P91 vs P92 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
P91 and P92 are creep-resistant, chromium–molybdenum–vanadium (Cr–Mo–V) martensitic steels widely used in high-temperature power-generation and petrochemical equipment such as boiler tubes, headers, and steam piping. Engineers, procurement managers, and fabricators commonly face the selection dilemma between P91 and P92 when balancing high-temperature strength and long-term creep resistance against weldability, fabrication ease, and overall lifecycle cost. Typical decision contexts include upgrading steam temperature capability, optimizing maintenance intervals for high-pressure components, or selecting materials for new high-efficiency boilers.
The key metallurgical distinction that drives performance differences is the alloying strategy—particularly how tungsten (W) and molybdenum (Mo) are balanced alongside other microalloying elements (V, Nb, B). This substitution strategy influences carbide chemistry, precipitate stability, and hardenability, which in turn affect creep strength, toughness, and weldability. That is why P91 and P92 are often compared for elevated-temperature components.
1. Standards and Designations
- Common standards and specifications:
- ASME/ASTM: ASME SA-335 / P91 and P92 (seamless ferritic alloy piping for elevated temperatures), ASTM A213, ASTM A387 (plate variants), and related ASME boiler and pressure vessel codes.
- EN: Equivalent grades often appear under EN and EN-modified designations (e.g., X10CrMoVNb9-1 for P91-like and X10CrWMoVNb9-2 for P92-like chemistry).
- JIS/GB: Local standards provide similar compositions under different numbering; confirm with supplier certification.
- Classification:
- Both P91 and P92 are alloy steels designed for high-temperature service; they are sometimes grouped with HSLA/martensitic creep-strength-enhanced ferritic steels (not stainless steels or tool steels).
2. Chemical Composition and Alloying Strategy
The following table shows typical compositional ranges for P91 and P92 in weight percent. These are representative of commercial normalized-and-tempered material specifications; actual values depend on specific subgrades and standards.
| Element | P91 (typical range, wt%) | P92 (typical range, wt%) |
|---|---|---|
| C | 0.08–0.12 | 0.08–0.12 |
| Mn | 0.30–0.60 | 0.30–0.60 |
| Si | 0.20–0.60 | 0.20–0.60 |
| P | ≤0.02 | ≤0.02 |
| S | ≤0.01 | ≤0.01 |
| Cr | 8.0–9.5 | 8.5–9.5 |
| Ni | ≤0.50 | ≤0.50 |
| Mo | 0.85–1.05 | 0.20–0.50 |
| W | trace–0.3 | 1.7–2.0 |
| V | 0.15–0.25 | 0.18–0.25 |
| Nb (Cb) | 0.06–0.12 | 0.06–0.12 |
| Ti | ≤0.01 | ≤0.01 |
| B | 0.0005–0.003 | 0.0005–0.005 |
| N | 0.03–0.07 | 0.03–0.07 |
How alloying affects properties - Chromium provides oxidation and corrosion resistance at elevated temperature and forms M23C6 carbides that affect creep and tempering behavior. - Molybdenum (Mo) increases solid-solution strengthening and contributes to stable carbide formation; Mo is traditionally central to P91’s creep strength. - Tungsten (W) in P92 is used as a partial substitute for Mo: W forms more stable, slowly coarsening carbides and contributes to higher long-term creep strength at very high temperatures. - Vanadium (V) and niobium (Nb) form fine MX carbonitrides that pin grain boundaries, inhibit recovery and recrystallization, and improve creep rupture strength. - Very low boron additions improve creep strength by segregating to prior austenite grain boundaries and delaying cavitation during long-term exposure.
3. Microstructure and Heat Treatment Response
Typical microstructure - Both P91 and P92 produce a tempered martensitic microstructure after a standard heat-treatment cycle (normalizing above Ac3 followed by quenching and tempering). - The tempered microstructure consists of lath martensite with distributed carbides and carbonitrides: M23C6 (Cr-rich) along prior austenite grain boundaries and lath boundaries, and MX (V,Nb) precipitates within laths.
Effects of alloying and heat treatment - P91: With higher Mo and slightly lower W, the carbide distribution is favorable for required creep strength in the original design window (typically up to about 600–620 °C). Mo contributes to matrix strengthening and precipitation stability, but Mo-rich carbides can coarsen over long exposures. - P92: The partial substitution of Mo by W yields carbides and intermetallics with slower coarsening kinetics at elevated temperature. P92 often develops a finer and more stable dispersion of carbides after proper tempering, which gives better long-term creep resistance at higher steam temperatures and longer life expectations. - Thermo-mechanical treatments: Both grades respond to normalizing + tempering and to specific thermomechanical processing that refines prior austenite grain size and promotes a desirable precipitate distribution. Tempering temperature controls toughness vs. strength trade-offs.
4. Mechanical Properties
The following table gives representative properties for normalized-and-tempered material in the as-supplied condition. Actual values depend on precise chemistry, heat treatment, thickness/form, and test standards.
| Property | P91 (representative) | P92 (representative) |
|---|---|---|
| Yield strength (0.2%, MPa) | ~415 (typical lower bound) — up to ~500–600 depending on tempering | higher: ~500–650 (wide range depending on temper) |
| Tensile strength (MPa) | ~550–700 | ~650–800 |
| Elongation (%) | ~18–25 | ~12–20 (often slightly lower than P91) |
| Charpy V-notch impact (room temp, J) | moderate to good (depends on heat treatment; commonly ≥40–60 J) | somewhat lower or similar depending on tempering; more sensitivity to heat treatment |
| Hardness (HB) | ~180–260 (typical N&T condition) | ~200–300 (can be higher due to alloy and tempering) |
Interpretation - Strength: P92 generally offers higher creep-rupture strength and higher tensile/yield strength in many commercial heat treatments due to W addition and refined precipitate stability. - Toughness and ductility: P91 tends to be a bit more ductile and forgiving in fabrication; P92 can be less ductile and requires stricter control of heat treatment and PWHT to secure toughness. - Hardness: P92 often shows higher hardness in comparable conditions; that helps high-temperature strength but can increase crack susceptibility in welding if not managed.
5. Weldability
Weldability considerations for both grades center on carbon equivalent/hardenability, microalloying, and PWHT requirements.
Typical weldability formulas to assess preheat and PWHT needs: - IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - International Pcm: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation - Both P91 and P92 have appreciable hardenability (Cr, Mo/W, V, Nb, B), leading to a high propensity for forming hard martensite in the HAZ if welded without proper preheat and PWHT. - P92’s higher W and slightly different alloy balance increase hardenability further, so it often requires more conservative preheat/PWHT controls and matching filler materials formulated for P92 chemistry. - PWHT (typically 700–760 °C for these steels, specific to code and thickness) is mandatory to temper martensite in the HAZ and relax residual stresses. Hydrogen control, controlled interpass temperatures, and low-hydrogen welding consumables are necessary. - Weld filler selection: Use consumables specified for P91 or P92 as appropriate (matching or approved overmatching consumables). For P92 welding, specialized filler wires and procedures are more common to minimize microstructural mismatches and to prevent Type IV cracking in long-term creep zones.
6. Corrosion and Surface Protection
- Neither P91 nor P92 is a stainless steel; both rely on alloying for oxidation resistance at high temperature rather than general aqueous corrosion resistance.
- In aqueous or corrosive environments, surface protection is typically required: painting, metallizing, cladding, or appropriate coatings. For atmospheric protection, industrial coatings or hot-dip galvanizing (where applicable by service) are options, but galvanizing may not be suitable for high-temperature steam service.
- PREN (Pitting Resistance Equivalent Number) is defined as: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$ This index applies to stainless steels and localized corrosion resistance and is not meaningful for P91/P92 because their compositions and intended uses are for high-temperature mechanical performance rather than chloride pitting resistance.
- High-temperature oxidation: Chromium content (≈9%) gives some oxidation resistance for steam-side service, but long-term oxidation scales and carburization behavior must be considered; P92 is often preferred at higher steam temperatures because W-bearing carbides slow scale growth and maintain mechanical integrity longer.
7. Fabrication, Machinability, and Formability
- Machinability: Both steels are less machinable than plain carbon steels due to alloy content and hardness. P92’s higher hardenability and potential for higher hardness can reduce tool life and increase required cutting forces.
- Formability/bending: Cold forming is limited; hot forming/pressing and controlled tempering are common. Both grades respond poorly to severe cold forming without subsequent heat treatment.
- Surface finish and grinding: Harder P92 requires more aggressive tooling/grinding. Residual stress control and avoiding over-tempering are important during finishing.
- Heat treatment after fabrication: Correct normalizing and tempering cycles or post-weld heat treatment are required to achieve intended properties and to avoid embrittlement.
8. Typical Applications
| P91 Typical Uses | P92 Typical Uses |
|---|---|
| Steam headers, piping and tubes in subcritical and early supercritical boilers (≤ ~600 °C service) | High-temperature steam piping and headers in advanced/ultra-supercritical boilers (higher creep-demand environments) |
| Heat-exchanger tubing, pressure vessel components for moderate temperature steam | High-pressure, high-temperature components in power plants designed for higher steam temperatures and longer life (HRSGs, reheaters) |
| Petrochemical piping where good high-temperature strength is required but cost sensitivity exists | Service where extended creep life and superior long-term stability are prioritized despite higher material and processing cost |
| Valve bodies and fittings for elevated temperature service | Critical components requiring maximum creep-rupture performance for design life extension |
Selection rationale - Choose P91 when the application requires proven creep strength at moderate elevated temperatures with good availability and slightly easier fabrication. - Choose P92 when the design temperature and required creep life exceed P91 capability, or when operators seek longer maintenance intervals and higher allowable stress at service temperature.
9. Cost and Availability
- Cost: P92 is generally more expensive than P91 due to higher alloy content (notably W), specialized melting controls, and more limited demand/supply. Fabrication and welding procedures for P92 can also increase installed cost.
- Availability: P91 has a longer history of widespread use and is more commonly stocked in many product forms (pipe, plate, forging). P92 availability depends on regional market and mill production; lead times can be longer, especially for large sections or special forms.
10. Summary and Recommendation
| Attribute | P91 | P92 |
|---|---|---|
| Weldability (practical) | Good with standard P91 procedures; less demanding than P92 | More demanding — higher hardenability requires stricter preheat/PWHT and matching consumables |
| Strength–Toughness at RT | Balanced — good toughness and adequate strength | Higher strength and creep resistance, but often slightly reduced ductility/toughness if not optimized |
| Creep performance at elevated T | Excellent up to design window (~up to ~600–620 °C typical) | Superior long-term creep resistance at higher temperatures and longer lives |
| Cost & availability | More economical and widely available | Higher material and processing cost; availability can be more limited |
Conclusions — choose based on service conditions: - Choose P91 if: you need a well-established, cost-effective creep-resistant steel for elevated-temperature service within the conventional P91 design envelope, desire somewhat easier fabrication and welding, and prioritize availability and lower purchase/fabrication cost. - Choose P92 if: the application demands superior long-term creep strength at elevated steam temperatures (or extended service intervals), if design life or higher allowable stresses justify the higher material and processing cost, and if your fabrication capability can manage stricter welding controls and qualification.
Final practical note: For either grade, success in service depends less on nominal grade name alone and more on correct chemistry verification, approved welding procedures, strict control of heat-treatment/PWHT, and quality assurance (NDT, mechanical testing, and traceability). When moving from P91 to P92, expect adjustments in welding procedure specifications, filler material selection, and possibly procurement lead-times and cost.