What Is ASTM A335?

Quick Answer

ASTM A335 covers seamless ferritic alloy steel pipe for high-temperature service. The grades (P1 through P91) contain varying amounts of chromium and molybdenum that provide superior creep resistance at temperatures from 400°C to 600°C+, well beyond the limits of carbon steel pipe (ASTM A106).

Chrome-moly (Cr-Mo) alloy pipes are required in power generation, refining, and petrochemical plants where piping operates continuously at elevated temperatures. The chromium provides oxidation and corrosion resistance; the molybdenum enhances high-temperature strength and creep resistance.

ASTM A335 Grades

Grade	Cr (%)	Mo (%)	Max Temp (~°C)	Yield (min)	Tensile (min)
P1	0.44-0.65 (Cr-free; C-Mo)	0.44-0.65	482°C	205 MPa	380 MPa
P5	4.0-6.0	0.45-0.65	593°C	205 MPa	415 MPa
P9	8.0-10.0	0.90-1.10	593°C	205 MPa	415 MPa
P11	1.0-1.5	0.44-0.65	538°C	205 MPa	415 MPa
P22	1.9-2.6	0.87-1.13	565°C	205 MPa	415 MPa
P91	8.0-9.5	0.85-1.05	593°C	415 MPa	585 MPa
P92	8.5-9.5	0.30-0.60	620°C	440 MPa	620 MPa

Max temperatures are approximate guidelines based on ASME B31.1/B31.3 allowable stress tables. Actual limits depend on design pressure and service conditions.

Most Common Grades

P11 (1.25Cr-0.5Mo) and P22 (2.25Cr-1Mo) are the two most widely used alloy pipe grades. P11 handles temperatures up to ~538°C and is common in:

• Boiler superheater and reheater piping
• Refinery heater transfer lines
• Steam distribution headers

P22 extends the range slightly higher and offers better resistance to hydrogen attack (per API 941 Nelson curves). It is the standard for:

• High-pressure steam mains in power plants
• Hydroprocessing reactors and associated piping
• Process piping in contact with hydrogen above 260°C

For a head-to-head comparison, see ASTM A335 P11 vs P22.

P91: Advanced Creep-Resistant Steel

P91 (9Cr-1Mo-V-Nb) offers significantly higher allowable stress at elevated temperatures than P11 or P22, allowing thinner wall thicknesses and lighter piping systems. This reduces dead weight on pipe supports and shortens welding time. However, P91 requires strict heat treatment control (normalizing + tempering) and qualified welding procedures.

Common Mistake

Welding ASTM A335 P91 without a precise post-weld heat treatment (PWHT) at 760±15°C can leave untempered martensite in the weld zone, leading to premature creep failures. Always verify that fabricators have qualified WPS/PQR per ASME Section IX specific to P91.

Heat Treatment

All ASTM A335 grades require specific heat treatment:

• P11: Normalized and tempered, or annealed
• P22: Normalized and tempered
• P91: Normalized (1040-1080°C) + tempered (730-780°C)

Heat treatment conditions directly affect the final mechanical properties and creep resistance. Mill test certificates must document the actual temperatures and hold times.

Read the full guide to pipe types