ASTM A335 P11 vs P22

P11 and P22 are the two most commonly specified ASTM A335 chrome-moly alloy steel pipe grades. Both are designed for elevated-temperature service, but they differ in chromium content, hydrogen resistance, and operating temperature range.

P11 (1.25Cr-0.5Mo) is the standard grade for moderate high-temperature applications. P22 (2.25Cr-1Mo) offers better performance at higher temperatures and in hydrogen-rich environments.

Head-to-Head Comparison

Property	P11 (1.25Cr-0.5Mo)	P22 (2.25Cr-1Mo)
Chromium	1.00-1.50%	1.90-2.60%
Molybdenum	0.44-0.65%	0.87-1.13%
Carbon	0.05-0.15%	0.05-0.15%
Manganese	0.30-0.60%	0.30-0.60%
Min. Yield Strength	205 MPa (30 ksi)	205 MPa (30 ksi)
Min. Tensile Strength	415 MPa (60 ksi)	415 MPa (60 ksi)
Max. design temp (~)	538°C (1000°F)	565°C (1050°F)
Hydrogen resistance	Moderate	Better (higher Cr)
Preheat for welding	150-200°C	200-250°C
PWHT	677-727°C	677-727°C
EN equivalent	13CrMo4-5 (1.7335)	10CrMo9-10 (1.7380)
Relative cost	Base	+5-10%

When to Use P11

P11 is the first step above carbon steel (ASTM A106) when operating temperatures exceed approximately 427°C. Typical applications:

• Boiler superheater piping (up to ~538°C)
• Refinery atmospheric and vacuum distillation unit piping
• Steam headers and laterals in power plants
• Process piping where moderate hydrogen partial pressures exist (below the P11 curve on the API 941 Nelson diagram)

When to Use P22

P22 is required when P11 reaches its limits:

• Operating temperatures between 538°C and 565°C
• Hydrogen service at elevated temperatures (hydrodesulfurization, hydrotreating, hydrocracking units)
• Applications above the P11 curve on the API 941 Nelson diagram
• Main steam piping in supercritical boilers

Pro Tip

Check the API 941 (Nelson curves) chart during material selection. If the operating temperature and hydrogen partial pressure fall above the P11 safe zone, P22 (or higher grades like P91) is mandatory to prevent high-temperature hydrogen attack (HTHA).

Weldability

Both grades require preheat and post-weld heat treatment (PWHT), but the parameters differ:

Welding Parameter	P11	P22
Min. preheat	150°C (302°F)	200°C (392°F)
Max. interpass	300°C (572°F)	300°C (572°F)
PWHT range	677-727°C	677-727°C
PWHT hold time	1 hr/25 mm (min 1 hr)	1 hr/25 mm (min 1 hr)
Filler metal	ER80S-B2 / E8018-B2	ER90S-B3 / E9018-B3
Hardness (max post-PWHT)	225 HBW	225 HBW

Dissimilar welding between P11 and P22 (or between either and carbon steel) requires specific filler metal selection and PWHT procedures. Consult qualified welding procedures per ASME Section IX and the project piping specification.

For grades beyond P22 (P91, P92), see the full ASTM A335 alloy pipe guide.

Read the full guide to pipe types