01 What Is the Difference Between P92 and P91?
For nearly three decades, P91 was the gold standard for high-temperature components in advanced power plants. Standardised as X10CrMoVNb9-1 / 1.4903 under EN 10222-2 and as ASTM A182-F91, it replaced older 2.25Cr-1Mo steels across main steam systems worldwide, enabling higher temperatures, thinner walls, and longer design lives.
But the drive toward higher thermal efficiency pushed plant designers into operating conditions P91 was never engineered for. Today's ultra-supercritical (USC) and advanced ultra-supercritical (AUSC) plants target steam temperatures of 600–640 °C and pressures of 25–30 MPa. At those conditions, P91's allowable stress becomes too low: required wall thicknesses in main steam piping and valves grow to dimensions that are structurally problematic and economically unacceptable.
P92 — X10CrWMoVNb9-2 / 1.4901 under EN 10222-2, ASTM A182-F92 / ASME SA182-F92 under North American codes — was developed specifically to address this limitation. The defining innovation: replace approximately half of P91's molybdenum (from 0.85–1.05% down to 0.30–0.60%) with tungsten (1.50–2.00%). Tungsten's larger atomic radius and far slower diffusion rate in iron at elevated temperature produce a more stable, stronger microstructure at 600 °C and above.
P92 forgings for European projects are governed by EN 10222-2. For North American projects, the applicable standard is ASTM A182 / ASME SA182 (Grade F92), covered under ASME Code Case 2179. Projects subject to PED 2014/68/EU typically require EN 10204 Type 3.1 mill certificates as a minimum; Type 3.2 (countersigned by a purchaser-nominated independent inspection body) can be arranged at the customer's request and cost.
02 The Metallurgical Mechanism: Why Tungsten Works
Understanding why P92 outperforms P91 at high temperature requires a brief look at how 9Cr martensitic steels fail in service — and precisely where the tungsten addition intervenes.
How 9Cr steels lose strength at high temperature
Both grades derive their strength from a tempered martensitic microstructure: a dense network of lath boundaries and finely dispersed carbide and carbonitride precipitates (primarily M₂₃C₆ carbides and MX carbonitrides) that physically obstruct dislocation movement. Long-term service at elevated temperature degrades these barriers through two diffusion-controlled processes:
- Carbide coarsening (Ostwald ripening): Fine precipitates dissolve and re-precipitate as larger, fewer particles — reducing the total precipitate surface area and the number of effective dislocation-pinning sites.
- Subgrain growth: The fine lath martensite gradually transforms toward an equiaxed subgrain structure, reducing grain-boundary area and the barriers it provides to dislocation climb.
Both processes accelerate with temperature and time, and both are governed by solid-state diffusion. This is where the choice between molybdenum and tungsten becomes decisive.
Why W outperforms Mo in solid-solution strengthening
- Atomic radius effect: Tungsten (atomic radius 193 pm) is significantly larger than molybdenum (139 pm). When W substitutes into the iron lattice, it creates greater local lattice distortion and stronger elastic interaction with moving dislocations.
- Diffusion rate: At 600 °C, the self-diffusion coefficient of W in α-Fe is roughly an order of magnitude lower than that of Mo. W atoms are essentially immobile on the timescale of 100,000 hours of service, preserving solid-solution strengthening that Mo cannot sustain.
- Carbide stability: W-enriched M₂₃C₆ carbides coarsen significantly more slowly than Mo-enriched equivalents at 600 °C. This maintains a finer, more numerous precipitate population — and the associated strengthening — over longer service lives.
Figure 1 — Operating Temperature Range: P91 vs P92 in Sustained Pressure Service
03 Creep Rupture Strength: Data and Code Values
| Temperature | P91 Allowable Stress (MPa) | P92 Allowable Stress (MPa) | P92 vs P91 Advantage |
|---|---|---|---|
| 550 °C | 109 | 117 | +7% |
| 575 °C | 94 | 107 | +14% |
| 600 °C | 71 | 94 | +32% |
| 625 °C | 48 | 70 | +46% |
| 640 °C | 34 | 55 | +62% |
Indicative values. Verify against EN 10222-2, ASME BPVC Section II Part D, or the applicable design code revision before use in formal calculations.
What these numbers mean in practice: a main steam pipe designed in P92 at 600 °C can use walls approximately 25% thinner than an equivalent P91 design for the same pressure. For a 600 MW USC plant main steam line operating at 27 MPa, this represents several tonnes of material savings per metre of pipework. Thinner walls also mean faster thermal response during startup and lower thermal fatigue stress during load cycling — both valuable for the flexible operation modern grids require.
The P92 allowable-stress advantage narrows significantly below 570 °C. For components that operate below 565 °C where P91 is already adequate, the additional cost and stricter welding requirements of P92 are not justified. See Section 8 for the full decision framework.
04 Full Grade Comparison: P91 vs P92 Side by Side
| Property / Criterion | P91 · 1.4903 · X10CrMoVNb9-1 | P92 · 1.4901 · X10CrWMoVNb9-2 |
|---|---|---|
| EN material number | 1.4903 | 1.4901 |
| ASTM equivalent | A182-F91 | A182-F92 |
| ASME code basis | Listed material | ASME Code Case 2179 |
| Cr content (wt%) | 8.0 – 9.5 | 8.5 – 9.5 |
| Mo content (wt%) | 0.85 – 1.05 | 0.30 – 0.60 (reduced) |
| W content (wt%) | None | 1.50 – 2.00 (key addition) |
| V content (wt%) | 0.18 – 0.25 | 0.15 – 0.25 |
| Nb content (wt%) | 0.06 – 0.10 | 0.04 – 0.09 |
| N content (wt%) | 0.030 – 0.070 | 0.030 – 0.070 |
| Creep rupture @ 600 °C / 100,000 h | ~71 MPa | ~94 MPa |
| Max. continuous service temp. | ~593 °C | ~625 °C |
| Normalising temperature | 1040 – 1080 °C | 1040 – 1080 °C |
| Tempering temperature | 700 – 760 °C | 730 – 800 °C (30 °C higher) |
| Min. preheat (welding) | 200 °C | 200 °C |
| Max. interpass temp. (welding) | 300 °C | 300 °C |
| PWHT temperature | 730 – 780 °C | 730 – 800 °C |
| Weld consumable availability | Widely available | Available — must specify W-bearing grade |
| Relative material cost (per kg) | Baseline | +8 – 15% over P91 |
| Wall thickness at 600 °C / 27 MPa | Baseline | ~25% thinner |
05 Heat Treatment: Protocols and Critical Differences
Both grades use a normalise-and-temper cycle. The framework is identical; the tempering window is critically different — and that 30 °C difference between grades has caused documented in-service failures when engineers applied P91 heat treatment parameters to P92 components.
Austenitising (Normalising): 1040–1080 °C
Heat the forging uniformly into the fully austenitic phase field. Temperature uniformity across the section must be within ±10 °C — under-temperature leaves undissolved carbides that weaken the final structure; over-temperature coarsens prior austenite grain size, degrading toughness. Hold for a minimum of 1 hour per 25 mm section thickness. Both P91 and P92 share this window exactly.
Cooling to Room Temperature
Air cool or force-cool to room temperature to produce a fully martensitic microstructure. For heavy sections above ~400 mm cross-section, accelerated cooling may be required to avoid delta-ferrite or bainite formation. Verify full transformation with metallographic examination or hardness survey on production coupons.
Tempering: 730–800 °C for P92 (vs 700–760 °C for P91) — CRITICAL DIFFERENCE
This higher tempering range is the most important procedural difference between the two grades. The W addition in P92 stabilises carbides and shifts the optimal softening and stress-relief temperature upward. Tempering P92 at P91 temperatures (700–760 °C) produces an under-tempered structure with excessive hardness (typically >270 HV), poor toughness, and elevated residual stress — a known precursor to stress-corrosion and reheat cracking. Minimum hold: 2 hours, scaled to section thickness.
Post-Weld Heat Treatment (PWHT): 730–800 °C — Mandatory for All Pressure Welds
PWHT at 730–800 °C is mandatory for every pressure-retaining weld on P92 components. Minimum hold: 1 hour per 25 mm of weld throat thickness. The PWHT cycle must be fully documented, conducted with calibrated thermocouples on or adjacent to the weld, witnessed by the relevant inspection authority, and retained in the component material dossier. Skipping PWHT on P92 is the leading cause of early-life heat-affected zone cracking on record in USC plant valve and piping systems.
For P92 forgings over 1,000 kg, attach thermocouples directly to the forging surface and geometric centre during heat treatment — not only to the furnace atmosphere. Jiangsu Liangyi's ten computer-controlled heat treatment furnaces maintain temperature uniformity within ±5 °C across the charge, with full digital records supplied with the EN 10204 3.1 certificate package.
06 Welding P92: Consumables, Preheat, and PWHT
Why P91 consumables must never be used on P92
P91 welding wire and electrodes contain no tungsten. Using them to weld P92 pressure components creates weld metal with creep rupture strength matching P91 — not P92. The resulting joint contains a local weak zone at the weld centre-line that is invisible to standard NDE and may not manifest as cracking for years. This is not a theoretical risk: multiple documented failure cases involve inadvertent use of P91 consumables on P92 welds.
Approved welding consumables for P92
- GTAW / TIG (root pass): Dedicated P92 filler wire with verified W: 1.5–2.0%, Mo: 0.3–0.6%, Nb + V present. Verify chemistry from consumable mill certificate — not just trade name.
- SMAW (stick): P92 low-hydrogen covered electrodes with matching W-Mo-Nb chemistry. Bake per manufacturer's instructions before use; use from a temperature-controlled oven.
- SAW (submerged arc): Dedicated P92 wire-flux combination. The flux chemistry affects the final weld metal W and Nb recovery — always specify and verify both wire and flux.
Preheat, interpass, and PWHT parameters
- Minimum preheat: 200 °C for wall thickness ≥ 20 mm
- Maximum interpass temperature: 300 °C — exceeding this promotes delta-ferrite formation in the weld metal
- PWHT: 730–800 °C, minimum 1 hour per 25 mm weld throat, mandatory without exception for pressure-retaining welds
- Cool-down after PWHT: Controlled slow cooling — avoid rapid cooling which can introduce new thermal stresses
07 USC Plant Applications for P92 Forgings
In a 600 °C-class ultra-supercritical plant, P92 forgings are specified across several critical steam-cycle locations (see full 1.4901 product specifications and dimensions):
- Main steam stop and control valves: Valve bodies, bonnets, stems, and seat rings in the main steam line between boiler outlet and HP turbine inlet. At 625–640 °C operating temperature, these are the most thermally demanding pressure-boundary components in the plant. Single-piece weights range from 500 kg to 3,000+ kg depending on nominal bore.
- Main steam and hot reheat pipe fittings: Tees, elbows, reducers, and weld-neck flanges. Open-die forged fittings are strongly preferred over fabricated equivalents at these temperatures — forged grain flow provides superior through-thickness creep and fatigue properties.
- Boiler outlet headers and terminal nozzle forgings: The final superheater and reheater outlet headers on supercritical once-through boilers can reach 610–625 °C at the outlet nozzle. P92 seamless rolled rings or hollow-bar forgings are the standard solution.
- HP turbine inlet steam chest and valve housings: The steam chest at the HP turbine first stage sees the highest combination of temperature and pressure in the entire turbine system — typical forging delivery lead times 12–20 weeks for complex shapes.
- Interconnecting pipework flanges (Class 2500 and above): Large-bore welding-neck flanges must be full-body forgings with fibre orientation perpendicular to the pressure boundary — no weld-on plate flanges or machined-from-bar shortcuts on Class 2500 P92 service.
Jiangsu Liangyi manufactures 1.4901 / X10CrWMoVNb9-2 forged components from 30 kg to 30,000 kg per piece, up to 6 metres in diameter and 15 metres in length, from its ISO 9001:2015 certified 80,000 m² facility in Jiangyin, China. Annual capacity: 120,000 metric tonnes. Visit jnmtforgedparts.com for the full dimensional range and technical data sheet.
Need 1.4901 / P92 Forgings for Your USC Project?
Valve bodies · Boiler headers · Pipe fittings · Seamless rings — ISO 9001:2015 certified, EN 10204 3.1 mill certificates as standard. Quote within 24 hours.
Send a Forging Enquiry Browse All Products08 When to Use P91 vs P92: Decision Guide
- Component design temperature is consistently below 593 °C and P91 allowable stress is adequate
- The plant is a conventional supercritical design at 540–565 °C / 24–25 MPa
- Welding subcontractors are qualified for P91 but have no P92 WPS/PQR on record
- The component is outside the main steam path — auxiliary steam, blowdown, attemperator systems
- Project budget constrains material upgrade and design temperature does not demand it
- Design steam temperature at the component exceeds 600 °C
- Design pressure is above 24 MPa and P91 wall thickness is uneconomically heavy
- Plant design targets USC or AUSC thermal efficiency improvements
- Client spec or EPC contract calls for EN 1.4901, F92, or X10CrWMoVNb9-2 explicitly
- Reducing component weight is critical for pipe hanger and support loads
09 Procurement Guidance: Standards, Certificates, and Lead Times
How to specify P92 on a purchase order (dual designation)
For European projects, always state both designations: "EN 10222-2, X10CrWMoVNb9-2, Mat. No. 1.4901 / ASTM A182-F92". Dual designation eliminates customs ambiguity and simplifies quality plan approval with EPC contractors. For projects subject to PED 2014/68/EU, consult your notified body or EPC contractor for the specific documentation requirements applicable to your equipment category.
Certificate types
- EN 10204 Type 3.1: Mill test report certified by the manufacturer's own qualified inspector, independent of production. This is what Jiangsu Liangyi issues as standard with every forging order. Accepted by the majority of power generation, oil & gas, and pressure vessel projects worldwide.
- EN 10204 Type 3.2: Type 3.1 content plus countersignature by an independent inspection body nominated and contracted by the purchaser (not the manufacturer). The purchaser arranges and pays for their chosen inspection agency to witness testing and countersign the certificate. This is required by many high-criticality power project quality plans and by some client specifications — confirm with your EPC or end-client.
Minimum NDE scope for P92 main steam forgings
- UT: 100% volumetric coverage to EN 10228-3 or ASTM A388 — specify acceptance level on your inquiry
- MT: Magnetic particle testing to EN ISO 10228-1 on all accessible post-machining surfaces
- Hardness: Brinell survey across the forging body to verify heat treatment uniformity (typical P92 finished hardness: 197–248 HB)
- Charpy impact: Testing at the design service temperature or a defined reference temperature, with minimum absorbed energy per the applicable standard
- PMI: Positive material identification — increasingly specified by EPC contractors for grade verification, especially for tungsten-bearing grades
Typical delivery lead times from China
- Standard P92 bars, rings, and discs: 4–6 weeks production + 2–4 weeks sea freight to Hamburg, Rotterdam, Antwerp, Felixstowe, or other major European ports
- Machined components with third-party 3.2 inspection: 8–12 weeks total
- Large valve bodies and boiler headers above 5 tonnes: 10–16 weeks — plan the procurement milestone accordingly on your project schedule
To receive a binding quotation within 24 hours, provide: material specification (EN and/or ASTM), drawing or dimensional sketch, single-piece weight estimate, quantity, certificate type required (3.1 or 3.2), NDE scope, machining state (as-forged / rough-machined / finish-machined to drawing), delivery port, and required delivery date. The more complete the inquiry, the faster and more accurate the response.
10 Frequently Asked Questions
P92 (X10CrWMoVNb9-2, 1.4901, F92) and P91 (X10CrMoVNb9-1, 1.4903, F91) are both 9% chromium tempered-martensitic steels for high-temperature pressure service. The critical difference is chemistry: P92 replaces approximately half of P91's molybdenum (0.85–1.05 wt%) with tungsten (1.50–2.00 wt%). Tungsten's larger atomic radius and far lower diffusion rate in iron at 600 °C produce a more stable microstructure, delivering approximately 33% higher creep rupture strength at 600 °C / 100,000 hours and raising the practical service ceiling from ~593 °C (P91) to ~625 °C (P92).
The indicative allowable stress of P92 (1.4901 / X10CrWMoVNb9-2) at 600 °C is approximately 94 MPa on a 100,000-hour creep rupture basis, based on ASME BPVC Section II Part D and ECCC 2017 datasets. P91 at the same temperature and time basis achieves approximately 71 MPa — a 32% difference. Always verify against the applicable code edition and revision for design purposes. The advantage widens to approximately 46% at 625 °C and 62% at 640 °C.
P92 (1.4901) forgings require: (1) Normalising at 1040–1080 °C, minimum 1 hour per 25 mm section thickness, followed by air or accelerated cooling to room temperature; (2) Tempering at 730–800 °C, minimum 2 hours hold — this is approximately 30 °C higher than the P91 tempering range of 700–760 °C and the difference is critical. Applying P91 tempering temperatures to P92 produces an under-tempered, excessively hard structure. PWHT after welding must be at 730–800 °C.
No — never. P91 welding consumables contain no tungsten and produce weld metal with P91-grade creep strength, not P92-grade. Using P91 consumables on P92 pressure-retaining welds creates a metallurgical weak zone at the weld centreline that cannot be detected by standard UT, MT, or PT methods. Multiple documented USC plant failures have been attributed to this error. Always specify and verify dedicated P92 welding consumables with confirmed W: 1.5–2.0%, Mo: 0.3–0.6%, and matching Nb and V content.
ASTM A182 Grade F92 is equivalent to EN 10222-2, X10CrWMoVNb9-2, material number 1.4901. Both refer to the same 9Cr-W-Mo-V-Nb tempered-martensitic steel for high-temperature pressure service. For European projects, specify both designations on the purchase order: "EN 10222-2, X10CrWMoVNb9-2, Mat. No. 1.4901 / ASTM A182-F92". This eliminates customs ambiguity and simplifies documentation under European pressure equipment regulations.
P92 forgings typically cost 8–15% more per kilogram than equivalent P91 forgings, reflecting the higher tungsten content in the alloy chemistry. However, because P92 allows wall sections approximately 25% thinner at 600 °C for the same design pressure, the total component weight — and therefore total material cost — is substantially lower than the per-kilogram premium suggests. In USC main steam systems, the P92 material premium is consistently offset by improved thermal efficiency over the plant's 30+ year operating life.
P92 steel (1.4901 / X10CrWMoVNb9-2 / F92) is used primarily in ultra-supercritical (USC) and advanced ultra-supercritical (AUSC) coal and gas power plants operating at steam temperatures of 600–640 °C and pressures of 25–30 MPa. Typical applications include main steam stop and control valve bodies and internals, main steam and hot reheat pipe fittings and flanges, superheater and reheater outlet header forgings, HP turbine steam chest components, and boiler pressure parts where P91 is insufficient. It is also used in high-temperature hydrogen service, and some oil and gas applications at elevated temperature, subject to API RP 941 limits.
Jiangsu Liangyi Co., Limited (ISO 9001:2015 certified, Jiangyin, China) manufactures 1.4901 / X10CrWMoVNb9-2 / ASTM A182-F92 forgings from 30 kg to 30,000 kg, up to 6 metres in diameter. EN 10204 Type 3.1 mill certificates are issued as standard with every order. EN 10204 Type 3.2 (countersignature by a purchaser-nominated independent inspection body) can be accommodated at the customer's request. Production lead times are typically 4–6 weeks for standard shapes, with sea freight delivery to major European ports in 2–4 weeks. Contact the sales team via the website for a detailed quotation.
11 References and Standards
Authoritative Standards and Data Sources Referenced in This Article
- EN 10222-2:2017 — Steel forgings for pressure purposes. Part 2: Ferritic and martensitic steels with specified elevated temperature properties. CEN, Brussels.
- ASTM A182 / A182M — Standard Specification for Forged or Rolled Alloy and Stainless Steel Pipe Flanges, Forged Fittings, and Valves and Parts for High-Temperature Service. ASTM International, West Conshohocken, PA.
- ASME BPVC Section II Part D — Materials: Properties (Customary). American Society of Mechanical Engineers. Includes allowable stress tables for Grade F92.
- ASME Code Case 2179 — Use of SA-182 Grade F92 and SA-335 Grade P92 in Section I Construction. ASME.
- ECCC Data Sheets (2017) — European Creep Collaborative Committee. Creep rupture strength values for X10CrWMoVNb9-2 and X10CrMoVNb9-1 at 100,000 hours.
- EN 10204:2004 — Metallic products. Types of inspection documents. CEN, Brussels. (Defines Types 2.1, 2.2, 3.1, and 3.2 material certificates.)
- PED 2014/68/EU — Directive of the European Parliament and of the Council on the harmonisation of the laws of the Member States relating to the making available on the market of pressure equipment.
- API RP 941 (8th Edition) — Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants. American Petroleum Institute.
- EN 10228-3:2016 — Non-destructive testing of steel forgings. Part 3: Ultrasonic testing of ferritic or martensitic steel forgings.
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