P11 vs P22 – Composition, Heat Treatment, Properties, and Applications

Introduction

P11 and P22 are two widely used Cr–Mo alloy steels for pressure-containing components, especially piping, headers, and boiler tubes in power generation and petrochemical plants. Engineers and procurement teams frequently weigh the trade-offs between higher strength and elevated-temperature capability versus cost, weldability, and toughness when choosing between them.

The principal distinguishing feature is the higher chromium and molybdenum content in P22 relative to P11, which shifts performance toward greater strength, creep resistance, and hardenability at the expense of somewhat reduced weldability and higher material cost. Because they occupy adjacent points on the Cr–Mo alloy spectrum, these grades are commonly compared when designing mid-temperature (up to several hundred °C) pressure systems.

1. Standards and Designations

• Common standards:
• ASTM/ASME: SA/SAE A335 P11, P22 (seamless ferritic alloy-steel pipe), A/SA 335 specified for high-temperature service.
• EN: Equivalent Cr–Mo grade families in EN standards (for example EN 10216/10222 series have similar Cr–Mo product families).
• JIS/GB: Japanese and Chinese standards include comparable Cr–Mo steels with analogous chemistry and properties.
• Classification:
• Both P11 and P22 are alloy steels (ferritic chromium–molybdenum grades), not stainless or tool steels. They are not classified as HSLA in the strict sense but are low-alloy pressure-vessel steels designed for elevated-temperature strength and creep resistance.

2. Chemical Composition and Alloying Strategy

Table: typical nominal composition ranges (wt%, representative of common specifications and commercial practice)

Element	P11 (typical)	P22 (typical)
C	0.05–0.15	0.05–0.15
Mn	0.25–0.60	0.25–0.60
Si	0.10–0.50	0.10–0.50
P	≤0.03	≤0.03
S	≤0.03	≤0.03
Cr	~0.9–1.4 (≈1.0–1.25)	~2.0–2.6 (≈2.25)
Mo	~0.4–0.6 (≈0.5)	~0.8–1.15 (≈1.0)
Ni	≤0.40	≤0.40
V, Nb, Ti, B, N	Trace/none (depends on manufacturer)	Trace/none (depends on manufacturer)

Notes: - Values shown are typical nominal ranges from pressure-vessel/pipe specifications rather than precision control limits for all product forms. Specific mill certifications and standards should be consulted for contract-critical projects. - The key alloying strategy is that P22 has substantial additional Cr and Mo relative to P11. This increases hardenability, high-temperature strength, and oxidation/corrosion resistance in some environments, while P11 provides a simpler chemistry with somewhat better weldability and lower material cost.

How alloying affects performance: - Chromium increases oxidation resistance, hardenability, and high-temperature strength; it also promotes carbide formation that stabilizes strength at elevated temperature. - Molybdenum strengthens the ferrite matrix, improves creep resistance, and increases hardenability, improving strength retention at elevated temperature. - Carbon, Mn, and Si set baseline strength and hardenability; higher carbon raises strength but can reduce weldability and toughness if not controlled.

3. Microstructure and Heat Treatment Response

Typical microstructures: - In normalized and tempered condition, both grades exhibit tempered martensite or bainitic tempered microstructures depending on cooling rates and alloy content. P22, with higher Cr and Mo, more strongly favors martensite/bainite formation and higher hardenability for the same thermal cycle than P11. - As-rolled microstructures for both are tempered pearlite/ferrite with alloy carbides (Cr–Mo carbides) dispersed.

Heat treatment responses: - Normalizing (air cooling from above A3) refines grain size and produces a uniform microstructure. P22 typically requires higher tempering temperatures to achieve comparable toughness due to increased hardenability. - Quenching and tempering: Both grades can be quenched and tempered; P22 will reach higher strength levels after the same quench due to Cr–Mo content, but tempering schedules must be tuned to restore toughness and reduce residual hardness. - Thermo-mechanical processing: Controlled rolling followed by tempering improves toughness and creep resistance; microalloying additions (Nb, V, Ti) if present further refine grain size and dislocation pinning.

Practical considerations: - P22’s higher hardenability means thicker sections and coarse heat sinks can form harder microstructures (risk of cold cracking in weld HAZ without preheat). - Tempering is critical to balance strength and toughness for both grades, particularly for operating temperatures where creep is a concern.

4. Mechanical Properties

Table: qualitative comparison (typical normalized & tempered product conditions)

Property	P11	P22
Tensile strength	Moderate	Higher
Yield strength	Moderate	Higher
Elongation (ductility)	Generally higher	Slightly lower
Impact toughness (at room temp)	Good when properly tempered	Good but can be more sensitive to heat treatment
Hardness (as-delivered)	Lower	Higher

Explanation: - P22 tends to achieve higher tensile and yield strengths primarily because of the elevated Cr and Mo content that increases strength and hardenability. P11 is generally more ductile and slightly easier to achieve tough HAZs after welding. - Impact toughness for both can be excellent when normalized and properly tempered; however, P22’s higher hardenability means improper thermal control can reduce toughness in thick sections or poorly welded joints. - The balance of strength vs. toughness is adjustable through tempering; design should specify required impact energy and heat treatment to ensure compliance.

5. Weldability

Weldability drivers: - Carbon equivalent and hardenability are principal indicators of susceptibility to HAZ hardening and cold cracking. Both grades require preheat and post-weld heat treatment (PWHT) for pressure applications, but P22 typically requires more conservative controls. - Use of carbon equivalent formulas provides a qualitative weldability assessment.

Common indices (for interpretation only): - IIW carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - International Welding Institute parameter: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - Because P22 has higher Cr and Mo, $CE_{IIW}$ and $P_{cm}$ values will be higher for P22 than for P11 with otherwise similar chemistry, indicating greater hardenability and a higher risk of HAZ hardening and cold cracking. - Practical consequences: increased preheat, controlled interpass temperatures, low-hydrogen consumables, and PWHT at specified temperatures are more critical for P22, especially in thicker sections.

6. Corrosion and Surface Protection

• Neither P11 nor P22 is stainless; corrosion resistance in wet or chemically aggressive environments is limited. Chromia-forming behavior improves with Cr content, so P22 offers modestly better oxidation resistance at elevated temperatures and may form a more protective oxide scale in high-temperature oxidizing atmospheres.
• For general atmospheric or aqueous corrosion, surface protection is required: painting, epoxy coatings, or galvanizing (where compatible) are typical. For buried or subsea service, epoxy coatings and cathodic protection are common.
• PREN is not applicable to these non-stainless steels; for context, PREN is: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Use of PREN is meaningful only for stainless grades containing significant nickel and nitrogen; for P11/P22, chromium and molybdenum content is low relative to stainless steels, so corrosion resistance planning must rely on coatings, inhibitors, and material selection.

7. Fabrication, Machinability, and Formability

• Machinability: Both are machinable in normalized/tempered condition; P22’s higher strength and hardness can reduce tool life and require heavier machining parameters compared with P11.
• Formability/bending: Cold forming is easier with P11 due to lower yield and higher ductility. P22 is less forgiving in tight bends and may require larger bend radii or elevated-temperature forming/controlled bending parameters.
• Surface finishing: Both respond well to standard finishing operations; grinding and polishing are straightforward when tempered to moderate hardness.
• Welding practice: For pressure systems, both require qualified filler metals compatible with tempering and PWHT to avoid embrittlement; P22 often needs tougher filler choices and stricter process controls.

8. Typical Applications

P11 (typical uses)	P22 (typical uses)
Low- to mid-temperature boiler tubes, headers, and piping where cost and weldability are prioritized	Higher-temperature service piping, steam lines, superheater/reheater headers and tubing where elevated-temperature strength is required
Structural components in power plants where moderate creep resistance is adequate	Components exposed to higher steam temperatures and pressures where creep and oxidation resistance are important
Heat exchangers and pressure vessels in petrochemical service with moderate temperature/pressure	Critical pressure-vessel components and piping in refineries and power plants requiring higher strength and better high-temperature oxidation resistance

Selection rationale: - Choose P11 when balance of cost, ease of fabrication, and acceptable elevated-temperature performance is needed for moderate service conditions. - Choose P22 when the service involves higher temperatures, higher design stresses, or when enhanced creep resistance and oxidation performance justify the higher material and fabrication cost.

9. Cost and Availability

• Cost: P22 is generally more expensive than P11 because of higher chromium and molybdenum content and tighter processing/heat-treatment requirements. Molybdenum is a relatively costly alloying addition.
• Availability: Both grades are common in seamless and weldable pipe product ranges; P11 and P22 are widely stocked for power and petrochemical markets. Availability by product form (pipe, plate, forgings) depends on mill inventory and regional supply. Lead times for P22 can be longer during times of molybdenum market tightness.

10. Summary and Recommendation

Table: comparative summary (qualitative)

Attribute	P11	P22
Weldability	Better (lower hardenability)	More demanding (higher hardenability)
Strength–Toughness balance	Moderate strength, good ductility	Higher strength, can be less ductile if not tempered correctly
Cost	Lower	Higher

Recommendations: - Choose P11 if: - You need a cost-effective Cr–Mo alloy for moderate temperature pressure systems. - Ease of welding, fabrication, and ductility are priorities. - Service temperatures and stresses are within P11’s capability and creep is not the controlling design criterion. - Choose P22 if: - The application requires higher elevated-temperature strength, improved creep resistance, or better oxidation resistance at higher steam/gas temperatures. - You can implement stricter welding controls, preheat/PWHT, and accept higher material cost for improved long-term performance.

Final note: Specify required heat treatment, impact energy, preheat, and PWHT in procurement documents; consult mill test certificates for actual chemistry and mechanicals for each lot. For critical pressure-vessel or elevated-temperature designs, validate the choice with creep data, long-term properties, and relevant code compliance.