SA671 CF66 Carbon Steel Boiler Steel Pipe
Introduction
ASTM A671 CF66 is an electric-fusion-welded carbon steel plate pipe designed for high-pressure and elevated temperature service, specifically categorized for its impact toughness at low temperatures. It is a distinct grade within the ASTM A671 specification.
The nomenclature is key:
SA671: The base specification for electric-fusion-welded pipe.
C: Indicates the pipe is fabricated from carbon steel plate.
F: A mandatory suffix indicating the pipe is suitable for service at low metal temperatures and that its impact properties are a specified requirement.
66: Represents the minimum specified yield strength of 66 ksi (455 MPa).
The "F" designation is the critical differentiator. It mandates that the pipe must meet specific Charpy V-notch impact toughness values at a designated low temperature, making it suitable for applications where the operating temperature can fall below ambient, but not necessarily to cryogenic levels.
Key Characteristics
Material: Carbon steel, typically made to fine-grain practice.
Manufacturing Process: Electric-Fusion-Welded (EFW) from steel plates.
Key Property: Guaranteed impact toughness at a specified low temperature, combined with good strength.
Mandatory Requirement: Charpy V-notch impact testing at a purchaser-specified temperature.
Typical Service Temperature Range: Suitable for both elevated temperature service (up to ~650°F / 343°C) and low-temperature service down to the specified impact test temperature (e.g., -20°F / -29°C, 0°F / -18°C).
Primary Application: Pressure vessels and piping systems that operate at high pressure but may be exposed to low ambient temperatures during shutdown, hydro-testing with cold water, or in cold climates.
Standards: ASTM A671.
Chemical Composition (ASTM A671 CF66, %)
The composition is controlled to ensure adequate strength and, crucially, good low-temperature toughness.
| Element | Maximum Percentage (%) | Notes |
|---|---|---|
| Carbon (C) | 0.24 | Limited to ensure weldability and toughness. |
| Manganese (Mn) | 1.20 | Primary strengthening element; Mn:C ratio is optimized for toughness. |
| Phosphorus (P) | 0.035 | Kept low to preserve toughness. |
| Sulfur (S) | 0.035 | Kept low to preserve toughness and weldability. |
| Silicon (Si) | 0.15 - 0.40 | Deoxidizer. |
| Carbon Equivalent (C.E.) | Typically ≤ 0.43 | Controlled to ensure good weldability without excessive preheat. |
*Note: The plate material is typically a normalized fine-grain steel conforming to specifications like ASTM A516 Grade 70 or similar.*
Mechanical Properties (Room Temperature)
| Property | Value (Imperial) | Value (Metric) |
|---|---|---|
| Yield Strength (min) | 66 ksi | 455 MPa |
| Tensile Strength (min) | 75 ksi | 517 MPa |
| Elongation (min, %) | 19 | 19 (in 2 in./50mm) |
Impact Toughness (Mandatory Requirement)
This is the defining requirement for "F" grades. The exact values and test temperature are specified by the purchaser.
| Parameter | Typical Requirement (Example) | Notes |
|---|---|---|
| Test Method | Charpy V-notch (CVN) | Standard per ASTM A370. |
| Test Temperature | Purchaser Specified (e.g., -20°F / -29°C, 0°F / -18°C) | Must be stated in the order. It defines the minimum design metal temperature. |
| Minimum Average Energy | Usually 20 ft-lb (27 J) for 3 specimens | Common requirement for many pressure vessel codes. |
| Minimum Single Value | Usually 15 ft-lb (20 J) | No single specimen shall be below this value. |
Note: The pipe may be supplied in the as-welded or stress-relieved condition, but the condition must be such that it meets the impact requirements.
Heat Treatment
Base Plate: The steel plate is typically supplied in the normalized condition to achieve a fine-grained microstructure conducive to good toughness.
Finished Pipe: Post-weld heat treatment (PWHT) or stress relief is often applied to the weld seam to ensure the weld and Heat-Affected Zone (HAZ) also meet the toughness requirements. The specific heat treatment is part of the qualified manufacturing procedure.
Non-Destructive Examination (NDE)
High-quality NDE is standard for these critical pipes.
Radiography (RT): The longitudinal seam weld is 100% radiographed (typically ASTM A671 Class 12).
Ultrasonic Testing (UT): May be specified for additional flaw detection.
Hydrostatic Testing: Mandatory for each pipe.
Typical Applications
Pressure Vessels and Boilers in Cold Climates: Where equipment is exposed to low winter temperatures.
Hydrotest of Hot Service Equipment: Using cold water, which can chill the metal below its design temperature during the test.
Refrigeration Plant Piping & Vessels: For ammonia or Freon systems.
Process Piping in Petrochemical Plants: That may see occasional low temperatures.
**Feedwater Heaters and Other
