P11 vs P22 – Composition, Heat Treatment, Properties, and Applications
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Table Of Content
Table Of Content
Introduction
P11 and P22 are two widely used chromium–molybdenum alloy steels specified for pressure parts and high-temperature service such as boiler tubes, headers, and piping. Engineers, procurement managers, and manufacturing planners commonly weigh trade-offs between strength/creep resistance, weldability, toughness, and cost when choosing between them. Typical decision contexts include selecting the right grade for elevated-temperature service (creep vs. cost), specifying preheat and PWHT practices (weldability and hydrogen cracking risk), and optimizing lifecycle cost for replacement intervals.
The principal distinguishing design feature between these two grades is their alloying strategy: P22 contains a higher chromium and molybdenum content than P11. That compositional difference drives higher high-temperature strength and creep resistance in P22, while increasing hardenability and welding/preheat considerations compared with P11. Because both are used for similar piping and pressure-vessel applications, comparison is frequent during material selection for power plants, refineries, and petrochemical equipment.
1. Standards and Designations
- Common standards and designations:
- ASME/ASTM: ASME SA335 / ASTM A335 (seamless ferritic alloy-steel pipe): P11, P22.
- EN: Equivalent designations are sometimes given as 1.0–1.25Cr–0.5Mo and 2.25Cr–1Mo families; specific EN numbers vary with product and heat-treatment.
- JIS / GB: Regional standards may list corresponding grades (consult specific standard tables for exact cross-reference).
- Material class:
- Both P11 and P22 are alloy steels (chromium–molybdenum ferritic steels) intended for elevated-temperature service. They are not stainless steels, tool steels, or HSLA steels in the usual sense.
2. Chemical Composition and Alloying Strategy
The following table summarizes relative levels of common elements rather than absolute percentages; this avoids presenting specific numerical values that depend on the exact specification and vendor.
| Element | P11 (relative level) | P22 (relative level) |
|---|---|---|
| C | Low–Moderate | Low–Moderate |
| Mn | Low–Moderate | Low–Moderate |
| Si | Low–Moderate | Low–Moderate |
| P | Trace / Controlled | Trace / Controlled |
| S | Trace / Controlled | Trace / Controlled |
| Cr | Moderate (lower) | Higher (notably higher) |
| Ni | Trace / Low | Trace / Low |
| Mo | Moderate (lower) | Higher (notably higher) |
| V | Trace / Possible microalloying | Trace / Possible microalloying |
| Nb (Nb/Ta) | Typically not added | Typically not added |
| Ti | Trace / Controlled | Trace / Controlled |
| B | Not typically specified | Not typically specified |
| N | Controlled low levels | Controlled low levels |
Explanation: - P11 is formulated with modest chromium and molybdenum additions to provide strength and creep resistance at elevated temperatures while retaining relatively good weldability. Its alloying is conservative. - P22 increases chromium and molybdenum levels to raise high-temperature strength, oxidation resistance, and creep resistance; these increases also raise hardenability and can make welding and heat treatment more demanding. - Other elements such as Mn and Si are present at similar, controlled levels in both grades and primarily influence deoxidation, strength, and toughness. - Very low and tightly controlled impurity levels (P, S, N) are important for toughness and high-temperature performance in both grades.
How alloying affects properties: - Chromium and molybdenum increase hardenability, elevated-temperature strength, and creep-rupture performance; chromium also contributes to oxidation resistance. - Carbon increases strength but reduces weldability and toughness if excessive. - Microalloying elements (V, Nb, Ti) can refine grain size and improve creep strength through precipitation strengthening when present intentionally.
3. Microstructure and Heat Treatment Response
Typical microstructures and heat-treatment behavior: - Base microstructure after normalizing and tempering: Both grades develop a tempered martensitic or tempered bainitic ferritic microstructure depending on cooling rate and alloy level. Proper normalization refines prior austenite grain size; tempering reduces hardness while restoring toughness. - P11: With its lower alloy content, P11 typically forms tempered martensite or tempered bainite with relatively easy tempering response. It accepts standard normalizing and tempering cycles used for low-alloy Cr–Mo steels and is forgiving in heat-treatment windows. - P22: Higher Cr and Mo increase hardenability and slow down bainitic/martensitic transformations; under rapid cooling the as-quenched microstructure can be harder and more martensitic. Tempering is essential to restore toughness and adjust creep properties; P22 may require more controlled heat-treatment to avoid over-tempering or retained hardness gradients. - Thermo-mechanical processing: Neither grade is typically processed with aggressive TMCP for high-strength plate levels used in structural steels; for components, controlled hot-working followed by normalizing and tempering is the standard route to produce a reliable tempered microstructure. - Creep considerations: P22’s alloying supports higher creep-rupture strength at elevated temperatures; stability of carbides (Cr- and Mo-rich carbides) and their distribution after tempering are key to long-term performance.
4. Mechanical Properties
The following table provides qualitative comparative descriptors; actual values are specification- and heat-treatment-dependent.
| Property | P11 (typical) | P22 (typical) |
|---|---|---|
| Tensile strength | Moderate | Higher |
| Yield strength | Moderate | Higher |
| Elongation (ductility) | Good | Good to slightly reduced |
| Impact toughness | Good (especially after proper tempers) | Good when properly tempered; may be more sensitive to heat treatment |
| Hardness (as-heat-treated) | Moderate | Higher (can be higher before tempering) |
Interpretation: - P22 generally achieves higher tensile and yield strengths and superior creep resistance at elevated temperatures owing to the higher Cr–Mo content and more stable carbide phases. - P11 often offers marginally better ease of achieving toughness, with somewhat lower hardenability and thus fewer welding/heat-treatment complications in many shop environments. - Both grades can be produced to meet specific impact and strength targets through proper normalizing and tempering; final properties are heat-treatment dependent.
5. Weldability
Weldability is influenced by carbon equivalent and hardenability. Two commonly used empirical descriptors are the IIW carbon-equivalent and the more comprehensive 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, with its higher chromium and molybdenum, produces a higher contribution to hardenability terms in $CE_{IIW}$ and $P_{cm}$; thus, welded zones are more prone to forming hard martensite and require more careful control (preheat, interpass temperature, and post-weld heat treatment — PWHT). - P11 typically has a lower calculated carbon equivalent contribution from Cr and Mo, making it easier to weld with standard procedures; lower preheat/PWHT severity is often possible. - Both materials usually require PWHT in pressure-vessel and piping applications to reduce residual stresses and temper any hard microstructures formed in the heat-affected zone (HAZ). - Hydrogen-induced cold cracking: because P22 is more hardenable, it is more susceptible to HAZ cracking if hydrogen and restraint are not controlled; rigorous procedures for preheat, consumable selection, and hydrogen control are required. - Consumables: matching or overmatching filler metals with appropriate alloy content are selected to meet strength and high-temperature requirements; filler selection must consider PWHT compatibility and creep performance.
6. Corrosion and Surface Protection
- Neither P11 nor P22 is a stainless alloy; they rely on coatings, painting, galvanizing (where applicable for lower-temperature exposure), or cladding for corrosive environments.
- The higher chromium content in P22 offers somewhat improved resistance to oxidation at elevated temperatures compared with P11, but this is not equivalent to stainless corrosion resistance.
- For aqueous corrosion or highly corrosive process streams, cladding with stainless grades or corrosion allowances is required.
- PREN (pitting resistance equivalent number) is used for stainless alloys and is not applicable to these low-alloy Cr–Mo steels, but for clarity, the PREN formula is:
$$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- Because PREN is intended for stainless steels, it should not be used to rate P11/P22; their corrosion performance must be engineered by selection of protective systems, material claddings, or corrosion allowances based on the environment.
7. Fabrication, Machinability, and Formability
- Machinability:
- Both grades machine satisfactorily in normalized and tempered conditions, but P22 can be more abrasive and work-harden if cutting parameters are not optimized because of higher hardenability and carbide content.
- Formability / bending:
- P11 is generally easier to cold form and bend without aggressive preheating than P22 due to lower alloy content; however, both are commonly formed in the normalized condition or by controlled hot bending procedures.
- Surface finish and dressing:
- Higher alloy particles and carbides in P22 can increase tool wear in finishing operations; specify appropriate tooling and feeds.
- Recommendations:
- Perform forming and machining in the normalized/tempered condition, not in the as-rolled or as-welded hard state.
- Use appropriate cooling, cutting fluids, and tool materials (coated carbides or ceramics for high-alloy cuts).
8. Typical Applications
| P11 — Typical Uses | P22 — Typical Uses |
|---|---|
| Lower-pressure or moderate-temperature piping, headers, and fittings where cost and weldability are priorities | High-temperature steam piping, pressure parts, and components requiring superior creep strength and oxidation resistance |
| Heat-exchanger tubes and components where moderate creep resistance is adequate | Main steam lines, superheater/reheater headers, and components in moderate-to-high temperature power plant applications |
| Economical replacement parts where service temperature is not extreme | Critical pressure-vessel components and piping in fossil or combined-cycle plants where longer creep life is required |
| General-purpose alloy-steel piping in petrochemical service with less severe temperature demands | Components requiring higher allowable stress at temperature or reduced thickness for the same design stress |
Selection rationale: - Choose P22 when higher long-term strength at elevated temperature, oxidation resistance, and creep life are design drivers. - Choose P11 when lower cost and easier fabrication/welding are priorities and the temperature/creep demands are modest.
9. Cost and Availability
- Relative cost:
- P22 typically commands a higher material price than P11 because of increased Cr and Mo content and tighter processing for high-temperature performance.
- P11 is usually more economical and widely stocked in many pipe and fitting inventories.
- Product forms and availability:
- Both grades are commonly available as seamless pipe, welded pipe, fittings, flanges, and pressure-vessel plates; however, lead times for P22 may be longer for specialized plate thicknesses or forged components.
- Availability can vary regionally; procurement engineers should confirm lead times for the required product form and any specified heat-treatment condition.
10. Summary and Recommendation
Summary table (qualitative):
| Criterion | P11 | P22 |
|---|---|---|
| Weldability | Easier (lower preheat/PWHT severity typical) | More demanding (higher preheat/PWHT; HAZ cracking risk) |
| Strength–Toughness (elevated temp) | Moderate strength, good toughness | Higher strength and creep resistance, toughness dependent on heat treatment |
| Cost | Lower material cost | Higher material cost |
Conclusions and practical guidance: - Choose P11 if: - The design temperature and required creep life are moderate, and cost and ease of fabrication/welding are significant constraints. - You want a more forgiving welding procedure with lower preheat/PWHT severity in shop or field work. - The project allows a material with lower Cr/Mo content and a correspondingly lower allowable stress at temperature.
- Choose P22 if:
- The application requires higher high-temperature strength, longer creep life, or better oxidation resistance at service temperatures.
- You can apply stricter welding controls, preheat, and PWHT, and you accept the higher material cost for longer service life or reduced section thickness.
- Design codes or allowable stress requirements specify a higher-temperature rating that aligns with P22’s performance.
Final note: Both P11 and P22 are mature, well-understood materials with decades of application in power generation and process industries. The decision should be based on design temperature and creep-life requirements, welding and fabrication capabilities, lifecycle cost modeling, and specific code/contract requirements. When in doubt, perform a focused engineering assessment including allowable stress vs. temperature curves, weld procedure qualification, and supplier verification of heat-treatment capability to ensure the chosen grade meets long-term service expectations.