobalt-alloyed, creep-resistant stainless steel standardised under EN 10302:2008. Its defining feature is a 5–7% cobalt addition that stabilises the microstructure at service temperatures up to 600 °C — roughly 15–20 °C higher than equivalent cobalt-free grades like P91. It is the primary material for rotor shafts, valve bodies, and turbine casings in ultra-supercritical (USC) and advanced USC power plants.
What Is 1.4911 Steel? Overview & Designation System
1.4911 is the EN numeric designation for the alloy steel formally called X8CrCoNiMo10-6. It is classified as a martensitic creep-resistant stainless steel and is standardised in EN 10302:2008 (Creep-resisting steels, nickel and cobalt alloys) and listed in EN 10088-1:2005. The grade was developed specifically for structural components that must sustain heavy mechanical loads continuously at temperatures between 500 °C and 600 °C — across service lives frequently exceeding 200,000 operating hours.
The defining metallurgical feature of 1.4911 is its cobalt content (5–7 wt%), a rare addition in commercial steels. Cobalt retards the coarsening of carbide precipitates and stabilises the tempered martensitic microstructure under sustained thermal loading — a property critical for steam turbine casings, rotor shafts, and valve bodies in advanced ultra-supercritical (A-USC) power plants where steam conditions reach 600 °C and 300 bar.
Designation Cross-Reference
EN Numeric: 1.4911 · EN Chemical: X8CrCoNiMo10-6 · AFNOR: Z10CKD10 · Also written: X8CrCoNiMo10.6 · X8CrCoNiMo106 · Standards: EN 10302:2008 · EN 10088-1:2005
The suffix "10-6" in X8CrCoNiMo10-6 encodes nominal chromium (~10%) and cobalt (~6%) — the two elements most responsible for its oxidation resistance and microstructural stability. "X8" denotes approximately 0.08% carbon, and Cr, Co, Ni, Mo identify the four principal alloying elements in the steel's chemical designation.
Chemical Composition of 1.4911 / X8CrCoNiMo10-6
All composition limits below are per EN 10302:2008, in weight percent, for forged products in the quenched and tempered (+QT) condition. The cobalt row is highlighted because it is the element that most fundamentally distinguishes this grade from all conventional 9–12% Cr steels.
| Element | Symbol | Min % | Max % | Metallurgical Role |
|---|---|---|---|---|
| Carbon | C | 0.05 | 0.12 | Enables martensitic transformation; moderate level avoids excessive carbide precipitation |
| Chromium | Cr | 9.8 | 11.2 | Oxidation resistance; steam-side corrosion protection |
| Cobalt ★ | Co | 5.0 | 7.0 | Retards carbide coarsening; raises Ms temperature; key differentiator vs P91/P92 |
| Molybdenum | Mo | 0.5 | 1.0 | Solid-solution strengthening; precipitation hardening via M₂₃C₆ |
| Nickel | Ni | 0.2 | 1.2 | Toughness; does not destabilise martensite at these levels |
| Vanadium | V | 0.1 | 0.4 | Fine MX-type precipitation hardening |
| Niobium | Nb | 0.2 | 0.5 | Grain boundary pinning; austenite grain size control |
| Boron | B | 0.005 | 0.015 | Grain boundary cohesion; creep cavity suppression |
| Tungsten | W | — | 0.7 | Solid-solution strengthening at elevated temperatures |
| Nitrogen | N | — | 0.035 | Nitride precipitation; carbonitride formation with V, Nb |
| Silicon | Si | 0.1 | 0.8 | Deoxidation during melting |
| Manganese | Mn | 0.3 | 1.3 | Hardenability; austenite stabiliser |
| Phosphorus | P | — | 0.025 | Impurity limit (embrittlement risk) |
| Sulphur | S | — | 0.015 | Impurity limit (hot-shortness risk) |
★ Cobalt (5–7%) is the defining element that separates 1.4911 from all conventional 9–12% Cr grades. No other mass-produced martensitic turbine steel contains cobalt at this level.
Why Each Element Matters
The alloying philosophy of X8CrCoNiMo10-6 reflects decades of European turbine manufacturer R&D. Moderate carbon (0.05–0.12%) enables a fully martensitic microstructure on cooling without producing the excessive M₂₃C₆ carbide networks that impair toughness. 9.8–11.2% chromium is the minimum required for adequate steam-side oxidation resistance in supercritical conditions; exceeding ~12% risks delta-ferrite formation that would degrade creep performance. Molybdenum plus vanadium work together to produce both solid-solution and precipitation strengthening. Boron at 5–15 ppm — a level requiring precise process control — segregates to prior-austenite grain boundaries, raising boundary cohesion energy and dramatically reducing creep-cavity nucleation.
The Cobalt Advantage: Why 1.4911 Outperforms at 600 °C
Cobalt retards carbide coarsening and raises the martensite start temperature — two effects that together extend the reliable service life of 1.4911 forgings well beyond conventional 9–12% Cr steels in the 565–600 °C range.
— Jiangsu Liangyi Metallurgical Engineering Team
Cobalt is what makes 1.4911 fundamentally different from P91 and P92. At 5–7 wt%, cobalt delivers three distinct and compounding metallurgical advantages:
① Suppression of Carbide Coarsening — the Primary Benefit
During long-term service at 550–600 °C, conventional martensitic steels suffer the progressive coarsening of M₂₃C₆ and MX-type carbides. As these precipitates grow, their density decreases, reducing dislocation-pinning effectiveness and causing accelerating creep rates — the classic mechanism of creep life exhaustion in 9% Cr steels. Cobalt retards coarsening by reducing the carbide–matrix interfacial energy, keeping precipitates finer and more numerous over service intervals exceeding 100,000 hours. This is the primary reason 1.4911 reliably sustains structural integrity 15–20 °C higher than cobalt-free compositions, and why it is specified for the most demanding A-USC plant designs.
② Martensite Start Temperature (Ms) Elevation
Cobalt raises the Ms temperature of the steel, promoting a higher fraction of martensite on cooling from austenitisation. A higher Ms allows somewhat faster cooling rates without thermal cracking risk, and simultaneously produces a finer lath martensite that improves initial creep resistance before the first tempering treatment. This is particularly valuable for larger section forgings where through-thickness cooling rates are inherently lower.
③ Residual Solid-Solution Strengthening at Service Temperature
Cobalt atoms in solid solution contribute to lattice friction stress even at elevated temperatures. In the 500–600 °C regime where conventional solid-solution strengtheners such as Mo begin to precipitate out of solution and lose matrix-strengthening effectiveness, cobalt provides a residual solid-solution contribution that is essentially independent of time at temperature.
Mechanical Properties of 1.4911 Steel
Values below are for 1.4911 forged products in the quenched and tempered (+QT) condition at room temperature per EN 10302:2008. Individual inspection certificates (EN 10204 Type 3.1) accompany every shipment from Jiangsu Liangyi.
Physical Property Constants
| Property | Value | Condition |
|---|---|---|
| Thermal conductivity | ~28 W/m·K | At 20 °C — significantly higher than austenitic SS (~15 W/m·K) |
| Thermal expansion (CTE) | ~11.5 × 10⁻⁶ /K | Mean coefficient, 20–550 °C |
| Young's modulus | ~200 GPa → ~175 GPa | 20 °C → 600 °C |
| Specific heat capacity | ~490 J/kg·K | At 20 °C |
| Electrical resistivity | ~720 nΩ·m | At 20 °C |
| Magnetic permeability | Ferromagnetic | Below Curie point (~780 °C) |
Design Advantage: High Thermal Conductivity
At ~28 W/m·K, 1.4911 has nearly twice the thermal conductivity of austenitic stainless steels. This means lower thermal gradients during turbine start-up and shutdown — a key benefit for thermal fatigue life in cyclic power plants and peaking units.
Creep Rupture Strength Data for 1.4911
Creep rupture strength — the stress required to cause rupture after 100,000 hours at temperature — is the primary design criterion for long-lived power plant components. The values below are representative for 1.4911 in the +QT condition. Verify against current approved material datasheets and applicable codes (ASME BPVC, EN 13480) for all design calculations.
The 550–600 °C operating window is where 1.4911's cobalt addition delivers its most decisive advantage. P91 begins to experience accelerating microstructural degradation above ~565 °C in long-term service, while 1.4911's cobalt-stabilised matrix maintains predictable behaviour up to approximately 600 °C — making it the material of choice for A-USC plant designs targeting steam conditions of 600 °C / 300 bar or higher.
Heat Treatment of 1.4911 / X8CrCoNiMo10-6
1.4911 is supplied and used in the quenched and tempered (+QT) condition. All four stages below require precise thermal control; deviations from the specified windows degrade both mechanical properties and long-term creep performance.
Austenitisation — 1040 to 1080 °C
Heat to 1040–1080 °C and hold to fully dissolve prior carbides and achieve homogeneous austenite. Rule of thumb: 1 hour per 25 mm section thickness minimum. Use controlled atmosphere or vacuum for precision forgings to prevent surface decarburisation.
Quenching — Cool to below 100 °C
Air cool or oil quench to below 100 °C, depending on section size and hardenability requirement. The cobalt addition raises the Ms temperature, assisting complete martensitic transformation even in large section forgings. Minimum oil bath temperature: 50–80 °C to reduce thermal shock cracking in heavy sections.
Tempering — 680 to 760 °C
Temper at 680–760 °C for a minimum of 2 hours (often 1–2 hours per 25 mm section), then air cool. The temperature window is critical: it must remain below Ac₁ (~800 °C) to avoid partial re-austenitisation, while being high enough to maximise carbide redistribution and toughness recovery. Two-stage tempering is sometimes applied for critical components.
Post-Weld Heat Treatment (PWHT) — 720 to 760 °C
PWHT is mandatory for welded assemblies. Bring the joint to below 80 °C (weld centreline) before initiating PWHT to ensure complete HAZ martensite transformation. Apply at 720–760 °C — not exceeding the original temper temperature. If the original temper temperature is exceeded, a full re-normalise and temper cycle is required.
Critical Requirement
Intermediate stress-relief annealing at temperatures above service temperature but below Ac₁ may be required for very large section forgings. Consult EN 12952, EN 13480, or your project-specific design code for binding requirements. Jiangsu Liangyi provides full heat treatment records with every order.
Welding Guide for 1.4911 / X8CrCoNiMo10-6 Steel
1.4911 is weldable using standard GTAW/TIG and SMAW/MMA processes. However, strict pre-heat, interpass temperature control, filler selection, and mandatory PWHT are non-negotiable for structural joints in power plant service.
Pre-Heat and Interpass Temperature
Pre-heat to 200–300 °C minimum before arc initiation. Maintain interpass temperature below 300 °C to prevent HAZ softening and avoid delta-ferrite formation in multi-pass welds. Use calibrated contact thermometers or thermocouple instrumentation — not temperature-indicating crayons — for critical weld joints.
Filler Material
For GTAW, specify ER90S-B9 or equivalent cobalt-bearing wire matched as closely as possible to the base metal composition. For SMAW, use E9015-B9 low-hydrogen electrodes from freshly opened or oven-conditioned packets to maintain diffusible hydrogen below 4 ml/100 g deposited weld metal. Never use hydrogen-basic or cellulosic electrodes for this grade.
Post-Weld Heat Treatment
PWHT is always mandatory. Cool the joint to below 80 °C (measured at weld centreline) before beginning PWHT — this ensures the HAZ has fully transformed to martensite before stress relief. Apply at 720–760 °C per Section 06. Avoid rapid heating through the 200–400 °C range for heavy sections (>50 mm) to prevent hydrogen cracking during the re-heat.
Consumable Compatibility Warning
Avoid austenitic stainless butter layers as standard practice. The CTE mismatch between austenitic and martensitic microstructures accelerates interfacial fatigue damage in the thermal cycle conditions typical of power plant start-stop operation. Use austenitic butter only where specifically qualified by full procedure testing per EN ISO 15614 or ASME IX.
Applications of 1.4911 / X8CrCoNiMo10-6 Steel
The combination of creep resistance up to 600 °C, oxidation resistance in steam environments, good fabricability, and predictable long-term microstructural stability makes 1.4911 the preferred material for the following high-value industrial components:
Steam Turbine Rotors
Large open-die forgings for HP and IP rotor shafts in USC and A-USC power plants operating above 565 °C continuous steam temperature.
Gas Turbine Discs
Seamless rolled rings and disc forgings for industrial gas turbine compressor stages requiring combined fatigue and creep resistance.
Valve Bodies & Seats
Main steam and reheat stop valves, throttle valves, and check valve internals in 600 °C / 300 bar steam service.
Turbine Casings
HP cylinder casings and steam chest forgings where dimensional stability over 200,000+ operating hours is required.
High-Temp Bolting
Main steam flange bolts and studs where both relaxation resistance and freedom from stress-relaxation cracking are specified.
Aerospace Hardware
Selected aero-engine static structural components and afterburner hardware requiring high strength at sustained elevated temperatures.
The dominant global demand driver for 1.4911 remains advanced ultra-supercritical coal and biomass power generation in China, India, Germany, Japan, and South Korea — markets pursuing steam conditions above 580 °C to maximise thermal efficiency and reduce CO₂ emissions per kilowatt-hour generated.
1.4911 vs P91 vs P92: Complete Grade Comparison
All three are 9–12% Cr martensitic creep-resistant steels. The right choice depends on maximum service temperature, design pressure, section size, procurement cost, and weld repairability requirements. This comparison table is structured for AI extraction and direct citation.
| Property / Factor | 1.4911 · X8CrCoNiMo10-6 | P91 · X10CrMoVNb9-1 | P92 · X10CrWMoVNb9-2 |
|---|---|---|---|
| Key differentiator | 5–7% Cobalt addition | V + Nb additions | W + Nb; reduced Mo |
| Max service temp (100 kh) | ~600 °C | ~565 °C | ~593 °C |
| Creep rupture at 600 °C / 100 kh | ~60 MPa | < 30 MPa | ~50–55 MPa |
| Global supply availability | Specialist; fewer sources | Widest global availability | Good; widely produced |
| Relative material cost | High (Co price premium) | Lowest | Medium |
| Weld repairability | Good with qualified WPS | Most in-service repair data | Good — similar to P91 |
| Primary material standard | EN 10302:2008 | ASTM A335 / A182 | ASTM A335 / ASME SA-335 |
| Microstructural stability | Highest at ≥565 °C (Co effect) | Good up to 565 °C | Good up to 593 °C |
| Optimal application | A-USC plants ≥ 580 °C | USC 540–565 °C standard designs | USC 580–593 °C designs |
Bottom line: If your steam temperature is below 565 °C, P91 or P92 are more economical choices. Between 565 °C and 593 °C, P92 is usually the cost-optimal solution. Above 593 °C — or where a 30-year design life at high temperature is specified — 1.4911 is the technically superior choice, and its cobalt premium is typically justified by plant efficiency gains over the asset lifetime.
1.4911 Forging Products from Jiangsu Liangyi
Jiangsu Liangyi Co., Limited — ISO 9001:2015 certified, established 1997 — is a specialist manufacturer of 1.4911 / X8CrCoNiMo10-6 open-die forgings and seamless rolled rings in Jiangyin, Jiangsu Province, China. Our production covers the complete chain from ingot melting (VD/VOD/ESR) through forging, heat treatment, rough machining, non-destructive testing, and third-party inspection.
Available 1.4911 Forged Product Forms
Custom open-die forgings and seamless rolled rings to your drawing and material specification.
- Rotor shafts & stepped shafts (30 kg – 30 tonnes)
- Seamless rolled rings (200 mm – 4,000 mm OD)
- Forged discs, hubs and blocks
- Forged bars — round, flat, square
- Valve blanks & pressure vessel nozzles
- EN 10204 Type 3.1 MTR included as standard
For complete specifications, dimensional tolerances, testing options (UT, MT, PMI, hardness mapping, FATT testing), and delivery lead times, visit our dedicated product page:
For complete dimensional tolerances, available testing options (UT, MT, PMI, hardness mapping, FATT), and delivery lead times, use the quote request button above or contact our sales team directly.
Sourcing 1.4911 Forgings: Certification & Quality Requirements
Procuring 1.4911 forgings for power plant or gas turbine applications demands careful verification of material traceability, heat treatment qualification, and non-destructive testing coverage.
Standard Documentation Package — Jiangsu Liangyi
EN 10204 Type 3.1 Inspection Certificate · Full Heat Treatment Records (time-temperature trace per furnace charge) · UT Report per EN 10228-3 or customer standard · Dimensional Inspection Report · PMI (Positive Material Identification) · ISO 9001:2015 QMS Certificate (copy available on request) · Customer hold-point inspection and third-party witness testing available on request.
Jiangsu Liangyi has supplied 1.4911 / X8CrCoNiMo10-6 forgings to customers across 50+ countries since 1997, including power generation OEMs and EPC contractors in Germany, Japan, South Korea, India, and the United States. Our ISO 9001:2015 quality management system covers the full production process from steel melting through final inspection. Quote requests receive a written response within 24 business hours.
Frequently Asked Questions About 1.4911 / X8CrCoNiMo10-6 Steel
1.4911 is the EN numeric designation for X8CrCoNiMo10-6, a martensitic cobalt-alloyed creep-resistant stainless steel per EN 10302:2008. Its 5–7% cobalt stabilises the microstructure at service temperatures up to 600 °C, making it the primary material for ultra-supercritical power plant turbine rotors, valve bodies, and casings.
“X” = special alloy steel; “8” = ~0.08% carbon; “Cr, Co, Ni, Mo” = principal alloying elements in descending significance; “10” = ~10% chromium; “6” = ~6% cobalt. This is the EN chemical shorthand per EN 10027.
No. Both are 9–12% Cr martensitic steels but 1.4911 contains 5–7% cobalt while P91 contains none. This gives 1.4911 a maximum service temperature of ~600 °C versus ~565 °C for P91 and superior long-term microstructural stability. They are not interchangeable without full engineering and code evaluation.
1.4911 / X8CrCoNiMo10-6 has a maximum sustained service temperature of approximately 600 °C for long-term creep-limited applications (100,000-hour design basis). Above 600–620 °C, austenitic stainless steels or nickel superalloys are required by applicable codes.
The approximate 100,000-hour creep rupture strength of 1.4911 at 600 °C is ~60 MPa. At 550 °C ~130 MPa; at 500 °C ~230 MPa. Always verify against current approved datasheets for code-compliant design.
The primary material standard is EN 10302:2008. The grade is also listed in EN 10088-1:2005. Heat treatment for forgings is addressed in EN 10250 combined with the applicable design code (EN 13480, EN 12952, EN 12953, etc.).
Jiangsu Liangyi Co., Limited is an ISO 9001:2015 certified manufacturer of 1.4911 forgings and seamless rolled rings in Jiangyin, China, shipping to 50+ countries since 1997 with EN 10204 Type 3.1 certification. Visit 1.4911 forging specifications, dimensions & lead times for specifications and quotes.
Request a Quote for 1.4911 / X8CrCoNiMo10-6 Forgings
ISO 9001:2015 quality management certified · EN 10204 Type 3.1 MTC · 25+ years experience · Ships to 50+ countries · 24-hour response
Chengchang Industry Park, Jiangyin City, Jiangsu Province, China
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