At a Glance — Key Facts
Section 1 — Grade Profiles
What Are P91 and P92 Steel? Definitions and Background
P91 (1.4903 / X10CrMoVNb9-1 / Grade 91) and P92 (X10CrWMoVNb9-2 / Grade 92 / NF616) are both 9% chromium tempered martensitic heat-resistant steels. P92 is a direct evolution of P91, replacing approximately half the molybdenum with tungsten and adding a boron micro-alloying element, resulting in 15–32% higher creep rupture strength above 580°C at the cost of a 20–35% higher material price and a more complex global welding supply chain.
P91 and P92 belong to the same family of 9% chromium tempered martensitic heat-resistant steels. They share a common microstructural basis — a tempered lath martensite matrix stabilised by fine M₂₃C₆ carbide and MX-type carbonitride precipitates — but differ in one critical alloying decision: the partial replacement of molybdenum with tungsten.
1.4903 / X10CrMoVNb9-1
ASTM A182 F91 · ASME SA182 F91 · EN 10302 · EN 10222-2 · X10CrMoVNb91
Developed at Oak Ridge National Laboratory (USA) in the late 1970s and commercialised through the 1980s, P91 is the world's most widely specified alloy for supercritical and sub-critical power plant steam piping. Vanadium (V) and niobium (Nb) additions precipitate as ultra-fine MX-type carbonitrides that pin dislocation motion at elevated temperature, delivering creep rupture strength approximately three times higher than the older P22 (2.25Cr-1Mo) grade at 550 °C. P91 has well-established weldability, a deep global consumable supply, and a repair infrastructure built over four decades of industrial deployment.
X10CrWMoVNb9-2 / NF616
ASTM A182 F92 · ASME SA182 F92 · NF616 (Japanese designation) · EN 10302 equivalent
P92 — also designated NF616 from its Japanese development programme — evolved directly from P91 in the early 1990s through a collaborative research programme between major Japanese steel manufacturers and the international power generation research community. The defining change: approximately half the molybdenum is replaced by tungsten (0.5 Mo + 1.8 W instead of 1.0 Mo), and a small boron addition (0.001–0.006%) is introduced. Tungsten's larger atomic radius produces stronger solid-solution strengthening, and W-bearing precipitates coarsen far more slowly under sustained high-temperature loading — preserving creep resistance across 100,000-hour design lifetimes. P92 delivers 15–32% higher creep rupture strength than P91 at temperatures above 580°C.
Section 2 — Chemical Composition
P91 vs P92 Chemical Composition: Side-by-Side Comparison
The table below compares nominal chemical composition ranges for both grades per EN 10302 (European standard for creep-resisting steels and alloys) and equivalent ASTM A182 specifications. All values are in weight percent (wt%). Highlighted rows mark the most significant differences between the two grades.
| Element | Symbol | P91 / 1.4903 (wt%) | P92 / X10CrWMoVNb9-2 (wt%) | Significance |
|---|---|---|---|---|
| Carbon | C | 0.08 – 0.12 | 0.07 – 0.13 | Similar — controls martensite hardness |
| Chromium | Cr | 8.00 – 9.50 | 8.50 – 9.50 | Similar — oxidation & corrosion resistance |
| Molybdenum | Mo | 0.85 – 1.05 | 0.30 – 0.60 | ↓ Reduced ~55% in P92 |
| Tungsten | W | — (absent) | 1.50 – 2.00 | ↑ Added only in P92; key to creep advantage |
| Vanadium | V | 0.18 – 0.25 | 0.15 – 0.25 | Similar — fine MX precipitates |
| Niobium | Nb | 0.06 – 0.10 | 0.04 – 0.09 | Similar — grain refinement + MX |
| Nitrogen | N | 0.030 – 0.070 | 0.030 – 0.070 | Identical — MX carbonitride formation |
| Boron | B | — (absent) | 0.001 – 0.006 | ↑ Added only in P92; retards grain-boundary carbide coarsening |
| Manganese | Mn | 0.30 – 0.60 | 0.30 – 0.60 | Identical |
| Silicon | Si | ≤ 0.50 | ≤ 0.50 | Identical |
Section 3 — Mechanical Properties
Room-Temperature Mechanical Properties: P91 vs P92
Both grades are delivered in the quenched and tempered (Q+T) condition. Room-temperature mechanical properties are broadly similar — the meaningful engineering difference emerges only at elevated temperature under long-term creep loading, as shown in Section 4 below.
Section 4 — Long-Term Performance
Creep Rupture Strength (100,000 h): The Number That Determines Wall Thickness
For components operating at elevated temperature under sustained load — boiler headers, main steam pipes, turbine casings — structural design is governed not by room-temperature yield strength but by creep rupture strength: the stress that causes fracture after 100,000 hours (~11.4 years) of continuous loading at a given temperature. The values below are mean creep rupture strengths consistent with ECCC (European Creep Collaborative Committee) data assessments for both grades. This is where P91 and P92 diverge most significantly.
Section 5 — Heat Treatment
Heat Treatment Windows: P91 vs P92 Requirements
Correct heat treatment is absolutely critical for both grades. An incorrectly heat-treated P91 or P92 forging — even one with a passing chemical analysis — can display creep rupture strength as low as 30% of design expectation, with no visible defect on conventional non-destructive testing. Heat treatment traceability and documented furnace records are therefore a key qualification criterion when selecting a forging supplier for either grade.
| Heat Treatment Step | P91 / 1.4903 | P92 / X10CrWMoVNb9-2 | Engineering Note |
|---|---|---|---|
| Austenitising / Normalising | 1040–1080 °C | 1050–1080 °C | Hold ≥ 30 min per 25 mm section |
| Post-austenitise cooling | Accelerated air or oil quench | Accelerated air or oil quench | Must pass Ms (~380 °C) before tempering begins |
| Tempering temperature | 750–780 °C | 730–800 °C | P92 window wider but requires tighter furnace control (±10 °C recommended) |
| Minimum tempering hold | ≥ 2 h | ≥ 2 h | Heavy sections (>300 mm) need proportionally longer holds |
| AC₁ upper limit | ~820 °C | ~830 °C | Never temper above AC₁ — re-austenitising required if exceeded |
| PWHT after welding | 730–780 °C mandatory | 730–780 °C mandatory | PWHT must never be skipped for structural welds in either grade |
At Jiangsu Liangyi, all 1.4903 / X10CrMoVNb9-1 P91 steel forgings and P92 forgings pass through ten computer-controlled heat treatment furnaces with full time-temperature data logging. Every heat treatment cycle is documented with traceable records that form part of the EN 10204 Type 3.1 material test certificate supplied as standard. EN 10204 Type 3.2 certificates — independently countersigned by TÜV, DNV, BV, ABS, LRS, or other nominated third-party inspectors — are available on request for pressure equipment and other regulated applications. Our forgings are suitable for use in systems required to comply with PED 2014/68/EU; customers are responsible for confirming applicable conformity assessment routes with their own notified body.
Section 6 — Fabrication
Weldability Comparison: Where the Real Practical Difference Lies
Both grades are weldable but demand strict procedural discipline. Neither grade tolerates skipped pre-heat, rushed inter-pass temperature control, or omitted post-weld heat treatment. P92 introduces one additional challenge: its tungsten content means that matching filler metal is significantly less available globally than the well-established P91-matching consumables — a material supply-chain factor with real consequences for field repair and maintenance welding over a 40-year plant life.
Section 7 — Application Fit
Which Components and Industries Use P91 vs P92?
| Application / Component | P91 Fit | P92 Fit | Recommendation |
|---|---|---|---|
Main Steam Piping (>600 °C · USC Boilers) Ultra-supercritical plant at 600–625 °C steam conditions |
⚠ Marginal | ✔ Preferred | P92 — wall-thickness efficiency and thermal margin are decisive |
Steam Headers and Collectors (≤600 °C) Large-bore thick-wall headers in sub- and supercritical plant |
✔ Standard | ✔ Suitable | P91 preferred — lower cost, extensive code data, proven QC history |
Turbine Valve Bodies and Bonnets HP/IP stop valves, control valves, NRVs in steam turbine systems |
✔ Standard | ✔ For T>600°C | P91 for ≤600 °C designs; P92 for advanced USC valve applications |
Pressure Vessel Forgings Reactor heads, nozzles, flanges in high-pressure process equipment |
✔ Common | ⚠ Less common | P91 dominant; P92 specified only for extreme-duty USC vessels |
HP Turbine Casings and Seamless Rings Ring rolling for turbine cylinder and diaphragm flanges |
✔ Widely used | ✔ Increasingly used | P92 ring rolling gaining adoption in 620°C+ turbine designs |
Oil & Gas Process Piping (≤570 °C) Refinery hot piping, hydrogen reformer circuits, hydrotreater headers |
✔ Specified | ✗ Over-specified | P91 meets all O&G temperature requirements; P92 is cost-unjustified here |
Petrochemical Reactor Nozzles High-pressure reactor inlet/outlet nozzles, manways, vessel flanges |
✔ Standard | ✗ Rarely needed | P91 with ESR remelting recommended for critical nozzle forgings |
Section 8 — Selection Framework
How to Choose Between P91 and P92: A Structured Decision Framework
Grade selection between P91 and P92 is not purely a technical question — it also involves procurement lead times, fabrication capability, site welding infrastructure, design code compatibility, and long-term maintenance planning across a 30–40 year plant life. The checklist below consolidates the key decision criteria into a practical two-column guide.
- Design steam temperature stays at or below 600 °C continuously
- Your EPC/fabricator has P91 welding procedures but not yet P92
- Lowest total project material cost is the primary driver
- The component is a thick-wall vessel where wall reduction has limited value
- The plant is in a region with limited P92 consumable and inspector infrastructure
- Your governing code edition provides stronger P91 allowable stress data
- The application is oil & gas or petrochemical (rarely exceeds P91's thermal limits)
- Insurers or owner engineers require extensive field-failure case history
- Steam temperature exceeds 600 °C continuously at any system point
- Wall-thickness reduction will meaningfully lower weight or weld volume
- The project targets thermal efficiency gains via elevated steam conditions
- Your EPC has P92-qualified WPS/PQR procedures and consumable supply secured
- The project is a new-build ultra-supercritical plant (>600 °C / >280 bar)
- The client specification explicitly calls for P92 / Grade 92 / NF616
- Long-term creep margin must be maximised for a 40+ year design life
- A-USC (700 °C programme) design requires the highest available allowable stress in 9Cr class
Section 9 — FAQ
Frequently Asked Questions: P91 vs P92 Steel
P91 (EN 1.4903 / X10CrMoVNb9-1 / Grade 91) and P92 (X10CrWMoVNb9-2 / Grade 92 / NF616) are both 9% chromium tempered martensitic creep-resistant steels. The key difference is that P92 replaces approximately half the molybdenum (1.0% Mo → 0.45% Mo + 1.75% W) and adds a small boron addition (0.001–0.006%). This gives P92 approximately 11–32% higher creep rupture strength above 550°C (22% higher at 600°C over 100,000 hours) and extends the maximum continuous service temperature from 620°C (P91) to 650°C (P92). P91 is more widely available, 20–35% less expensive per kilogram, and has a larger global welding infrastructure built over 40+ years of industrial deployment.
Yes. P91-to-P92 dissimilar welds are made in practice and are specified in plants that upgrade specific high-temperature sections. A P91-matching ER90S-B9 filler is generally preferred over P92 filler for such joints, with pre-heat (200–300°C), inter-pass temperature control (max 300°C), and PWHT (730–780°C) requirements identical to same-grade welding. Hardness verification across both heat-affected zones is mandatory. The joint is designed with the lower-grade side's allowable stress governing at the transition point.
The maximum recommended continuous service temperature for P91 steel (EN 1.4903 / X10CrMoVNb9-1 / Grade 91) is 620°C, based on EN 10302 creep design data and ECCC (European Creep Collaborative Committee) creep rupture strength assessments. For sustained operation above 600°C, P92 (X10CrWMoVNb9-2 / Grade 92) is the preferred grade, with a maximum service temperature of 650°C. Both temperature limits are based on 100,000-hour creep rupture life criteria as codified in EN 10302 and applicable pressure equipment design standards.
Broadly yes in chemistry and minimum mechanical properties, but they are not automatically interchangeable in regulated applications. EN 10302/EN 10222-2 and ASTM A182 specify different test requirements, certification routes, and heat treatment documentation. For PED 2014/68/EU regulated applications in Europe, EN-certified material with EN 10204 Type 3.1 or 3.2 certificates is required. For ASME Section I or B31.1 code applications, ASME SA182 F91 with a Manufacturer's Test Report (MTR) is required. Jiangsu Liangyi can supply forgings compliant with EN, ASME, or both standards simultaneously on request.
P92 carries a 20–35% material price premium over P91 for three reasons: (1) Tungsten alloy cost — tungsten is significantly more expensive per kilogram than molybdenum; (2) Tighter chemistry control — the 1.50–2.00% W and 0.001–0.006% B ranges require more precise steelmaking, often including vacuum degassing and ESR; (3) Smaller global production volumes reducing economy-of-scale benefits. Whether the premium is justified depends on the application. For oil & gas and sub-600°C power applications, the premium is rarely recoverable. For large ultra-supercritical main steam systems at 620°C+, the reduced wall thickness and improved creep margin typically generate project-level savings that substantially exceed the material price difference.
Section 10 — About the Manufacturer
About the Author: Jiangsu Liangyi Co., Limited Technical Team
Jiangsu Liangyi Co., Limited is an ISO 9001:2015 certified open die forging manufacturer based in Jiangyin, Jiangsu Province, China, with over 25 years of experience supplying 9Cr martensitic heat-resistant steel forgings to power generation, oil & gas, and petrochemical customers across 50+ countries. This comparison guide was prepared by our technical engineering team to support engineers and procurement professionals in making informed material selection decisions between P91 and P92. For complete product specifications, dimensional capability, available certifications, and quotation, please visit our dedicated product page.
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