A335 P11 vs P22 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
ASTM A335 P11 and P22 are two widely used chromium–molybdenum alloy steels for high-temperature pressure parts such as piping, headers, and boiler tubing. Engineers, procurement managers, and manufacturing planners commonly weigh trade-offs between cost, high-temperature strength, weldability, and long-term creep resistance when selecting between them. Typical decision contexts include matching material strength and toughness to service temperature, specifying weld procedures for fabrication, and balancing lifecycle cost against initial purchase price.
The primary technical distinction between these grades is their alloying level of chromium and molybdenum: P22 contains substantially more chromium and molybdenum than P11, which directly affects hardenability, high-temperature strength, and oxidation resistance. Because both steels are Cr–Mo ferritic alloys designed for elevated-temperature service, they are frequently compared during material selection for boilers, superheated steam piping, and pressure vessels.
1. Standards and Designations
- ASTM/ASME: ASTM A335 / ASME SA-335 (Seamless ferritic alloy-steel pipe for high-temperature service)
- Grade P11 (often noted as 1.25Cr–0.5Mo nominal)
- Grade P22 (often noted as 2.25Cr–1Mo nominal)
- EN: Comparable grades available in EN standard systems (e.g., P11 ≈ 13CrMo4-5 or similar families; P22 ≈ 2.25Cr–1Mo tempering steels)
- JIS/GB: National standards provide roughly equivalent Cr–Mo tempered steels used for high-temperature piping and vessels.
- Classification: Both P11 and P22 are alloy steels (ferritic Cr–Mo steels), not stainless or tool steels; they are used for high-temperature pressure-retaining components and are not HSLA in the modern sense.
2. Chemical Composition and Alloying Strategy
The table below lists typical nominal composition ranges (wt%) used in industry practice for A335 P11 and P22. Values are shown as representative ranges rather than exact guaranteed minimums/maximums from a specific purchase spec.
| Element | A335 P11 (typical, wt%) | A335 P22 (typical, wt%) |
|---|---|---|
| C | 0.08 – 0.15 | 0.08 – 0.15 |
| Mn | 0.25 – 0.60 | 0.25 – 0.60 |
| Si | 0.10 – 0.50 | 0.10 – 0.50 |
| P | ≤ 0.025 | ≤ 0.025 |
| S | ≤ 0.025 | ≤ 0.025 |
| Cr | ~0.90 – 1.30 | ~2.00 – 2.50 |
| Ni | ≤ 0.40 (trace) | ≤ 0.40 (trace) |
| Mo | ~0.40 – 0.65 | ~0.85 – 1.05 |
| V | typically trace | typically trace |
| Nb (Cb) | typically trace | typically trace |
| Ti | typically trace | typically trace |
| B | typically trace | typically trace |
| N | trace | trace |
How alloying affects performance: - Chromium: improves oxidation/corrosion resistance at elevated temperature and contributes to hardenability and high-temperature strength. The higher Cr in P22 boosts its resistance to scaling and improves strength retention at service temperatures. - Molybdenum: strengthens ferrite at elevated temperature, improves creep strength, and increases hardenability. P22’s higher Mo content yields better creep and high-temperature strength versus P11. - Carbon and manganese: primary contributors to strength and hardenability; both grades maintain moderate carbon to balance weldability and strength. - Minor elements and microalloying (V, Nb, Ti) may be present in trace amounts and can refine grain structure and precipitation-strengthen the matrix under specific processing.
3. Microstructure and Heat Treatment Response
Typical microstructures: - As-manufactured and normalized: both grades generally show a tempered martensite/bainitic–tempered ferrite microstructure after normalizing and tempering cycles commonly specified for pressure parts. - After quench & temper (where applicable for heavy sections or forgings): a tempered martensitic structure with carbide precipitation (Cr/Mo-rich carbides) provides high strength and creep resistance. - Thermo-mechanical processing: controlled rolling can refine grain size and improve toughness for both grades, though alloy content controls the ease of refinement.
Heat treatment effects: - Normalizing followed by tempering refines grain size, transforms as-rolled microstructures into a uniform tempered martensite/bainitic matrix, and precipitates Cr–Mo carbides that contribute to high-temperature strength. - P22’s higher Cr and Mo promote a greater volume fraction of alloy carbides and slow softening at elevated temperatures; it typically requires tempering regimes tailored to balance hardness and toughness for thicker sections. - P11 is less alloyed and thus somewhat easier to obtain appropriate toughness after standard normalizing/tempering cycles, but has lower long-term strength at very high temperatures versus P22.
4. Mechanical Properties
Below are representative property ranges for normalized and tempered condition commonly used in design. Actual guaranteed properties depend on product form and specific heat treatment.
| Property (room temp unless noted) | A335 P11 (typical) | A335 P22 (typical) |
|---|---|---|
| Tensile strength (MPa) | ~415 – 550 | ~415 – 620 |
| Yield strength (0.2% offset, MPa) | ~240 – 360 | ~260 – 400 |
| Elongation (%) | ~20 – 25 | ~18 – 22 |
| Charpy V-notch impact (J, normalized) | Typically good at ambient; retains toughness with proper heat treatment | Comparable or slightly lower ductility for same strength due to higher alloying |
| Hardness (HB or HRC) | Moderate (e.g., HB 150–220 range depending on heat treat) | Slightly higher for equivalent tempering due to alloying |
Interpretation: - P22 generally provides higher high-temperature strength and creep resistance because of its greater Cr and Mo contents; this often translates into the ability to operate safely at higher temperatures or with reduced wall thickness for a given design life. - P11 tends to be more ductile and marginally easier to heat-treat for toughness in certain thicknesses, but it provides lower long-term strength at elevated temperatures.
5. Weldability
Weldability depends on carbon equivalent, hardenability, and microalloying. Two widely used empirical measures are the IIW carbon equivalent and Pcm:
$$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr + Mo + V}{5} + \frac{Ni + Cu}{15}$$
$$P_{cm} = C + \frac{Si}{30} + \frac{Mn + Cu}{20} + \frac{Cr + Mo + V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation: - P22 has higher Cr and Mo, which increases $CE_{IIW}$ and $P_{cm}$ relative to P11, implying higher susceptibility to hardening in the heat-affected zone and a greater need for preheat and controlled interpass temperatures. - Both grades are commonly welded in industry; recommended practices include low-hydrogen electrodes, specified preheat (and sometimes post-weld heat treatment — PWHT), and weld procedure qualification. P22 more frequently requires more conservative preheat/PWHT regimes for thicker sections due to higher hardenability. - Control of hydrogen, interpass temperature, and PWHT are crucial to avoid hydrogen-assisted cracking and to temper HAZ hardness.
6. Corrosion and Surface Protection
- Neither P11 nor P22 is stainless; both will corrode if exposed to aqueous or aggressive environments. Selection is driven by mechanical and high-temperature oxidation behavior rather than general corrosion resistance.
- For external or atmospheric protection: galvanizing, painting/coating systems, or thermal spray overlays can be applied depending on service.
- For internal protection in corrosive process streams: cladding (e.g., weld overlay with corrosion-resistant alloys), linings, or corrosion allowance design are typical strategies.
- PREN (pitting resistance equivalent number) is used for stainless alloys and is not applicable for Cr–Mo ferritic steels. For stainless alloys, one would use:
$$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
But this index is irrelevant for A335 P11 and P22 because their Cr contents are below stainless thresholds and they lack the high N and Ni content typical of stainless grades.
7. Fabrication, Machinability, and Formability
- Machinability: Both grades machine moderately well in normalized/tempered condition. P22 may be slightly tougher on cutting tools because of higher alloying and resulting precipitation-hardened carbides. Carbide-forming elements (Cr, Mo) increase abrasion of tooling.
- Formability: Both are less formable than plain carbon steels when cold; forming is typically done in normalized or annealed conditions with attention to bend radii. For piping and tubing shapes, forming is routine but springback and risk of cracking in heavily cold-worked regions are considerations.
- Surface finishing: Grinding, turning and welding finishing are standard; attention to residual stresses and avoidance of overheating during machining on P22 is advised to prevent local hardness increases.
8. Typical Applications
| A335 P11 – Typical Uses | A335 P22 – Typical Uses |
|---|---|
| Lower-to-moderate-temperature steam piping, headers, and fittings where cost sensitivity is a factor and temperatures are moderate | High-temperature steam piping, superheater/reheater tubing, and pressure vessels where higher creep strength and oxidation resistance are needed |
| Boilers and heat exchangers in less severe high-temperature zones | Power plant main steam lines, hot reheat piping, and components for service up to higher design temperatures |
| Petroleum and chemical plant piping for moderate temperatures | High-duty petrochemical process heaters and elevated-temperature pressure-retaining components |
Selection rationale: - Choose P11 for applications where nominally lower maximum service temperature, lower material cost, and easier fabrication are prioritized. - Choose P22 when the application demands higher creep strength, better retention of mechanical properties at elevated temperature, or improved resistance to scaling/oxidation.
9. Cost and Availability
- Availability: Both grades are commonly stocked as seamless and welded pipes, fittings, and some forgings; P11 and P22 are standard grades in many supply chains for power generation and process plants.
- Cost: P22 is typically more expensive than P11 due to higher Cr and Mo content. The incremental cost may be justified by longer life, thinner wall design possibilities, or reduced maintenance.
- Product form: Both are readily available in pipe, tube, and pressure-vessel plates; lead times are generally short for standard sizes but may increase for large forgings or special heat treatments.
10. Summary and Recommendation
| Aspect | A335 P11 | A335 P22 |
|---|---|---|
| Weldability | Better (lower alloy content → lower CE) | Slightly more demanding (higher Cr/Mo → higher CE; needs more preheat/PWHT) |
| Strength–Toughness at RT | Moderate strength, good ductility | Higher strength at temperature, comparable or slightly lower ductility at equivalent strength |
| High-temperature / Creep Resistance | Adequate for moderate elevated temperatures | Superior for higher temperatures and longer creep lives |
| Cost | Lower | Higher |
Recommendations: - Choose P11 if you need a cost-effective Cr–Mo alloy for moderate elevated-temperature service where extensive PWHT or aggressive high-temperature creep resistance is not required, and where slightly better cold ductility and easier weld procedures are advantageous. - Choose P22 if the design requires higher long-term strength and creep resistance at elevated temperatures, improved scaling resistance, or the possibility of thinner walls for a given design life — and you can accommodate the stricter welding and post-weld heat treatment requirements and slightly higher material cost.
Concluding note: For critical pressure-retaining components, always qualify the material spec, required heat treatment, and welding procedure with the project’s metallurgist or code authority (ASME) to ensure compatibility with design temperature, allowable stresses, and fabrication constraints.