P22 vs P91 – Composition, Heat Treatment, Properties, and Applications
Share
Table Of Content
Table Of Content
Introduction
P22 and P91 are two widely used pressure-vessel and piping steels in power generation, petrochemical, and heavy-industry equipment. Engineers and procurement professionals frequently weigh cost, fabricability, and long-term performance when choosing between them—for example, balancing initial material cost and weldability against required high-temperature strength and creep life.
The principal engineering distinction is that P91 is formulated and processed to deliver substantially greater high-temperature strength and creep resistance than P22, largely through higher chromium content and controlled microalloying plus heat treatment. These differences make them common alternatives for components exposed to elevated temperatures and stresses, which is why comparison is frequent in component specification and lifecycle cost analysis.
1. Standards and Designations
- Common standards:
- ASTM/ASME: ASTM A335 / ASME SA-335 (seamless ferritic alloy-steel pipe) — P22, P91
- EN: EN 10216 / EN 10222 equivalents (various EN steel grades map to these P grades)
- JIS / GB: national standards often provide approximate equivalents (consult specific conversions)
- Material classification:
- P22: low-alloy ferritic steel (1.25% Cr — commonly called 1.25Cr-0.5Mo). Classified as an alloy steel for high-temperature service.
- P91: high-chromium, martensitic, creep-strength-enhanced ferritic steel (nominally 9Cr-1Mo with V/Nb additions). Often treated as an alloy/HSLA (high-strength low-alloy) martensitic steel optimized for creep resistance.
2. Chemical Composition and Alloying Strategy
Table: Typical composition ranges (wt%). Values shown are representative ranges from common specifications; exact limits depend on the specific standard and heat.
| Element | P22 (typical range) | P91 (typical range) |
|---|---|---|
| C | 0.04–0.12 | 0.08–0.12 |
| Mn | 0.25–0.60 | 0.25–0.60 |
| Si | 0.10–0.50 | 0.20–0.60 |
| P | ≤0.025 | ≤0.02 |
| S | ≤0.015 | ≤0.01 |
| Cr | 0.9–1.5 | 8.0–9.5 |
| Ni | trace | trace–low |
| Mo | 0.38–0.65 | 0.85–1.05 |
| V | — | 0.08–0.25 |
| Nb (Cb) | — | 0.03–0.12 |
| Ti | trace | trace |
| B | — | trace (ppm level) |
| N | trace | 0.02–0.06 |
How alloying affects properties: - Chromium (Cr) increases oxidation resistance and hardenability; the much higher Cr in P91 is a major contributor to improved high-temperature strength and oxidation resistance. - Molybdenum (Mo) improves strength at elevated temperature and creep resistance in both grades; P91 typically has ~1% Mo vs ~0.5% in P22. - Vanadium (V) and niobium (Nb) in P91 form fine carbides/nitrides that stabilize the martensitic microstructure and inhibit creep deformation by pinning dislocations and grain boundaries. - Carbon provides strength via martensite/tempered martensite, but higher carbon also raises hardenability and risk of cracking; P91 uses a controlled C content to balance strength and weldability. - Small additions of B and controlled N in P91 can further refine properties by influencing precipitation and hardenability.
3. Microstructure and Heat Treatment Response
- P22: Typical microstructure after normalization and tempering is tempered bainite/tempered ferrite with dispersed Mo-rich carbides. It does not form a fully martensitic structure in the same way as P91 after typical heat treatments. The microstructure is stable for moderate elevated-temperature service but less resistant to long-term creep than P91.
- P91: Designed to form a fine martensitic structure after normalizing and rapid cooling, followed by a tempering step that precipitates fine carbides and nitrides (e.g., M23C6, MX-type precipitates). Thermomechanical processing and controlled tempering are essential to obtain the optimized tempered martensitic microstructure that delivers high creep strength.
- Processing effects:
- Normalizing: refines prior austenite grain size; P91 typically requires a higher-temperature normalization than P22 to dissolve alloy carbides and promote proper martensite formation.
- Quenching and tempering / Normalizing and tempering: both grades require tempering after hardening. P91 tempering is particularly critical to stabilize the martensitic structure and achieve toughness while mitigating residual stresses.
- Thermo-mechanical treatments and controlled cooling rates are more critical for P91 to avoid coarse precipitates and to control long-term creep performance.
4. Mechanical Properties
Table: Qualitative comparison (typical values after normalization & tempering; actual properties depend on exact heat treatment, thickness and specification).
| Property | P22 (typical) | P91 (typical) |
|---|---|---|
| Tensile strength (ultimate) | Moderate | High |
| Yield strength | Moderate | High |
| Elongation (ductility) | Good (higher than P91) | Moderate (lower than P22) |
| Impact toughness (RT) | Good when properly tempered | Good when properly tempered; can be sensitive to heat treatment |
| Hardness | Moderate | Higher (more wear-prone) |
Explanation: - P91 delivers substantially higher tensile and yield strengths and superior long-term creep resistance at elevated temperatures owing to its martensitic microstructure and microalloying (V, Nb) compared with the lower-alloy bainitic/tempered ferritic structure of P22. - P22 generally offers greater ductility and may present easier toughness control in some thicknesses; P91 can achieve good toughness but requires strict control of heat treatment and post-weld heat treatment (PWHT).
5. Weldability
Key factors: carbon-equivalent, hardenability, and microalloying content determine preheat/PWHT needs and HAZ cracking risk.
Common weldability indices (used to assess risk qualitatively): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Interpretation (qualitative): - P22: lower Cr and lower hardenability relative to P91—moderate carbon-equivalent values. Welding P22 typically requires preheat and PWHT to minimize HAZ hardness and susceptibility to hydrogen-assisted cracking, but standard PWHT cycles are well established and relatively forgiving. - P91: higher Cr, Mo, and microalloying elements increase hardenability and raise the risk of forming hard martensite in the HAZ; therefore, welding P91 is more demanding. Proper preheat, controlled interpass temperatures, and carefully prescribed PWHT cycles are essential to avoid HAZ embrittlement and to temper the martensitic HAZ. Use of matching or over-matching filler metals and strict procedure qualification is common. - Practical advice: P91 weld procedures require qualified WPS/PQR and experienced personnel; repair welding and post-weld tempering must follow manufacturer or code-approved cycles. P22 is more tolerant but still requires correct PWHT for pressure-service components.
6. Corrosion and Surface Protection
- Neither P22 nor P91 is stainless. Corrosion resistance in wet/acid environments must be managed by material selection, coatings, or inhibitors.
- Common protection strategies: painting, high-temperature aluminizing, thermal spraying, or specification of corrosion allowance. For outdoor or humid environments, standard coatings and cathodic protection are used as required.
- PREN (pitting corrosion index) is not applicable to these ferritic, non-stainless steels because PREN is used for stainless alloys: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- For high-temperature oxidation/scale resistance, the higher Cr in P91 provides improved oxidation resistance relative to P22, but neither provides stainless-level corrosion protection.
7. Fabrication, Machinability, and Formability
- Machining:
- P22: easier to machine relative to P91 due to lower strength and hardness; cutting speeds can be higher and tools have longer life.
- P91: harder and stronger, tends to work-harden; requires more robust tooling, lower cutting speeds, and rigid setups.
- Forming/bending:
- P22: better cold-forming characteristics; larger reductions possible without cracking.
- P91: limited cold formability—hot forming or greater caution and larger bend radii may be necessary.
- Surface finishing: P91 can require more aggressive grinding/polishing steps and can generate harder-to-machine chips; grinding is commonly used to remove HAZ decarburized layers after welding before PWHT in critical cases.
8. Typical Applications
| P22 (typical uses) | P91 (typical uses) |
|---|---|
| Steam piping, headers and pressure vessels in moderate-temperature boilers and refineries (up to ~540–565°C service, depending on design) | High-temperature headers, superheater/reheater tubing, steam lines and pressure components in ultra-supercritical and advanced fossil-power plants where high creep strength is required |
| Heat exchangers, drums, and non-critical high-temperature piping where cost and availability favor 1.25Cr-0.5Mo | High-stress, long-life components exposed to sustained high temperature and stress where reduced wall thickness or extended creep life is required |
| General industrial pressure components where conventional design lifetime is acceptable | New-build or retrofit applications that target higher steam temperatures/pressures and longer service intervals |
Selection rationale: - Choose P22 for moderate-temperature service where weldability, ductility, and lower material cost are priorities. - Choose P91 where long-term creep resistance, elevated-temperature strength, and potential for thinner sections or extended service life justify the higher material and fabrication cost.
9. Cost and Availability
- Relative cost: P91 is typically more expensive than P22 on a per-kilogram/foot basis because of higher alloy content and tighter processing/heat-treatment requirements.
- Availability: P22 is broadly available in many product forms (pipe, plate, fittings). P91 is widely available but may have longer lead times for specific product forms, tight-tolerance mill-processed components, or when higher-grade fabrication (e.g., welding consumables) is required.
- Procurement note: total installed cost should consider not only material price but welding procedure qualification, PWHT cycles, inspection, and potential lifecycle replacement intervals.
10. Summary and Recommendation
Summary table (qualitative):
| Criterion | P22 | P91 |
|---|---|---|
| Weldability | Easier; standard PWHT | More demanding; strict preheat/PWHT and qualified procedures |
| Strength–Toughness (room temp) | Moderate strength, good ductility/toughness | High strength, good toughness if properly treated; lower ductility |
| High-temperature creep resistance | Moderate; suitable for lower temp/high cycle service | Superior; designed for extended creep service |
| Cost | Lower material cost; simpler fabrication | Higher material and fabrication cost |
| Availability | Widely available | Widely available but specialized processing may increase lead time |
Recommendation: - Choose P22 if you need a cost-effective alloy for moderate elevated-temperature service where standard PWHT is acceptable, ductility and easier fabrication are priorities, and long-term creep life beyond conventional design limits is not required. - Choose P91 if the design demands significantly higher strength and long-term creep resistance at elevated temperatures (for example, advanced power plant steam parameters), or where reducing wall thickness/weight and extending maintenance intervals justify higher material and fabrication costs and stricter welding controls.
Final note: Exact grade selection must consider component design stresses, required design lifetime, applicable codes (ASME/EN/JIS/GB), welding and inspection capabilities, and life-cycle cost analysis. For critical pressure-retaining components consult code requirements and material suppliers for certified chemical and mechanical data and for qualification of welding procedures.