What Is ASTM A335?
ASTM A335, formally A335/A335M, covers seamless ferritic alloy-steel pipe intended for high-temperature service. The pipe is designed for bending, flanging and welding in environments requiring elevated-temperature strength and corrosion resistance, and is supplied in the heat-treated condition.
The specification is the piping counterpart of A213 tubes, with P designations replacing the T designations of the tube standard.
Grade Family and Chemistry
| Grade | Composition | Application Note |
|---|---|---|
| P1 | 0.5Mo | Basic molybdenum steel |
| P2 | 0.5Cr-0.5Mo | Low chromium-moly |
| P5 | 5Cr-0.5Mo | Medium chromium, refinery service |
| P9 | 9Cr-1Mo | High chromium |
| P11 | 1.25Cr-0.5Mo | Standard Cr-Mo grade |
| P22 | 2.25Cr-1Mo | Most common grade |
| P91 | 9Cr-1Mo-V-Nb | High-strength, creep resistant |
| P92 | 9Cr-2W-0.5Mo-V-Nb | Advanced 9 percent Cr steel |
| P122 | 12Cr-1W-1Cu-Mo-V-Nb | Highest chromium grade |
Chromium provides oxidation and corrosion resistance while molybdenum provides creep strength, with vanadium and niobium added in the modern grades for precipitation strengthening.
Mechanical Properties
| Grade | Tensile, min MPa | Yield, min MPa | Hardness, max HB |
|---|---|---|---|
| P1, P2 | 380 | 205 | 163 |
| P5, P9, P11, P22 | 415 | 205 | 163 |
| P91 | 585 | 415 | 250 |
| P92 | 620 | 440 | 250 |
The P91 and P92 grades achieve their superior strength through normalization and tempering, which form a tempered martensite structure.
Heat Treatment and Testing
All A335 pipe is supplied heat treated. Lower grades are annealed or stress relieved, while P91, P92 and P122 require normalizing around 1040 to 1080 degrees C followed by tempering around 730 to 800 degrees C. Each pipe is hydrostatically tested, and tensile, hardness, flattening and optional non-destructive tests complete the inspection program.
Applications
A335 pipe serves main steam and hot reheat lines in power plants, refinery furnace and transfer lines, petrochemical process piping and high-temperature manifolds. Grade selection follows the service temperature: P1 and P2 to about 480 degrees C, P11 and P22 to about 580 degrees C, and P91, P92 and P122 to 625-650 degrees C.
FAQ
Q1. What is the difference between A335 and A213?
A335 covers pipe for high-temperature piping systems, while A213 covers smaller tubes for boilers and heat exchangers. The alloys are the same family with P and T prefixes.
Q2. Why is P91 used in modern power plants?
P91 offers high creep strength at 550-600 degrees C, allowing thinner walls and higher steam parameters than older grades.
Q3. Do all A335 grades require post-weld heat treatment?
Most Cr-Mo grades require PWHT after welding to restore toughness and relieve stress; the requirement depends on grade and wall thickness.
Q4. What is the maximum service temperature of A335 pipe?
It depends on grade: about 480 degrees C for P1 and P2, 580 degrees C for P11 and P22, and up to 650 degrees C for P92 and P122.
Q5. Is A335 pipe available as welded pipe?
No, A335 covers seamless pipe only. Welded alloy pipe for similar service is covered by specifications such as A691.
Q6. What documents accompany A335 pipe?
A mill test certificate with heat numbers, chemical analysis, tensile and hardness results, heat treatment records and hydrostatic test data.
