P22 vs P91 – Composition, Heat Treatment, Properties, and Applications

Introduction

Choosing between P22 and P91 is a common technical decision for engineers, procurement managers, and manufacturing planners working on pressure-retaining equipment and high-temperature piping systems. The selection dilemma typically centers on lifetime under steam/heat (creep performance), fabrication and weldability, and total installed cost (material plus welding and heat treatment). P22 (a 2.25Cr–1Mo class alloy) and P91 (a 9Cr–1Mo, microalloyed creep-strengthened steel) are frequently compared because both are used in power and process industries but target different temperature–stress envelopes and fabrication requirements.

The principal technical distinction is that P91 is engineered for substantially higher long-term strength and creep resistance at elevated temperatures, while P22 provides a balance of adequate high-temperature strength with easier fabrication and lower material cost. That difference drives material selection for components subjected to prolonged exposure to steam or high-temperature service.

1. Standards and Designations

• Common standards and designations:
• ASTM/ASME: A335 / SA335 P22 and P91 (seamless ferritic alloy-steel pipe grades), ASTM A213, A335, ASME SA335.
• EN: Equivalent steels are covered under EN standards for pressure equipment but may have different grade names (e.g., 22CrMo and 9CrMo variants).
• JIS/GB: National standards will list near-equivalents (e.g., 2.25Cr–1Mo and 9Cr–1Mo series).
• Classification:
• P22: low-alloy ferritic steel (often grouped with Cr–Mo heat-resistant steels).
• P91: alloyed/high-strength ferritic steel with microalloying (Cr–Mo–V–Nb) — considered a creep-strength-enhanced ferritic (HSLA-like in some contexts, but specifically engineered for high-temperature creep resistance).

2. Chemical Composition and Alloying Strategy

Table: Typical compositional ranges (mass %) for P22 and P91 (representative ranges per common specifications).

Element	P22 (approx. ranges)	P91 (approx. ranges)
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 – 0.60
P	≤ 0.03	≤ 0.02
S	≤ 0.03	≤ 0.01
Cr	2.00 – 2.60	8.00 – 9.50
Ni	≤ 0.40	≤ 0.40
Mo	0.80 – 1.10	0.85 – 1.05
V	– (trace)	0.15 – 0.25
Nb (Cb)	– (trace)	0.06 – 0.12
Ti	≤ 0.01	≤ 0.02
B	– (trace)	≤ 0.001
N	≤ 0.03	~0.03 – 0.09

Notes: - Ranges above are indicative of typical production chemistries used to meet ASME/ASTM-based specifications. Exact limits depend on the specific standard and supplier. - P91 includes purposeful microalloying (V, Nb, controlled N and B) to stabilize fine tempered martensitic microstructures and improve creep resistance. P22 relies mainly on moderate Cr and Mo additions for elevated-temperature strength.

How alloying affects properties: - Chromium increases oxidation/scale resistance and strength at temperature; higher Cr in P91 enables a tempered martensitic matrix stable at higher temperatures. - Molybdenum enhances high-temperature strength and hardenability in both grades. - Vanadium and niobium in P91 form carbide/nitride dispersoids that pin grain boundaries and retard creep/void growth enabling higher long-term strength. - Carbon and microalloying balance hardenability, strength, and weldability—the controlled C level in P91 is higher than in some low-alloy steels but is managed by tempering and alloy design.

3. Microstructure and Heat Treatment Response

P22: - Typical microstructure after normalizing and tempering: tempered bainite / ferritic-pearlitic microstructure depending on exact heat treatment. Conventional normalization and tempering produce a relatively coarse tempered microstructure suitable for moderate-temperature steam service. - Heat treatment response: P22 responds to normalizing and tempering; over-tempering lowers strength but improves toughness. It is not designed for the fine martensitic-tempered microstructure seen in P91.

P91: - Typical microstructure after normalizing and tempering: tempered lath martensite with a high dislocation density and a controlled dispersion of V/Nb carbides/nitrides; this fine, stable microstructure gives high creep resistance. - Thermo-mechanical processing and stringent control of normalizing temperature and tempering conditions are critical to develop the desired microstructure and to avoid temper embrittlement or over-tempering. - P91 is sensitive to post-weld heat treatment (PWHT) — correct PWHT is essential to recover toughness and relieve residual stresses without coarsening precipitates.

Comparison: - P91 achieves higher strength and creep resistance by forming a tempered martensitic structure stabilized by microalloy precipitates; P22 relies on Cr–Mo strengthening in a more ferritic/bainitic matrix. - Both require controlled heat treatment, but P91 typically demands tighter control (normalized at higher temperature and tempered at specific temperatures) and consistent PWHT to meet creep specifications.

4. Mechanical Properties

Table: Comparative descriptors for mechanical properties in commonly supplied (normalized & tempered) conditions.

Property	P22	P91	Commentary
Tensile Strength	Medium	High	P91 delivers substantially higher tensile strength in tempered condition due to martensitic structure and microalloying.
Yield Strength	Medium	High	P91 provides higher yield, which benefits thinner sections for the same load.
Elongation (ductility)	Good	Moderate	P22 tends to be more ductile; P91 trades some ductility for strength and creep resistance.
Impact Toughness	Good (at lower temps)	Good to excellent (when properly heat treated)	P91 can achieve good toughness but is more process-sensitive; mis-heat-treated P91 may show reduced toughness.
Hardness	Moderate	Higher	P91 exhibits higher hardness after tempering; hardness must be controlled to avoid embrittlement and to meet welding/heat treatment specs.

Interpretation: - P91 is the stronger and more creep-resistant material for elevated-temperature service, but achieving its mechanical properties requires controlled processing and PWHT. - P22 offers a balance of strength, toughness, and ductility suitable for many services up to moderate elevated temperatures and is generally more forgiving in fabrication.

5. Weldability

Weldability considerations include carbon content, alloying content, hardenability, and the presence of microalloying elements. Predictive formulas commonly used for qualitative assessment:

• Carbon Equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

• Pcm (weldability 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: - P22: moderate carbon equivalent; generally good to fair weldability with standard preheat and PWHT practices for thicker sections. Commonly welded in powerplant fabrication with established procedures. - P91: higher hardenability due to higher Cr and microalloying; has a higher effective CE and Pcm, meaning increased risk of hard, brittle HAZ if welded without strict controls. P91 requires carefully controlled filler metals, preheat, interpass temperatures, and mandatory PWHT to restore toughness and relieve residual stresses. - In practice, P91 welding procedures are more demanding and require qualified weld procedures and welders; dissimilar-metal joints (e.g., P91 to P22 or to standard carbon steel) require special transition weld procedures.

6. Corrosion and Surface Protection

• Both P22 and P91 are non-stainless ferritic alloy steels and rely on coatings or barrier protection for corrosion resistance in aqueous or aggressive atmospheres.
• Typical protections: painting, galvanizing (where compatible), cladding (e.g., weld overlay or corrosion-resistant liner), or inhibitors in closed systems.
• PREN (pitting resistance equivalent number) is not applicable to these non-stainless steels; for stainless grades the index is: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• For high-temperature oxidation/scale resistance, P91's higher Cr content gives improved performance compared with P22, but neither grade replaces stainless steels for corrosion-critical environments.

7. Fabrication, Machinability, and Formability

• Machinability: P22 is generally easier to machine due to lower hardness and simpler microstructure. P91's higher hardness and alloying can increase tool wear and require slower feeds/cutting speeds.
• Forming/bending: P22 is more ductile and forgiving in forming operations. Cold forming of P91 is limited and typically requires thermal/forming strategies or limited deformation; hot forming may be used but requires careful control.
• Finishing: Surface preparation and post-fabrication heat treatments (especially PWHT for P91) add steps and cost. Welding distortion control is more critical for P91 because of higher residual stresses and hardness in the HAZ.

8. Typical Applications

Table: Typical uses for each grade and selection rationale.

P22 (2.25Cr–1Mo)	P91 (9Cr–1Mo–V–Nb)
Boiler tubes and moderate-temperature steam piping (subcritical and lower-supercritical units)	High-temperature steam piping, headers, and components in ultra-supercritical and supercritical boilers
Pressure vessels and heat exchangers for moderate-temperature service	Components requiring long-term creep resistance at higher temperatures (e.g., high-pressure steam lines)
General process piping where moderate elevated-temperature strength is adequate	Power plant main steam piping, reheater and superheater headers, and components where design life under creep is critical

Selection rationale: - Choose P22 when service temperatures and stresses are moderate, when fabrication simplicity and cost control are priorities, and when long-term creep life requirements are less severe. - Choose P91 when design stress and temperature demand high creep strength and long-term stability; P91 allows reduced section thickness or extended component life under severe high-temperature conditions.

9. Cost and Availability

• Material cost: P91 is typically more expensive per kilogram than P22 due to higher alloying and processing requirements.
• Fabrication and lifecycle cost: P91 may require more expensive welding consumables, stricter procedure qualification, and mandatory PWHT—raising installed cost. However, for high-temperature service the lifecycle cost may favor P91 due to reduced replacement and maintenance driven by superior creep strength.
• Availability: P22 is widely available in many product forms (plate, pipe, fittings) and is commonly stocked. P91 is widely produced for power generation but may have longer lead times for certain product forms and large-diameter or specialty forgings.

10. Summary and Recommendation

Table: concise comparative summary.

Criterion	P22	P91
Weldability	Good to fair (standard procedures)	Fair to challenging (requires strict controls & PWHT)
Strength–Toughness (elevated temp)	Moderate	High (superior creep resistance)
Cost (material + fabrication)	Lower	Higher
Fabrication / Machinability	Easier	More demanding

Conclusions and selection guidance: - Choose P22 if you need a cost-effective, easier-to-fabricate Cr–Mo alloy for moderate elevated-temperature applications where long-term creep life is not the governing design driver. Typical contexts: conventional boiler tubing, moderate-temperature pressure vessels, and general process piping. - Choose P91 if the component must sustain higher stresses at elevated temperatures for long service times (e.g., superheater/reheater/headers in advanced power plants), when minimizing wall thickness or extending service life justifies higher material and fabrication costs. Ensure that qualified welding procedures, correct filler metallurgy, and controlled PWHT are in place.

Final practical note: Material selection should always be coupled with engineering assessments of operating temperature, stress, expected lifetime, welding and inspection capability, and lifecycle cost. When in doubt for high-temperature, long-life service, consult creep-data curves, applicable code rules (ASME BPVC/EN standards), and a materials specialist to validate the choice between P22 and P91.