1. Quick Summary
ASTM A335 is a standard specification for seamless ferritic alloy-steel pipe intended for high-temperature service. The "P22" designation refers to a specific grade of steel within this standard, commonly known as 2.25 Chrome - 1 Molybdenum steel.
In simple terms: ASTM A335 P22 is a type of seamless pipe made from a chromium-molybdenum alloy steel, designed to withstand high pressure and high heat for long periods.
2. Key Characteristics & Properties
Chemical Composition
The chemical composition is the primary factor defining Grade P22. It provides the corrosion resistance and strength at high temperatures.
| Element | Composition (%) |
|---|---|
| Carbon (C) | 0.05 - 0.15 |
| Manganese (Mn) | 0.30 - 0.60 |
| Phosphorus (P) | 0.025 max |
| Sulfur (S) | 0.025 max |
| Silicon (Si) | 0.50 max |
| Chromium (Cr) | 1.90 - 2.60 |
| Molybdenum (Mo) | 0.87 - 1.13 |
Chromium (Cr): Provides improved oxidation (scaling) and corrosion resistance at high temperatures.
Molybdenum (Mo): Enhances strength at high temperatures and improves resistance to creep (slow deformation under stress and heat).
Mechanical Properties
These properties are measured at room temperature.
| Property | Value |
|---|---|
| Tensile Strength | 415 MPa (60 ksi) min |
| Yield Strength | 205 MPa (30 ksi) min |
| Elongation | Varies with wall thickness, but typically ≥ 30% |
High-Temperature Performance
The main reason for using P22 is its excellent performance in elevated temperature environments (typically up to about 593°C / 1100°F). It has good creep strength, meaning it resists deforming over long periods under high stress and heat.
3. Common Applications
ASTM A335 P22 pipes are used in critical services where high temperature and pressure are present:
Power Plants: Superheaters, reheater tubes, main steam lines, and headers in fossil-fuel power stations.
Petrochemical Refineries: Pipelines in refinery process units, such as catalytic crackers and hydrocrackers, where temperatures are high.
Boiler Tubes: For both industrial and utility boilers.
Heat Exchangers: In services requiring resistance to thermal cycling and high pressure.
4. Manufacturing Process
As a seamless pipe, it is manufactured without a welded seam. The primary process involves:
Piercing: A solid cylindrical steel billet is heated and pierced by a mandrel to create a hollow shell.
Elongation: The hollow shell is then elongated and rolled to achieve the desired diameter and wall thickness.
Sizing & Finishing: The pipe is finished to precise dimensions, heat-treated, and subjected to various tests.
The seamless construction provides a more uniform structure around the entire circumference, making it stronger and more reliable for high-pressure applications compared to welded pipes.
5. Heat Treatment
A335 P22 pipes are required to be supplied in the fully annealed or normalized and tempered condition. This heat treatment is essential to:
Relieve internal stresses from the manufacturing process.
Refine the grain structure.
Achieve the desired combination of strength and ductility.
6. Testing and Inspection
To ensure quality and integrity, the pipes undergo rigorous testing, including:
Hydrostatic Test: Each pipe is tested to a specified water pressure without leaking.
Non-Destructive Electric Test: Often an eddy current or ultrasonic test to check for surface and subsurface defects.
Flattening Test, Hardness Test, Bend Test: Performed on samples from each heat lot.
Chemical Analysis and Tensile Tests: Conducted to verify composition and mechanical properties.
7. Comparison with Other Grades
ASTM A335 P11 (1.25Cr-0.5Mo): Lower chromium and molybdenum content than P22, meaning lower high-temperature strength and oxidation resistance. It's used for less severe service conditions.
ASTM A335 P91 (9Cr-1Mo-V): A newer, advanced alloy with significantly higher strength at elevated temperatures. Allows for thinner pipe walls at the same pressure rating, but requires more precise welding procedures.
ASTM A106 Grade B (Carbon Steel): Used for high-temperature service but without significant corrosion resistance. It's a cheaper alternative for applications where oxidation is not the primary concern.
Important Note on Welding
Welding P22 steel requires special procedures, including preheating (typically 200-300°C / 400-575°F) and post-weld heat treatment (PWHT). This is critical to prevent cracking and to ensure the weldment has properties matching the base metal. Welding rods or wires with a similar composition (e.g., ER90S-B3, E9015-B3) are used.
Summary Table
| Feature | Description |
|---|---|
| Standard | ASTM A335 (Seamless Ferritic Alloy-Steel for High-Temperature Service) |
| Grade | P22 (Equivalent to WP22 in ASTM A335) |
| Common Name | 2.25Cr-1Mo Steel |
| Key Feature | Excellent creep strength and oxidation resistance at high temperatures |
| Form | Seamless Pipe |
| Primary Applications | Power plants, refineries, high-temperature pressure vessels |
In conclusion, ASTM A335 P22 Seamless Steel Pipe is a workhorse alloy steel pipe chosen for its reliable performance in demanding high-temperature and high-pressure environments. Its combination of chromium and molybdenum makes it a durable and long-lasting choice for critical industrial infrastructure.
