What Are the Spec Differences Between A335 P11, P22, and P91?

All three grades fall under ASTM A335/A335M — Seamless Ferritic Alloy-Steel Pipe for High-Temperature Service. But the chemistry and mechanical gaps between them are enormous. The table below pulls every relevant value directly from the ASTM specification.

Why Does P91 Allow Thinner Walls Than P11 at the Same Design Pressure?

1. Strength — P91's Yield Is 2× P11's, and That Directly Reduces Wall Thickness

The yield strength difference between these grades is the single most important engineering distinction. P11 specifies 205 MPa minimum yield; P22 also specifies 205 MPa; P91 specifies 415 MPa — exactly double. In ASME B31.1 and B31.3 wall thickness calculations, allowable stress at design temperature directly determines required wall thickness. P91's higher allowable stress values mean that for the same design pressure and diameter, a P91 pipe requires roughly half the wall thickness of a P11 pipe at temperatures below 500°C.

At elevated temperatures, the gap widens further. P91's creep strength at 600°C is approximately 3× that of P11 at the same temperature, because the vanadium and niobium in P91 form MX carbonitride precipitates (V(N,C) and Nb(N,C)) that pin grain boundaries and prevent dislocation climb — the mechanism that drives creep deformation. P11 and P22, without V and Nb, rely solely on Mo carbides for creep resistance, which are less effective above 550°C.

The practical consequence: a supercritical boiler main steam line using P91 at 600°C can be designed with significantly thinner walls than the same line in P22. Thinner walls mean less weight, less welding volume, faster construction, and lower total installed cost — even though P91's material cost per kg is 2.5–3× P11's. The total installed cost of a P91 system is often lower than a P22 system for the same duty because the wall thickness savings offset the material premium.

2. Temperature Capability — Each Grade Has Its Own Ceiling

ASME B31.1 allowable stress tables define the effective temperature ceiling for each grade where the allowable stress drops to impractical levels:

• P11: Effective ceiling ~550°C (1022°F). Above this temperature, allowable stress drops rapidly — P11 is no longer economical for wall thickness calculations above 550°C because the required wall thickness becomes impractically thick.
• P22: Effective ceiling ~600°C (1112°F). P22's higher Cr and Mo give it ~50°C more than P11. It's the established refinery heater tube grade where temperatures run 500–600°C.
• P91: Effective ceiling ~650°C (1202°F). P91 extends the envelope another 50°C beyond P22 and maintains useful allowable stress values up to that ceiling. For ultra-supercritical designs approaching 620–630°C, P91 is the only practical choice among these three grades.

For temperatures above 650°C, the industry moves to higher-alloyed materials — P92 (9Cr-2W-V-Nb) or austenitic stainless steels — but those are outside this comparison. Within the A335 chrome-moly family, P91 holds the high-temperature crown.

3. Weldability — P91 Demands Precision That P11 and P22 Do Not

This is where P91's complexity becomes real. All three grades require preheat and PWHT (post-weld heat treatment), but the requirements escalate dramatically from P11 to P91:

The critical difference: P91's PWHT must be held within a 45°C window (730–775°C) — overshooting above 775°C destroys the tempered martensite microstructure and reduces creep strength to P11 levels. Undershooting below 730°C leaves untempered martensite in the weld HAZ, creating brittle zones that crack under thermal cycling. This is not a flexible requirement — it demands qualified welders, calibrated equipment, and documented temperature records. P11 and P22 have much wider PWHT windows and are forgiving of minor deviations.

Our fabrication team's experience: P91 welding requires welders qualified specifically per ASME IX on P91 test coupons. A welder qualified on P22 (P-No. 5A) cannot automatically weld P91 (P-No. 5B) — the essential variables differ. We've seen projects where P22-qualified welders were deployed on P91 joints without requalification, and the resulting HAZ microstructures failed to meet the required hardness ceiling of 350 HBW. Corrective re-PWHT was necessary on every joint, adding 3 weeks to the schedule.

4. Application Mapping — Where Each Grade Belongs

5. Cost Comparison — Material Price vs. Total Installed Cost

The material cost progression is straightforward: P11 at baseline 1.0×, P22 at ~1.3×, P91 at ~2.5–3.0×. But the total installed cost picture is more nuanced because P91's higher allowable stress reduces wall thickness:

Note: Cost factors from HT PIPE 2025 procurement data for A335 alloy pipe. Wall thickness values are illustrative for a 12" NB pipe at 150 bar design pressure and 600°C per ASME B31.1 calculations. Actual values depend on specific design conditions.

6. Hydrogen Resistance — P22 Is the Refinery Standard for a Reason

In refinery hydrogen service (hydroprocessing reactors, shift converters), the API 941 Nelson curves define temperature limits where each alloy resists high-pressure hydrogen attack. P22 (2.25Cr-1Mo) sits on a more favorable Nelson curve position than P11 (1.25Cr-0.5Mo) — P22 can withstand hydrogen at higher temperatures and pressures without suffering hydrogen attack (decarburization and methane bubble formation at grain boundaries).

P91's 9Cr content actually gives it excellent hydrogen resistance per the Nelson curves — better than both P11 and P22. But P91 is rarely specified for refinery hydrogen service because (1) the welding complexity is a liability in refinery construction environments where weld quality control is less rigorous than power plant construction, and (2) the established code history for P22 in hydrogen service (decades of API 941 compliance data) provides regulatory comfort that P91 cannot yet match despite its superior chemistry.

HT PIPE's Real-World Selection Guide Based on A335 Alloy Pipe Exports

Our "ASTM A335 Alloy Pipe.xlsx" inquiry data over the past 18 months shows a clear geographic split:

• Middle East / Refinery projects: 75% P11 and P22, 25% P91. Refinery heater tube replacements and hydrogen service piping dominate. P22 is the default for anything above 500°C; P11 for moderate-temperature utility piping. P91 appears only when a refinery is upgrading to higher operating temperatures.
• South Asia / Power plant projects: 80% P91, 20% P11/P22. India's supercritical power plant construction program drives massive P91 demand — virtually every new 660MW and 800MW unit specifies P91 for main steam and reheat lines.
• Central Asia / Combined projects: Mixed P11/P22/P91 depending on whether the project is refinery or power plant.

We at HT PIPE supplied A335 alloy pipe for a refinery heater replacement project in the Middle East in 2024. The original heater tubes were P11 (installed in the 1990s) operating at 520°C. The refinery upgraded to P22 for the replacement because the new operating temperature was 560°C — above P11's practical ceiling but within P22's comfort zone. P91 was considered but rejected because the refinery's maintenance welders were qualified on P22, not P91, and the schedule didn't allow for welder requalification and procedure development.

This is a common decision pattern: P91 wins on paper for strength and wall thickness optimization, but P22 wins in practice when welding infrastructure and qualified personnel aren't available for P91's demanding requirements. Choose the grade your project can actually weld correctly — not the grade that looks best in a calculation.

Frequently Asked Questions

Q1: Can P91 be substituted for P11 or P22?

Yes — P91 meets all P11 and P22 service requirements and exceeds them in strength and temperature capability. S32205 meets all S31803 requirements; similarly, P91 exceeds both P11 and P22 in every mechanical and corrosion parameter. However, the substitution introduces P91's demanding welding requirements — PWHT within a strict 730–775°C window, P-No. 5B qualified welders, and hardness testing to 350 HBW max. If your fabrication team cannot meet these requirements, substituting P91 for P11/P22 creates a quality risk that outweighs the performance advantage.

Q2: Why is P91's PWHT window so narrow?

P91 is a tempered martensite alloy — its creep strength depends on the specific microstructure created by normalizing (1040–1080°C) and tempering (730–775°C). PWHT above 775°C overtempers the martensite, dissolving the fine MX carbonitride precipitates that provide creep strength — the result is a microstructure with creep performance no better than P11. PWHT below 730°C leaves untempered martensite in the HAZ, which is brittle and cracks under thermal cycling. The 45°C window is the range where tempering produces the correct microstructure — it's not a specification convenience, it's a metallurgical requirement.

Q3: What filler metal should I use for P91 welds?

Use ER90S-B9 (GTAW) and E9018-B9 (SMAW) — these are matching filler metals with chemistry designed to produce the same tempered martensite microstructure as the base metal after PWHT. Do not use ER80S-B2 (P11 filler) or ER90S-B3 (P22 filler) on P91 — the resulting weld chemistry will be deficient in V and Nb, producing a weak HAZ. For dissimilar welds between P91 and P11/P22, use ER90S-B9 filler on the P91 side with a transition weld procedure per ASME B31.1.

Q4: Does P91 require hardness testing after welding?

Yes — ASME B31.1 and the P91 welding guidelines (EPRI guidelines, AWS D10.10) require Vickers hardness testing of the weld HAZ after PWHT, with a maximum of 350 HV (approximately 350 HBW). Any HAZ location exceeding 350 HV indicates insufficient PWHT — the martensite has not been properly tempered and the joint is at risk of brittle cracking under thermal cycling. For P11 and P22, hardness testing is recommended but not as strictly enforced because these alloys don't have the same brittleness risk in the HAZ.

Q5: Is P91 more corrosion-resistant than P11 or P22?

Yes — P91's 9% chromium content provides significantly better oxidation resistance at elevated temperatures than P11's 1.25% Cr or P22's 2.25% Cr. In steam environments above 550°C, P91 forms a more protective oxide scale that limits steam oxidation and exfoliation. P11 and P22 suffer from steam-side oxide exfoliation at temperatures above 540°C, which can block turbine inlet screens with spalled oxide flakes. P91's higher Cr also provides better resistance to sulfidation and fuel ash corrosion in coal-fired boiler environments.

Q6: When should I choose P22 over P11?

Choose P22 when: (1) Design temperature exceeds 540°C — P22 maintains useful allowable stress up to 600°C where P11 drops off. (2) Hydrogen service per API 941 — P22's 2.25Cr sits on a more favorable Nelson curve. (3) Higher creep strength is needed — P22's 1Mo content provides ~30% more creep strength than P11 at 540°C. The cost premium for P22 over P11 is typically 30%, which is justified for any application above 540°C or in hydrogen environments. For service below 540°C without hydrogen, P11 is adequate and more economical.

Related Comparisons & Resources

• A333 Gr.6 vs A420 WPL6: Low Temperature Carbon Steel Pipe & Fittings — the other end of the temperature spectrum
• Carbon Steel vs Stainless Steel: Cost, Corrosion & Application Guide
• A335 Alloy Seamless Pipe — All Grades P5 through P91
• A234 Alloy Butt Weld Fittings — WP11, WP22, WP91
• A182 Alloy Flanges — F11, F22, F91
• HT PIPE Recent Export Projects — refinery and power plant alloy pipe case studies