P22 vs P91 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
P22 and P91 are two widely used tempered alloy steels for pressure-containing components in power generation, petrochemical, and heavy process industries. Engineers, procurement managers, and manufacturing planners commonly face a trade-off when choosing between them: lower material and fabrication cost with acceptable high-temperature strength versus higher-temperature capability and long-term creep resistance requiring tighter fabrication controls.
The principal distinguishing factor is their designed performance in elevated-temperature service: one grade is optimized for moderate high-temperature strength with simpler processing, while the other is engineered for substantially higher creep resistance at steam and process temperatures through increased alloying and microalloy stabilization. Because both appear in similar ASME/ASTM product specifications (pipes, fittings, forged components), comparison is frequent when upgrading systems, specifying replacements, or designing new pressure equipment.
1. Standards and Designations
- ASTM / ASME:
- P22 — ASTM A335 / ASME SA335 P22 (often specified as 2.25Cr–1Mo)
- P91 — ASTM A335 / ASME SA335 P91 (9Cr–1Mo–V–Nb, also known as Grade 91)
- EN / European: equivalents are typically given as steels in the 13Cr and martensitic 9–12%Cr families; direct numeric equivalents are not one-to-one.
- JIS / GB: national standards may list close functional equivalents, but specification of chemical limits and heat treatment must be checked.
- Classification: both are alloy steels (not stainless or tool steels). They are high-strength, heat-resistant alloy steels designed for elevated-temperature service. P22 is a low-alloy Cr–Mo grade; P91 is a high-chromium, creep-strength-enhanced martensitic alloy with microalloying additions.
2. Chemical Composition and Alloying Strategy
The following table shows typical composition ranges (wt%) used by industry and standards bodies for P22 and P91. Values are representative; consult the applicable material specification or mill test certificate for procurement.
| Element | P22 (typical range, wt%) | P91 (typical range, wt%) |
|---|---|---|
| C | 0.05 – 0.15 | 0.08 – 0.12 |
| Mn | 0.30 – 0.60 | 0.30 – 0.60 |
| Si | 0.10 – 0.50 | 0.20 – 0.50 |
| P | ≤ 0.025 | ≤ 0.020 |
| S | ≤ 0.015 | ≤ 0.010 |
| Cr | 2.0 – 2.6 | 8.0 – 9.5 |
| Ni | ≤ 0.40 | ≤ 0.40 |
| Mo | 0.85 – 1.05 | 0.85 – 1.05 |
| V | trace – low | 0.10 – 0.25 |
| Nb (Nb+Ta) | trace – low | 0.06 – 0.12 |
| Ti | — | ≤ 0.02 (if specified) |
| B | — | ≤ 0.001 (microalloying) |
| N | ≤ 0.015 | 0.03 – 0.07 |
How alloying affects properties: - Chromium and molybdenum increase hot-strength and hardenability; increasing Cr from ~2.3% (P22) to ~9% (P91) is the major chemical step that raises elevated-temperature strength and oxidation resistance. - Vanadium and niobium in P91 form stable carbides/nitrides that pin grain boundaries and temper embrittlement, improving creep strength and stability at high temperature. - Carbon and nitrogen levels are controlled to balance strength and weldability; higher carbon raises strength but increases hardenability and cold cracking risk. - Microalloying elements (V, Nb, B) in P91 are specifically added to improve creep resistance via precipitation strengthening and grain refinement during tempering.
3. Microstructure and Heat Treatment Response
Typical microstructures: - P22: In normalized and tempered condition, P22 generally shows a tempered bainitic/ferritic structure with dispersed carbides (Cr–Mo carbides). It is less prone to forming a highly hardened martensite microstructure under conventional processing, so it is more forgiving in welding and heat treatment. - P91: P91 is a martensitic steel after quenching; the delivered condition is normally normalized and tempered to develop a tempered martensitic microstructure with fine, dispersed M23C6 and MX-type (V/Nb carbides/nitrides) precipitates that provide creep resistance and stability at elevated temperatures.
Effect of heat treatment: - Normalizing (air cooling from a specified austenitizing temperature) and tempering are essential for both grades but more critical for P91 to obtain the intended tempered martensitic microstructure and to precipitate strengthening carbides/nitrides. - Quenching & tempering (for forged components) must be carefully controlled for P91 to avoid excessive hardness that impairs weldability and to ensure post-weld heat treatment (PWHT) parameters are met. - Thermo-mechanical treatments and stability aging: P91 benefits from controlled processing and tempering to stabilize the creep-resistant precipitates; over-tempering or incorrect PWHT can reduce strength or cause temper embrittlement.
4. Mechanical Properties
Representative mechanical property ranges (room temperature, normalized & tempered; specific values depend on product form and exact heat treatment):
| Property | P22 (typical) | P91 (typical) |
|---|---|---|
| Tensile strength (MPa) | ~415 – 585 | ~620 – 850 |
| Yield strength (0.2% offset, MPa) | ~250 – 350 | ~450 – 650 |
| Elongation (%) | ~20 – 25 | ~15 – 25 |
| Impact toughness (Charpy V-notch, J) | moderate; good at ambient | generally good; engineered for high-temp toughness |
| Hardness (HRC / HB) | ~170–220 HB (varies) | ~200–300 HB (varies with condition) |
Interpretation: - P91 is materially stronger in both yield and tensile strength when properly normalized and tempered; this is primarily due to higher Cr and microalloying precipitates. - Ductility (elongation) is similar or slightly reduced in P91 versus P22 in some conditions because of higher strength and tempered martensite. - Toughness can be excellent in both grades if appropriate heat treatment and post-weld heat treatment are performed; P91 must be processed correctly to avoid brittle martensite zones.
5. Weldability
Weldability considerations derive from carbon equivalence, hardenability, and microalloying. Common empirical indices are used for qualitative assessment.
Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
Alternative index 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: - P22 has a lower carbon equivalent than P91 owing to lower Cr and microalloying content, making it comparatively easier to weld with less stringent preheat and post-weld heat treatment (PWHT) requirements. - P91 has higher hardenability because of greater Cr and microalloying (V, Nb, B). That increases susceptibility to forming hard martensitic and/or untempered regions in weld heat-affected zones (HAZ), elevating cold-cracking risk if not preheated and PWHTed correctly. - Welding P91 typically requires controlled bevels, matched filler metallurgy (e.g., P91 filler or stabilized variants), precise interpass temperatures, and a PWHT to temper the martensite and restore ductility and creep strength. Procedures are more complex and often subject to strict procedure qualification. - For both grades, hydrogen control, low-hydrogen procedures, and proper PWHT are essential for reliable long-term service.
6. Corrosion and Surface Protection
- Both P22 and P91 are non-stainless alloy steels; general corrosion resistance in ambient conditions is moderate but not comparable to stainless alloys. Surface protection is commonly used:
- Protective coatings: painting, epoxy, high-temperature paints.
- Metallurgical coatings: thermal spray (Al/Al-silicate), cladding for corrosion-intensive service.
- Hot-dip galvanizing is possible for some P22 applications in ambient conditions but is not typical for high-temperature service; galvanizing is not suitable for sustained high temperature.
- PREN (Pitting Resistance Equivalent Number) is used for stainless alloys: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- PREN is not applicable to P22 and P91 because they are not stainless steels designed to form protective chromium-rich passive films. Their corrosion management focuses on coatings, cladding, and material selection for the specific environment (oxidation, sulfidation, pitting).
7. Fabrication, Machinability, and Formability
- Machinability:
- P22 is easier to machine than P91 in comparable conditions due to lower hardness and lower alloy content.
- P91, being higher-strength and more heavily alloyed, is tougher on cutting tools and may require slower cutting speeds and more robust tooling.
- Formability and cold working:
- Both grades are not intended for extensive cold forging or forming after final heat treatment; forming is generally performed in softer (as-rolled or normalized) conditions before final heat treatment.
- Finishing:
- Grinding and surface preparation for P91 demand attention to avoid introducing surface defects that can serve as creep or fatigue initiation sites.
- Fabrication planning:
- P91 requires qualified welding procedures and experienced welders. Distortion control and residual stress management are important because PWHT cycles are necessary.
8. Typical Applications
| P22 (common uses) | P91 (common uses) |
|---|---|
| Fossil power plant piping and headers operating at moderate steam temperatures (up to ~540–565°C) | Ultra-supercritical and advanced fossil power plant steam piping, headers, and components with higher steam temperatures (often >550°C) |
| Pressure vessels and boilers where cost and simpler fabrication are priorities | High-temperature steam turbines, reheaters, superheaters, and high-pressure piping requiring long-term creep resistance |
| Petrochemical heater tubes, process piping at moderate temperatures | Components in power plants and chemical plants requiring high creep strength and microstructural stability |
| Economical replacement materials for older low-alloy steels | New designs targeting extended life at elevated temperature; critical weldments requiring PWHT |
Selection rationale: - Choose P22 when service temperatures and expected creep requirements are moderate and when procurement/fabrication simplicity and cost are important. - Choose P91 when long-term exposure at higher temperatures and pressures demands superior creep resistance, finer precipitate stabilization, and higher allowable stresses.
9. Cost and Availability
- Relative cost: P91 is typically more expensive per kilogram than P22 due to higher alloy content (notably chromium and microalloying additions) and tighter process controls. Fabrication and weld procedure qualification costs increase total installed cost for P91.
- Availability:
- P22 is widely available in pipes, fittings, fittings, and forgings from many mills and distributors.
- P91 is broadly available but certain product forms (large forgings, specialized diameter/thickness combinations) may have longer lead times and are more likely to be sourced from specialized suppliers. Availability can vary by region and market demand.
10. Summary and Recommendation
Summary table — qualitative comparison:
| Attribute | P22 | P91 |
|---|---|---|
| Weldability | Easier; lower CE, less stringent PWHT | More difficult; higher CE/hardenability; requires controlled PWHT and qualified procedures |
| Strength–Toughness (elevated temp) | Moderate high-temp strength; adequate up to ~540–565°C | High elevated-temperature strength and creep resistance; suitable for higher temp/longer life |
| Cost (material + fabrication) | Lower | Higher |
Recommendation: - Choose P22 if you need a lower-cost, easier-to-weld alloy steel for pressure equipment and piping operating at moderate elevated temperatures, when long-term creep at high temperature is not the governing design criterion. - Choose P91 if the application demands significant creep resistance and higher allowable stresses at elevated temperatures (long-life, high-pressure steam systems), and if the project can support the necessary fabrication controls, qualified welding procedures, and cost premium.
Final note: Both grades require specification of exact material grade, heat treatment condition, and welding procedure in procurement and design documents. For safety-critical and long-term high-temperature service, involve materials and welding engineers early to confirm design stress allowable, required PWHT parameters, and qualification testing to ensure reliable in-service performance.