P91 vs P92 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
Engineers and procurement professionals selecting steels for high-temperature power and process applications must balance strength, creep resistance, weldability, and life-cycle cost. P91 and P92 are two widely used 9% chromium heat-resistant steels developed for steam-generating and high-pressure piping systems; the decision between them is commonly a trade-off between long-term high-temperature performance and fabrication/inspection cost.
The principal metallurgical distinction is that P92 evolves the classic 9Cr–1Mo family by shifting part of the strengthening strategy toward heavier refractory strengthening (tungsten and optimized microalloying), which improves creep resistance at elevated temperatures. Because of that directional alloy change, P92 generally offers higher long-term strength and creep rupture performance at the expense of slightly more demanding welding and fabrication practice compared with P91.
1. Standards and Designations
- Common standards and specifications where P91 and P92 appear:
- ASME/ASTM: typically as P91 and P92 in SA-335 (seamless ferritic alloy-steel pipe) and related boiler/piping codes.
- EN: these steels are available under European designations in EN equivalents and detailed product standards for tubes and fittings.
- GB (China): widely produced under corresponding GB/T grades for heat-resistant steels.
- JIS: Japanese standards sometimes reference equivalent 9Cr steels for high-temperature service.
- Classification: both P91 and P92 are low-alloy, heat-resistant ferritic–martensitic steels (neither stainless nor tool steels). They are best categorized as high-strength, creep-resistant alloy steels (HSLA type for elevated-temperature service).
2. Chemical Composition and Alloying Strategy
The following table shows typical composition ranges (approximate, rounded to reflect common specifications and industrial practice). Exact allowable ranges are defined in the applicable material standard or vendor datasheet.
| Element | P91 (typical ranges, wt%) | P92 (typical ranges, wt%) |
|---|---|---|
| C | 0.08–0.12 | 0.08–0.12 |
| Mn | 0.3–0.6 | 0.2–0.6 |
| Si | 0.2–0.5 | 0.2–0.6 |
| P | ≤0.02 | ≤0.02 |
| S | ≤0.01 | ≤0.01 |
| Cr | 8.0–9.5 | 8.5–9.5 |
| Ni | ≤0.40 | ≤0.40 |
| Mo | ~0.85–1.05 | ~0.4–0.7 |
| W | trace to 0.5 | ~1.7–2.5 |
| V | 0.18–0.25 | 0.18–0.25 |
| Nb (Cb) | 0.06–0.12 | 0.06–0.12 |
| Ti | ≤0.02 | ≤0.02 |
| B | very low ppm levels | very low ppm levels |
| N | 0.02–0.07 | 0.03–0.07 |
Notes: - P92 reduces overall Mo content and introduces purposeful tungsten (W) to increase solid-solution and precipitation strengthening at elevated temperature. Microalloying with V and Nb is retained and optimized in both grades to stabilize fine carbide/nitride precipitates that control creep. - Boron at parts-per-million levels is often used to improve hardenability; nitrogen is controlled to stabilize carbides/nitrides and to influence tempering behavior. - These ranges are indicative; always confirm with mill certificates and relevant code requirements.
How alloying affects properties: - Chromium provides oxidation resistance and matrix stability at elevated temperature. - Mo and W are key for solid-solution strengthening and for forming stable carbides/complex precipitates that retard creep; replacing some Mo with W shifts the temperature-strength equilibrium in favor of better long-term creep performance. - V and Nb form fine MX (carbonitride) precipitates that pin dislocations and grain boundaries, which improves creep strength and controls temper embrittlement when properly balanced. - Carbon controls hardness and strength but increases hardenability and susceptibility to martensite formation—hence tight control and appropriate heat treatment are required.
3. Microstructure and Heat Treatment Response
Typical microstructures: - Both grades are produced to a tempered martensitic/ferritic microstructure after normalized and tempered heat treatment. The as-normalized structure is martensite lath packets with prior austenite grain boundaries; tempering produces tempered martensite with various carbides and carbonitrides (M23C6, MX-type precipitates). - P92 microstructure tends to exhibit a higher stability of precipitates at service temperature due to W-containing carbides and a refined distribution of Nb/V carbonitrides (designed to resist coarsening).
Heat treatment routes: - Normalizing: heating to an austenitizing temperature to dissolve alloy carbides, followed by air cooling to form martensite; typical temperatures are set by code/spec and must be adhered to for dimensional control and metallurgical properties. - Quenching is not typical—these steels are normalized and then tempered rather than hardened by quenching in the sense used for tool steels. - Tempering: performed to reduce brittleness, relieve stresses, and precipitate strengthening carbides. Tempering temperature and time significantly affect creep strength, toughness, and hardness. - Thermo-mechanical processing: some product forms (plates, forgings) benefit from controlled rolling and accelerated cooling to refine prior austenite grain size and to distribute precipitates more uniformly.
Effect differences: - P92’s tungsten-containing precipitates and slightly different Nb/V balance reduce precipitate coarsening at service temperatures, leading to superior long-term creep strength compared with P91. Tempering windows and PWHT cycles must be selected and controlled to avoid over-tempering or under-tempering in either grade.
4. Mechanical Properties
Table — qualitative and typical ranges (after proper normalization and tempering; specific values depend on exact heat treatment and code requirements):
| Property | P91 (typical) | P92 (typical) |
|---|---|---|
| Tensile strength (Rm) | ~600–750 MPa (room temp, typical) | ~650–800 MPa (room temp, typical) |
| Yield strength (Rp0.2) | ~415–520 MPa | ~480–560 MPa |
| Elongation (A%) | ~18–25% | ~15–25% (similar ductility) |
| Impact toughness (Charpy V-notch) | Moderate to good (depends on tempering) | Good, comparable but sensitive to heat treatment |
| Hardness (HRC/HBW) | Typically ~180–250 HB | Typically ~190–260 HB |
Interpretation: - P92 is generally designed to provide higher tensile and creep strength, especially at elevated temperatures and long exposure times. - Ductility and impact toughness can be similar at room temperature when proper heat treatment is applied, but both grades require careful tempering to maintain required toughness, particularly across welds. - Hardness is comparable; differences are governed by tempering temperature and final microstructure.
5. Weldability
Weldability considerations: - Both P91 and P92 are weldable but require controlled preheat, inter-pass temperature limits, and mandatory post-weld heat treatment (PWHT) to temper the martensitic weld and heat-affected zone (HAZ). - Higher alloy content and hardenability make both susceptible to HAZ hardening and cold cracking if welding procedures are not controlled.
Useful formulas (qualitative interpretation only):
- Carbon equivalent (IIW):
$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
Higher $CE_{IIW}$ indicates greater hardenability and a larger risk of HAZ martensite and cracking; both P91 and P92 produce relatively elevated values compared with low-alloy steels.
- Pcm (weldability parameter):
$$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
$P_{cm}$ helps assess cold cracking susceptibility; microalloying elements and boron in P91/P92 can raise the index.
Practical implications: - P92’s increased tungsten (and adjusted Mo) slightly raises hardenability relative to P91, so welding controls tend to be more critical (higher preheat, careful interpass and PWHT profiles, use of matching filler metals). - Use of matched or overmatching filler metals, strict hydrogen control, and qualified welding procedures are required. Post-weld heat treatment is essential to achieve required mechanical properties and to reduce residual stresses and martensitic hardness. - Weld repair and multi-pass welding require particular attention to PWHT temperature/time cycles specified in the code or manufacturer’s welding procedure.
6. Corrosion and Surface Protection
- Neither P91 nor P92 are stainless steels; they rely on Cr content (~9%) for improved oxidation resistance at elevated temperatures rather than corrosion resistance in wet or chloride environments.
- For general atmospheric, aqueous, or chemically aggressive exposures, standard surface protection practices apply: coatings, paints, thermal spray, or galvanizing may be used where appropriate (but galvanizing onto high-temperature service components is not typical).
- PREN (pitting resistance equivalent number) is not relevant for these non-stainless heat-resistant ferritic steels; for reference, PREN is calculated as:
$$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ but this index applies to stainless alloy selection and does not meaningfully characterize P91/P92.
Practical guidance: - For long-term steam-side oxidation and fireside corrosion in boilers and superheaters, material selection (P91 vs P92) should be driven by operating temperature and expected deposit/oxidation behavior, with coatings and water chemistry controls applied as needed.
7. Fabrication, Machinability, and Formability
- Machinability: both grades are more difficult to machine than low-alloy steels due to higher strength and hardenability; P92 can be slightly more challenging because of tungsten content and associated carbide stability. Use sharp tooling, rigid setups, and adjusted cutting parameters.
- Formability/bending: cold forming is limited; forming is usually performed on normalized or annealed product forms where possible. Bending radii and methods should follow vendor guidance and allow for subsequent heat treatment.
- Grinding, drilling, and finishing operations require attention to heat generation to avoid tempering or work hardening of the surface.
- Weld fabrication requires qualified procedures and personnel familiar with PWHT requirements.
8. Typical Applications
| P91 — Typical Uses | P92 — Typical Uses |
|---|---|
| Main steam piping, headers, superheater tubes, reheater sections in conventional and subcritical plants (up to ~600–620°C service ranges depending on design life) | High-pressure, ultra-supercritical boiler and turbine piping, advanced superheater/reheater tubes, components where higher creep strength is needed for longer service or higher temperatures (typically higher end of 9Cr family) |
| Boiler tubes and fittings in fossil power plants | Thick-section components or those requiring improved long-term rupture strength and reduced creep rates |
| Heavy-wall pressure vessels where cost-constrained, well-understood material behavior is acceptable | New-build plants or retrofits where extended life at higher metal temperatures justifies higher material and fabrication cost |
Selection rationale: - Choose based on required design temperature, required creep-rupture life, thickness (W increases strength in thicker sections), and the acceptable welding/fabrication strategy.
9. Cost and Availability
- P92 is typically more expensive than P91 because of the additional tungsten and manufacturing controls; it may also have longer lead times and more limited availability in certain product forms or sizes.
- P91 is widely available worldwide in tubes, fittings, plates, and forgings and often represents the baseline for code-approved components.
- Availability varies with market cycles, mill capabilities, and geographic region; procurement must confirm lead times for seamless versus welded tube, forgings, and fittings.
10. Summary and Recommendation
Summary table (qualitative):
| Attribute | P91 | P92 |
|---|---|---|
| Weldability (procedure complexity) | Good — standard PWHT required | More demanding — higher hardenability, stricter control |
| Strength–Toughness (room temp) | Strong, good toughness | Higher strength, comparable toughness if properly treated |
| Creep resistance (long-term, high T) | Good up to typical service limits | Better long-term creep resistance at higher temperatures |
| Cost & Availability | Lower cost, broader availability | Higher cost, more restricted supply in some forms |
Recommendations: - Choose P91 if: - The application falls within conventional 9Cr–1Mo service temperatures and design life expectations, - Fabrication speed, cost, and easier availability are priorities, - Proven code experience and existing welding procedures are preferred.
- Choose P92 if:
- Design requires improved long-term creep strength, operation at the higher end of 9Cr temperature capabilities, or longer guaranteed life under steam/pressure,
- The project can accommodate more stringent welding controls, higher material cost, and potentially longer lead times,
- Improved performance in thick sections or aggressive high-temperature creep regimes is a decisive factor.
Final note: Material selection should always be supported by project-specific creep-rupture data, weld procedure qualification records, proper heat-treatment schedules, and consultation with material suppliers and fabricators. Confirm exact composition and guaranteed properties from the mill test certificate and follow code (ASME/EN/GB/JIS) prescriptions for design, welding, and inspection.