What Is P22 Pipe?
ASTM A335 P22 is a seamless ferritic alloy-steel pipe for high-temperature service, with a composition of about 2.25 percent chromium and 1 percent molybdenum. The chromium provides oxidation and corrosion resistance while the molybdenum delivers creep strength, allowing the pipe to sustain high stress at elevated temperatures for long service lives.
P22 is supplied in the heat-treated condition, normally annealed or normalized and tempered, and is the default material for many high-pressure, high-temperature piping systems.
Chemical Composition
| Element | Typical Range |
|---|---|
| Chromium | 1.90-2.60% |
| Molybdenum | 0.87-1.13% |
| Carbon | 0.05-0.15% |
| Manganese | 0.30-0.60% |
| Phosphorus, max | 0.025% |
| Sulfur, max | 0.025% |
The low carbon content keeps the alloy weldable while the chromium and molybdenum provide the elevated-temperature properties that define the grade.
Mechanical Properties and Service Limits
| Property | Requirement |
|---|---|
| Tensile strength, min | 415 MPa |
| Yield strength, min | 205 MPa |
| Hardness, max | 163 HB |
| Typical temperature ceiling | About 580 degrees C |
P22 exhibits excellent creep and stress rupture strength, significantly better than carbon steel, and resists oxidation and sulfur corrosion better than lower-alloy grades.
Applications
P22 pipe is used in fossil fuel power plants for main steam lines, boiler headers, superheaters and reheaters; in combined-cycle plants for heat recovery steam generators; in refineries for furnace tubes, heater transfer lines and process piping; and in chemical plants for high-temperature process streams.
Fabrication and Welding
P22 requires qualified welding procedures with preheating and post-weld heat treatment for most service conditions. PWHT tempers the hardened heat affected zone and relieves residual stress. The grade is well understood by fabricators, which contributes to its reliability and popularity.
FAQ
Q1. What does P22 mean?
P22 is the ASTM A335 designation for seamless chrome-moly pipe with about 2.25 percent chromium and 1 percent molybdenum for high-temperature service.
Q2. At what temperature can P22 operate?
P22 is typically used up to about 580 degrees C, where its creep strength and oxidation resistance are required.
Q3. Why is P22 preferred over P11 in power plants?
P22's higher chromium content gives better oxidation resistance and slightly higher strength, making it the standard for main steam and reheat lines.
Q4. Is post-weld heat treatment mandatory for P22?
For most power and refinery applications, yes. Codes require PWHT to restore heat affected zone toughness and prevent cracking in service.
Q5. What is the difference between P22 and P91?
P91 adds vanadium and niobium with 9 percent chromium, providing much higher creep strength for advanced power plants at higher cost.
Q6. What tests are performed on P22 pipe?
Chemical analysis, tensile and hardness testing, hydrostatic testing, flattening tests and optional non-destructive examination, with full documentation on the mill certificate.
