What A335 P22 Is
A335 P22 is a seamless ferritic alloy steel pipe Grade P22 under ASTM A335/A335M and its ASME counterpart SA335. The grade is the classic 2.25% chromium, 1% molybdenum steel, UNS number K21590, and it has been the workhorse material for main steam lines, hot reheat lines, superheater outlet headers and high pressure steam piping for decades.
Chromium supplies oxidation and corrosion resistance at temperature; molybdenum supplies creep strength. The combination is the reason P22 is still specified where a lower alloy steel such as P11 (1.25Cr-0.5Mo) would be overstressed and a 9Cr steel would be unnecessarily expensive.
Temperature Range and the Code Basis
The practical range for P22 piping runs from ambient start-up conditions up to about 650 degrees C (1200 degrees F). The figure is a code limit rather than a supplier preference: the allowable stress tables for 2.25Cr-1Mo in ASME B31.1 and in ASME Section II Part D are tabulated up to 1200 degrees F, and above roughly 550 degrees C the allowable stress is controlled by creep and rupture data rather than by the room temperature tensile values.
Two practical consequences follow. First, the room temperature tensile certificate does not by itself qualify the pipe for the top of the range. Second, because the strength above 550 degrees C comes from the heat treated microstructure, any repair or field weld must use a procedure that preserves it, including the correct post-weld heat treatment.
Chemical Composition
| Element | Requirement for P22 |
|---|---|
| Carbon | 0.05-0.15% |
| Manganese | 0.30-0.60% |
| Phosphorus | 0.025% maximum |
| Sulphur | 0.025% maximum |
| Silicon | 0.50% maximum |
| Chromium | 1.90-2.60% |
| Molybdenum | 0.87-1.13% |
The narrow carbon range and the 0.025% phosphorus and sulphur ceilings are typical of a pressure part grade. They exist to protect creep ductility and weldability, and they are also why the grade is more expensive to make than a structural steel with the same chromium level.
Mechanical Properties and Heat Treatment
| Property | Requirement |
|---|---|
| Minimum tensile strength | 415 MPa |
| Minimum yield strength | 205 MPa |
| Minimum elongation | 30% |
| Heat treatment | Normalized and tempered, or quenched and tempered |
A335 requires the grade to be heat treated, and the tempering treatment must be controlled because the properties used in the code at high temperature depend on it. Records of furnace charge, holding temperature and holding time are part of the documentation package for a power plant order, alongside the tensile test, the flattening or bend test and the hydrostatic test required by the general requirements of ASTM A1016/A1016M.
Size Range, Dimensions and Applications
Dimensions are normally ordered to ASME B36.10M. Common production covers outside diameters from 21.3 mm up to about 762 mm with wall thickness from roughly 3 mm to 160 mm, which allows both small bore superheater tube and large bore hot reheat pipe to be bought to the same grade. Because wall thickness drives the through-wall temperature gradient, thick wall P22 pipe in cyclic service is usually assessed for thermal fatigue in addition to the steady state creep check.
Typical duties are main steam and hot reheat piping, superheater and reheater outlet headers, steam drum internals, and high temperature process piping in refinery and petrochemical units where hydrogen or sulphur service does not require a higher alloy.
Frequently Asked Questions
Q: What is the maximum temperature for A335 P22 pipe?
A: About 650 degrees C (1200 degrees F), which is the upper limit of the code allowable stress tables; above roughly 550 degrees C creep governs the design.
Q: What is the minimum yield strength of P22?
A: 205 MPa, with minimum tensile strength of 415 MPa and minimum elongation of 30% under ASTM A335/A335M and ASME SA335.
Q: What is the chromium and molybdenum content of P22?
A: Chromium 1.90-2.60% and molybdenum 0.87-1.13%, the 2.25Cr-1Mo composition with UNS number K21590.
Q: Does P22 need post-weld heat treatment?
A: Yes. Wall thickness and chemistry place it in the range where post-weld heat treatment is required, and the treatment must not degrade the tempered microstructure that provides creep strength.
Q: Which dimensional standard applies to P22 pipe?
A: ASME B36.10M for wrought steel pipe dimensions, with the grade requirements from ASTM A335/A335M or ASME SA335.